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thesis entitled
Iron Oxidation Indices for Antarctic Meteorites

with Hypotheses for Associated Weathering Mechanisms

 

A Study of 19 Weathering Category C Ordinary Chondrites
from Allan Hills and Lewis Cliff
presented by

James Warren Ashley

has been accepted towards fulfillment
of the requirements for

Master of Science degree in Geological Science

 

 

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IRON OXIDATION IN DICES FOR ANT ARCTIC METEORITES
WITH HYPOTHESES FOR ASSOCIATED WEATHERING MECHANISMS

 

A STUDY OF 19 WEATHERING CATEGORY C ORDINARY CHONDRITES
FROM ALLAN HILLS AND LEWIS CLIFF

by

James Warren Ashley

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Geological Sciences

1995

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ABSTRACT

[RON OXIDATION IN DICES FOR AN TARCT IC METEORITES
WITH HYPOTHESES FOR ASSOCIATED WEATHERING MECHANISMS

 

A STUDY OF 19 WEATHERING CATEGORY C ORDINARY CHONDRITES
FROM ALLAN HILLS AND LEWIS CLIFF

By

James Warren Ashley

Preliminary findings show that negative weight loss (weight gain) on ignition (-
L01), may provide a viable alternative to the ABCe system for indexing iron rust
intensities among Antarctic meteorites. Further investigation is warranted.

Due to (1) fractures, plus (2) the interplay of secondary product stress versus
rock strength, in possible combination with petrologic type and/or recovered sample
mass, a heterogeneity in iron oxidation develops in highly weathered Antarctic ordinary
chondrites which may also characterize the distribution patterns of other types of
alteration products. In addition to weathering intensity, researchers should be mindful
of fracture density and recovered sample mass during sample selection for studies of
pre-terrestrial properties. H chondrites may be more susceptible to fragmentation in
the Antarctic environment than L Chondrites due to their higher relative abundance of
reduced metal grains. This tendency may help explain the mass frequency distribution
observed for the Antarctic ordinary chondrite population, which is skewed toward

lower masses for H Chondrites relative to L Chondrites.

This work is dedicated to my parents:
Leona Rose/135119!
and
RJ (150nm: flrfiléy
in commemoration of their
52MWefimgflnniversary

(May 15m, 1995)

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ACKNOWLEDGMENTS

For their participation in this project, I would like to
thank the following individuals and institutions. My
advisor and friend Dr. Michael A. Velbel for countless hours
of inspiration, brainstorming, support, and good-natured
cajoling, Dr. Thomas A. Vogel and Kris Huysken for helping
with loss on ignition sample preparation, Dr. Ann Yates for
advice on application of the -LOI method for meteorites, Dr.
Roy S. Clarke, Jr. and Dr. Eugene Jarosewich of the
Smithsonian Institution, Division of Meteorites, for
generously allowing me access (on the eve of a national
holiday) to the Smithsonian meteorite collection, and the
use of their microscope and curatorial facilities, Kurt
Zacharias for instruction on use of the scanning electron
microscope, Dr. Delbert L. Mokma and Dr. Shawel Haile-Mariam
for assistance with the citrate dithionite pre-treatment,
Dr. Mounir Saad for assistance with X-ray diffraction
procedures, and my friends and colleagues of the Geological
Sciences Department at M.S.U. I would like to thank my
former employer SSOE, Inc., and work associates Jeff
McCormack and David Schroeder for their understanding and

composure during my time on this project. I would also like

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to express my deep gratitude to Dan and Pat Voydanoff for
providing my wife and I with a microcomputer system as'a
wedding gift, as a result of which, this thesis was written
and printed at home. Additionally, I wish to recognize
Kevin S. Jung, John Foran, Hazel Bremmer, Gary Dannemiller
and Dr. Larry Barrows for their assistance with various

aspects of the work.

For any first list of acknowledgements, I feel it fitting to
include among those directly related to the project, any
additional individuals and institutions who are considered
instrumental to the author’s development as a scientist.
Accordingly, I would here like to name Dr. Norman W. Ten
Brink for friendly encouragement and guidance during my
undergraduate years. Also deserving of his own distinct
accolades is Mr. David L. DeBruyn, Chief Curator at the
Roger B. Chaffee Planetarium of the Grand Rapids Public
Museum where I worked part time for 15 years. I would like
to thank Mr. DeBruyn for half a lifetime of patient
mentoring and a contagious, and philosophically optimistic
enthusiasm for “life, the universe and everything,” as it
were. I will also mention Mike LeBaron and Jon Truax for
having the vision and drive to take on, in the face of
political opposition, what came to be known as the James C.
Veen Observatory Radio Telescope Project; a six-year, on—

going effort carried out in the spirit of research that

ultimately became an accessory to my personal scientific

growth.

Most of all, a shining acknowledgment to the object of my

affection, my wife Sandy, for superhuman coolness throughout

the extended operation of working full time and writing a
thesis “on the side,” an unfathomable number of home-cooked

meals, and an occasional karate kick for motivation.

vi

PREFACE

The weathering of meteorites has been recognized as a stick in the eye of
science doubtless since the first find was studied in the laboratory. Cosmically
significant interpretations often rely on the precise measurement of subtle
patterns in chemical composition and mineralogies. The terrestrial alteration of
these primary features is viewed almost as a sacrilegious corruption by many
meteoriticists — profaning the stone’s innocence like some kind of perverse
molestation by disallowing the successful delivery of all those shrouded secrets
from our distant past (as the stone had originally intended).

Meteorites are literally windfalls of serendipity — the most exciting
rocks on earth fall from the sky. Yet one must be about their swift recovery, or
Nature herself will, through weathering, ironically and murderously foil all
striving for primordial solar system enlightenment. Its “The Fix,” of Mr.
Murphy (his zeroth law) — like the bag of gold coins that fades in your hands
as you snap out of your dead-of-night influenza fever.

While sharing this sentiment (all humor aside) on a scientific level, I
have learned during this research to respect the quiet decay action of the
weathering force. There is a noble deliberateness and irreversibility of change
for minerals out of equilibrium with their birth-place environment. So it is for
all things; and in due time, weathering, or something very like it, will overtake
everything and everybody. Like any natural process, weathering (or the
increase of entropy) can be thought of simply as an on-going transformation,
witheffectsthatcanbeasbeautifulastheyaredamaging. Inthewordsof
Stevenson (1881):

"Right before us, at the southern end we saw the wreck of a ship in the last
stages of dikpidation. It had been a great vessel of three masts, but had Iain so long
exposed to the injuries of the weather, that it was hung about with great webs of
dripping seaweed, and on the deck of it share bushes had taken root, and now
flourished thick with flowers. "

-Robert Louis Stevenson
Treasure Island

vii

TABLE OF CONTENTS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LIST OF TABLES xi
LIST OF FIGURES xii
KEY TO ABBREVIATIONS AND NOMENCLATURE xvi
1 INTRODUCTION 1
1.1 The Discipline of Meteoritics l
1.2 Background on Meteorites 2
1.2.1 Chondrite Classifieation 2
1.2.1.1 Carbonaceous Chondrites 5

1.2.1.2 Ordinary Chondrites 6

1.2.1.3 Enstatite Chondrites 7

1.2.2 Petrologic Types 7
1.2.3 Post-accretionary Aqueous Alteration Products 9
1.2.3.1 Carbonaceous Chondrite Matrices 10

1.2.3.2 Ordinary Chondrite Matrices 11

1.2.4 Meteorite Weathering 14
1.2.5 Fusion Crusts and Fractures 15

1.3 Antarctic Meteorites 16
1.3.1 ReconnaissanceandReoovery 16
1.3.2 Antarctic Meteorite Weathering 18
1.3.3 Signifieance of Extraterrestrial Aqueous Alteration for Antarctic Meteorites 21
1.3.4 The Antarctic and non-Antarctic Meteorite Source Controversy 22
1.3.4.1 Age Differences 24

1.3.4.2 Type Differences 25

1.3.4.3 Mass Differences 26

1.3.4.4 Chemieal Differences 27
1.3.4.4a Major Elements 27

1.3.4.4b Trace Elements 28

 

viii

3A

4!:

1.3.4.5 Cause and Implication of Differences

29

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.3.4.5a Terrestrial Causes (Weathering and Contamination) 29
1.3.4.5b Extraterrestrial Causes (Meteoroid Streams) 30

1.4 The Weathering Index 32
1.5 Scope of Experimentation 35
2 EXPERIMENTAL METHODS 37
2.1 Photomicrography 37
2.2 Modal Analysis 37
2.2.1 Difliculties in Modal Analysis 38
2.3 X-ray Diffraction 41
2.3.1 Sample Preparation and General Pretreatment 41
2.3.2 Citrate Dithionite Pretreatment 43
2.4 Negative Loss on Ignition 44
3 ANALYTICAL RESULTS 49
3.1 Petrographic Characterization of Oxides 49
3.1.1 Antarctic Samples 49
3.1.2 Carichic 65
3.2 Modal Analysis 66
3.3 X-ray Diffraction 70
3.4 Negative Loss on Ignition 72
4 DISCUSSION 77
4.1 Graphical Representation of Data 77
4.1.1 Modal Analysis 77
4.1.2 Negative Loss on Ignition 82
4.1.3 Experimental Oxidometer Contrast and Comparison Diagrams 84
4.1.4 Recovered Sample Mass Diagrams , 93
4.2 Interpretation of Data 105
4.2.1 Secondary Mineral Identifieaticn 105
4.2.1.1 Theoretical Secondary Mineralogy 105
4.2.1.2 X-ray diffraction 107
Carichic 107
ALHA84075,7 108
ALHA77271,27 109
ALHA77182,21 110

4.2.2 Modal Analysis Ternary Diagrams and the CPL-based Index 111
4.2.2.1 Assumptions 111

 

ix

51

AI
A

1 Ir...

 

4.2.2.2 ABCe Index Inadequacies

113

 

4.2.2.3 Residual Metal Grains and Pressure Equilibrium

114

 

4.2.3 Negative Loss on Ignition and the -LOI Index

122

 

4.2.4 Experimental Oxidometer Contrast and Comparison

123

 

4.2.5 Non-index-related Anomalies

125

 

4.2.5.1 Cariclric, Fractures and the FIMAHE

126

 

4.2.5.2 Associated Weathering Mechanisms

133

 

A. Sample-specific factors

133

 

(1) Class and petrologic type weathering
(11) Mass-dependent weathering

134
139

 

B. Curatorial weathering

5 CONCLUSIONS

 

APPENDIX A: PETROGRAPHIC DESCRIPTION OF SAMPLES

APPENDIX B: DESCRIPTIVE CATALOGUE OF

COLOR TRANSPARENCIES

 

 

APPENDIX C: X-RAY DIFFRACTION DATA

 

LIST OF REFERENCES

141

148

151

193

232

253

1“?"
:C

 

III

 

Table

2.1
3.1
3.2
3.33
3.3b
4.1

4.2

4.3

LIST OF TABLES

 

 

 

 

 

 

 

Page
Crucible weights before and after cooling 46
Modal analytical results 67
Modal opaque mineral percentages 68
Negative loss on ignition data for Autumn of 1990 73
Negative loss on ignition data for Spring of 1992 74
Summary of sample-specific information 88
Calculation of primary and secondary
mineral volume percent differences 120
Determination of delta -LOI means 146

 

Figure
1.1
1.2
1.3
1.4
1.5
3.1
3.2
3.3
3.4
3.5
3.6

3.7

3.8
3.9

'3.10

LIST OF FIGURES

Barred olivine chondrule

Page

 

Excentroradial pyroxene chondrule

 

Granular olivine chondrule

Excentroradial pyroxene chondrule cross section

 

Sample recovery locations

Iron oxide stain in chondrule

17

51

 

Iron oxide stain in cleavage planes

51

 

Iron oxide stain crystallinity; frame #1

52

 

Iron oxide stain crystallinity; frame #2

52

 

Iron oxide stain crystallinity; frame #3

53

 

Opaque-pseudomorphic limonite haloes

53

 

Void space partially filled with amorphous oxide and lined
by opaque-pseudomorphic limonite

55

 

55

 

Figure 3.7 view in crossed polarized light

Expanding opaque-pseudomorphic limonite deposit

Figure 3.9 view in crossed polarized light

56

56

 

xii

 

In

 

 

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3.11

3.12

3.13

3.14

3.15

3.16

3.17

3.18

3.19

3.20

3.21

3.22

4.1

4.2

4.3

4.4

4.5

4.6

Expanding opaque-pseudomorphic limonite deposit

57

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.11 view in crossed polarized light 57
Disintegrating opaque grain 59
Transparent limonite fracture fillings 60
Figure 3.14 view in crossed polarized light 60
Botryoidal void space fillings 61
Sharp contact phase 62
Figure 3.17 view in crossed polarized light 62
Sharp contact phase 63
Figure 3.19 view in crossed polarized light 63
Carichic 64
Theoretical opaque metal grain replacement process 70
Modal analytical data plot in terms of

three most basic components 78
Theoretical distribution of current Antarctic

meteorite weathering categories 80
Redistribution of modal analytical data

with non-OPL separated from OPL 81
Reproducibility of -LOI results 83
Reproducibility graph with meteorite class

and petrologic type indicated 85
Reproducibility graph with fracture index indicated 86

 

xiii

 

 

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

. on Figure 4.3

Geometrical relationship illustrating false modal value
generation for NNL stain

1990 gravimetric and modal index values

87

 

1992 gravimetric and modal index values

91

 

Plot of both 1990 and 1992 -LOI data

92

 

Figure 4.11 showing taxonomic group and petrologic type

94

 

Recovered sample mass versus modal index

96

 

Recovered sample mass versus gravimetric index

97

 

Recovered sample mass versus 1992 gravimetric index

98

 

Reproducibility graph illustrating mass distribution

 

Figure 4.9 with recovered sample mass illusuated

100

 

Mass distribution of 1990 -LOI data reillustrated

Difference in -LOI behavior between high and low recovered
mass samples

Delta -LOI vectors versus recovered sample mass

Possible scenarios of movement for data points

102

103

104

116

 

Representation of unweathered ordinary chondrite
sample in thin section

Initial stage of weathering

Intermediate stage of weathering

128

128

129

 

The Carichic sample (advanced stage of weathering)

129

 

xiv

4.25

4.26

4.27

Example of fracture-induced metal alteration
heterogeneity effect 131

Mass frequency distribution among Antarctic

ordinary chondrites 136

 

Recovered mass distribution plotted on Figure 4.3 140

 

XV

BFF

CI

CM

CO

CV

EOC

OLFF
OPL

SCP

KEY TO ABBREVIATIONS AND NOMENCLATURE

Allan Hills, Antarctica

Botryoidal fracture fillings

Carbonaceous chondrite

Ivuna C group

Murchison C group

Ornans C group

Vigarano C group

Al Rais and Renazzo C group

Equilibrated ordinary chondrite
Fracture—induced metal alteration heterogeneity effect
High iron ordinary chondrite

Low iron ordinary chondrite

Lewis Cliff, Antarctica

Low total iron, low reduced iron ordinary chondrite
Negative loss on ignition

Neoformed non-pseudomorphic limonite
Oxidation index

Opaque limonite fracture and void space fillings
Opaque-pseudomorphic limonite

Poorly characterized phase

Sharp contact phases

SEM Scanning electron microsc0pe
UOC Unequilibrated ordinary chondrite
XRD X—ray diffraction

xvii

1 INTRQDUCTION

The primary goals of this research are to determine if clay
minerals are present in highly weathered Antarctic
meteorites, and, more generally, to examine the potential
for applicability of an experimental weathering index to the

Antarctic meteorite weathering problem.

1.1 The Discipline of Meteoritics

With the exception of Lunar samples recovered by the Luna
and Apollo programs, meteorites are the only
extraterrestrial rock materials that can be studied in the
laboratory. A.formidable body of information on some of the
earliest inner solar system processes —— including nebula
condensation and differentiation, planetesimal accretion,
and incipient geochemical evolution —— is preserved in
meteorites. Determined largely through recent studies of
oxygen isotope ratios, it is currently accepted that
meteorites sample a diverse assemblage of parent bodies, and
that their study can therefore lead to a better
understanding of a wide variety of geological processes.

The most likely parent bodies include as many as 70 or more

of the minor planets (e.g., Dodd, 1986; Lipschutz et al.
1989) possibly a small number of comets (e.g., McSween,
1987), the Moon (Lindstrom, 1989), and even the planet Mars
(Laul, 1986). Furthermore, because they are preserved
remnants of the earliest epochs of nebular solar system
history, meteorites may even provide insights into the
cosmochemistry of the formation and evolution of other star
systems. Meteorites are thus a tangible bridge between the
sciences of astronomy and geology, and unite the sub-
disciplines of geochemistry and comparative planetology with

astrophysics.

1.2 Background on Meteorites

The research reported in this thesis examines the nature of
meteorite weathering in the Antarctic environment. Some
general background on meteorites is therefore necessary.
The following sections constitute a very broad overview at
best, and focus on ordinary chondrites for the purpose of

preparing the reader for this research.

11L1(Jummrhe(flmmflflanhnr

Meteorites are classified into three main groups; irons,
stony-irons and stones, based on iron and silicate

fractions. Stones are subdivided into chondrites and

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Figure 1.1. Barred olivine chondrule. Photographed in polarized 1191, this droodrule from ALHA77004,22 displays Iarnellae of olivine
inoptical contnmhymgtnmwnfiomhmmddestammg)whhdukglassyzmesbdwemflnhmelhe ancdimensionsare400by
600 microns

 

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1.2. Empradialpymxenedrondrule. Thischondr'uleirrALHAB1031J9lmbeenformedbytherapidcoolingFofadmpIetof
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olivineande ' yroxene droudrules, which are typically monoa'ystalline. Photographedinplaneuansmiuedlight. The
dominant 'I'hiscbondnrlewasidanifiedinALHA8103l, 19. Frame dimmsiomare 1.0by 1.5 mm.

 

xcentrondral p
urina’al is olivine.

   
     

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Figure 1.4. Excentroradial pyroxene chondrule in cross section. Another example of the chondnrle type depicted in Fig 1.2., but cut
perpendicularly to the plane of crystal growth. Photographed in polarized light. Notice inter-tonguing of pyroxene platelets. Frame
dirnarsions are 1.0 by 1.5 mm.

achondrites, distinguished by the presence in the former of
chondrules of various types (millimeter-sized silicate
spheroids, Figures 1.1 through 1.4), and other textural and
chemical criteria. Chondrites are further subdivided into
carbonaceous, ordinary and enstatite categories according to
their redox states and refractory lithophile ratios
(McSween, 1979). Meteorites are also classified by their
mode of recovery. A "fall" is a meteorite whose atmospheric
passage was observed and which was recovered shortly
thereafter. A "find" is a meteorite that was found at some

indeterminate time following its arrival on Earth.

1.2. 1.1 Carbonaceous chondrites

Apart from the usual presence of chondrules and sedimentary
textures, chondrites are related by their compositions,
which are close to that of the solar photosphere, excluding
highly volatile elements (Dodd, 1981). Of the chondrites,
the carbonaceous (C) chondrite§ contain nearly perfect
solar abundances of major and trace elements and the highest
proportions of volatile elements, and so are believed to be
the most primitive of this group. As such, they were long
thought to represent the starting material from which all
subsequent terrestrial planets and their rock types formed

and evolved. It is now realized that a great deal of

 

' Carbonaceous chondrites are so named due to their sometimes elevated levels of carbon, though this is
not necessarily a diagnostic characteristic (McSween, 1979).

aqueous alteration has taken place within the matrices of C
chondrites (see Section 1.3.3), and so at least part of
their mineralogies are something less than primary (Dodd,

1981; McSween, 1979).

C chondrites are subdivided into CI, CM, CV, CO and CR
classes based on lithophile element ratios. Each class
refers to a specific fall and only occasionally have
subsequent finds added to the classes. Thus C chondrites
are among the least abundant samples in the collections,
comprising only some 4.2 percent of all non-Antarctic falls

(Sears and Dodd, 1988).

The CIs contain no chondrules and are essentially 100
percent matrix material. CIs and CMs contain the highest
abundances of volatile elements and lowest abundances of

high temperature minerals of all meteorites.

1.2.1.2 Ord_1rm Chondrites

Only ordinary chondrites were examined in the present study.
Ordinary chondrites comprise some 82 percent of all
meteorite samples recovered (Dodd, 1981). The dominant
minerals in ordinary chondrites are, in order of decreasing
abundance, olivine, pyroxene, plagioclase feldspar, reduced
iron—nickel metal (kamacite and taenite), and iron sulfide

(troilite); with apatite, chromite, ilmenite, and various

accessory minerals and glass comprising the remaining 1
percent fraction (Dodd, 1981). Texturally, ordinary
chondrites are similar to sedimentary rocks, with
chondrules, mineral clasts and metal grains set within a
matrix of finer material of similar mineralogy. The matrix
is a clastic accretion of fine-grained high temperature
silicates, and metal and sulfide particles, which together

act as a “cement” for the larger grains.

1.2.1.3 Enstatite Chondrites

Enstatite chondrites are the most reduced of the chondrite
group of meteorites. Their name is based on their typically
high abundance of enstatite (Dodd, 1981). They are
mentioned here only for the purposes of completeness. None
were examined in the present research. They are relevant to
the body of this thesis only in that they have been
recovered from Antarctica, and so are in need of proper

weathering indexing.

1.2.2 Petrologic Types

Chondrites are assigned a petrologic type number that
corresponds to relative intensities of aqueous alteration
and thermal metamorphism. Petrologic type numbers range
from 1 to 7, and were originally intended to represent a

continuous spectrum of metamorphic intensity, measured

according to degrees of textural integration and
compositional homogenization for specific mineral phases,
from unaffected to highly altered (Van Schmus and Wood,
1967). Petrologic types 1 and 2 (currently known only for
carbonaceous chondrites) have been separated from types 3
through 7 for ordinary chondrites (types 3 and 4 also apply
to carbonaceous chondrites) because they are now thought to
depict stages of hydrothermal processing only and not
thermal metamorphism (McSween, 1979). Petrologic types 3

through 7 can be distinguished texturally in thin section.

Metamorphism in meteorites is based on subtle changes in
mineral chemistry and texture rather than on empirically
derived mineral assemblages, as with terrestrial metamorphic
rocks (Best, 1982). Textural and mineralogical_
characteristics change with increasing metamorphic grade.
According to Dodd (1981), olivine compositions vary widely
within type 3 chondrites, even within individual chondrules
(0 to z30 mol% Fa). However, olivines in types 4 through 7
are homogeneous and consistently calcium-poor. Pyroxenes
are also inhomogeneous in type 3 chondrites, but this
variation persists to type 4 due to the diffusion rates of
iron and magnesium. Chemical variations decrease with
increasing metamorphic intensity. Hence type 3 chondrites
are referred to as unequilibrated ordinary chondrites
(UOCs), and those of higher, more chemically homogeneous

petrologic types, as equilibrated ordinary chondrites

(EOCs). Also, Fe/Mg ratios increase with petrologic type.
Petrologic type 7 is identified based on the presence of
calcic orthopyroxene (2 1.0% CaO). Quartzofeldspathic glass
changes to plagioclase in types 4 through 7. Shock loading
can then restructure the plagioclase to form maskelynite.
Homogenization occurs through small-scale diffusion
processes between the grains and the matrix, which tends to
preserve "relict" textures. The primary (pre-accretionary
and accretionary), secondary (post-accretionary aqueous and
thermal), and tertiary (shock), processes are believed to

overlap in space and time.

1.2.3 Post-accretionary Aqueous Alteration Products

As stated above, petrologic types 1 and 2, which belong
exclusively to the carbonaceous chondrite group, are no
longer considered part of the thermal and shock metamorphic
range. Rather, they are thought to be representative of a
range in aqueous alteration processing alone, based on
variations in secondary mineralogy of the matrix of
carbonaceous chondrites (McSween, 1979).‘ These and other
pre-terrestrial alteration products (Hutchinson et al. 1987)
are considered to form under low—temperature hydrothermal
conditions. The following is a brief introduction to the
aqueous alteration of high temperature meteoritic materials

in the extraterrestrial environment.

10

1.2.3.1 Cgbonaceous Chondrite Matrices

It has been known since 1864 that the CI and CM carbonaceous
chondrite matrices are composed partly of low temperature
phyllosilicate materials (Barber, 1985). This matrix is
very fine-grained and consists of magnetite, iron sulfides,
gypsum, epsomite, carbonates (dolomite in C18, calcite and
aragonite in CNS), serpentine, chlorite, montmorillonite,
and glass fragments (Barber, 1985); with serpentine
comprising the dominant matrix phase in CM meteorites
(Zolensky and McSween, 1988). Montmorillonites have also
been detected in the less primitive CV carbonaceous
chondrites and are suspected to exist in the CO carbonaceous
chondrites (Tomeoka and Buseck, 1982). CM meteorites are
also noted for possessing a poorly characterized greenish
and hydrated poorly crystalline silicate known
affectionately as "spinach" (Fuchs et al. 1973). This
material does not lend itself well to examination by
conventional methods because of its fine grain size and
opacity, the superimposition of X-ray diffraction lines, and
the weak intensity of non-basal reflections (Barber, 1985).
The material is inferred to be a phyllosilicate and is found
in association with olivine grains, which led Olsen and
Grossman in 1978 to suggest the spinach to be an alteration
product of anhydrous silicates and glass. Two components,
one of iron-rich phyllosilicates and a second rich in iron,

nickel, sulfur and carbon, comprise this spinach. Gooding

11

(1985) determined that iron-rich chlorite is the likely

phyllosilicate mineral.

Transmission electron microscopy patterns reveal radiating,
fern-like fibrous morphologies of the magnesium-rich
serpentines in the CI and CM chondrites, as well as disk-
shaped growths with circular and spiral patterns of
chrysotile (Barber, 1981). Higher resolution images reveal
interlayered serpentine and brucite with tubular and sheet-

like morphologies (Fuchs et al. 1973).

These hydrous phases in the CI and CM carbonaceous
chondrites (termed poorly characterized phases, or PCPs by
Fuchs et al. 1973) are thought to have formed in situ by the
aqueous alteration of fine-grained high temperature phases
and in direct contact with liquid water of uncertain origin
(Barber, 1985; Mackinnon and Zolensky, 1984). CM chondrites
are similar in texture to hydrothermally altered terrestrial
mafic rocks and volcanoclastics, and indicate post-

accretionary alteration (e.g., McSween, 1979).

1.2.3.2 Ordum Qhondn'te Matrices

Recall that ordinary chondrites differ from carbonaceous
chondrites by their lower oxidation states, higher relative
abundances of total iron, greater abundances of chondrules,

depletion of volatile elements, and lack of low temperature

12

phases. Recent work by Hutchison et al. (1987), and
Alexander et al. (1989) on the Semarkona and Bishunpur
ordinary chondrites (both LL3 UOCs) has revealed
phyllosilicate phases in their matrices. Extensive
replacement of interchondrule matrix and chondrule rims by
iron-rich nontronitic sodium-smectite occurs in Semarkona.
This smectite is very fine-grained but occurs locally as a
coarse-grained phase with radial growth patterns. As a
result of this alteration, very few of the original primary
matrix features remain, with the exception of a few
inclusions (Barber, 1981), and only the most magnesium-rich
mafic minerals are preserved. The Bishunpur ordinary
chondrite was found to contain the calcium-smectite
saponite, formed from the alteration of amorphous material
of feldspathic composition (Hutchison et al. 1987). Again,
this alteration was confined to the interchondrule matrix.
In addition, dissolution features were detected in matrix
silicates (olivine and pyroxene), and in association with
"maghemite and other poorly described Fe-oxide phases,"

(Zolensky and McSween, 1988).

Although the equilibrium composition of the volatiles
present during these alterations was determined to be
dominated by water, it was emphasized by Hutchison et al.,
(1987) that liquid water need not have been present at the
time because grain-boundary fluids could have been

sufficient to mobilize elements. These findings moved the

13

authors to suggest that, due to comparable high temperature
mineralogies between carbonaceous and ordinary chondrites,
the low temperature phases found in abundance in the
carbonaceous chondrites might represent an assemblage toward
which ordinary chondrites would evolve given appropriate
alteration conditions for an extended time period.
Interestingly, the smectite chemistries in both meteorites
are similar and uniform throughout the specimens, but have
no known terrestrial analogues apart from a similarity to

nontronite.

These studies indicate that incipient alteration in
unequilibrated ordinary chondrites takes place almost
exclusively within the interchondrule matrix and on
chondrule rims. Huss et al. (1980) examined ordinary
chondrite matrices and found there to be two types, (1) a
fine-grained, porous, clastic matrix of angular fragments of
silica minerals and amorphous interstitial material, and (2)
a coarser, less porous, non-clastic matrix. Matrix type 2
was interpreted by the authors to be the recrystallized
complement of matrix type 1.

The matrix materials most susceptible to hydrous alteration
under terrestrial conditions are the amorphous, glassy
phases whose free energies of formation are much less than
the well crystallized phases. Tazaki et al. (1989) studied
incipient clay formation on volcanic, alkalic igneous and

synthetic glasses and showed through high resolution

l4

transmission electron microsc0pic analysis that glass phases
contain well-ordered crystalline domains which can trigger

clay mineral growth.

Additional examples of extraterrestrial aqueous alteration
include the occurrence of iddingsite in nakhlites (Gooding,
1989), one of the associated SNC group of achondrite
meteorites suspected to have originated on the planet Mars
(Reid and Bunch, 1975); and recent evidence of clay-like

minerals on interplanetary dust particles (e.g. Barber,

1985).

1.2.4 Meteorite Weathering
Unfortunately for cosmochemists, meteorites —— being
dominantly composed of high temperature minerals —— weather

rapidly in the terrestrial environment where pressure-
temperature-concentration (PTX) conditions are far from the
conditions of formation, and where liquid water and oxygen
are abundant?. .However, while constituting a nuisance to
the study of meteorites on Earth, low temperature
mineral/water interactions may be a significant
extraterrestrial process. It has been suggested that
meteoritic weathering scenarios in Antarctica (discussed

below) may be analogous to low temperature "hydrocryogenic"

 

2Buchwald and Clarke (1989) stated that the "Corrosive processes begin as a meteorite enters the
.mmmmMmHmdammmmumflnmhmgmmmmsflmfismagmzmhf

15

alteration mechanisms on Mars, asteroid regoliths and
interiors, within comets, and within the surfaces of icy
moons of the Jovian planetary systems (Gooding, 1986a;

Zolensky and McSween, 1988).

1.2.5 Fusion Crusts and Fractures

Fractures in meteorites are present largely due to shock
(Dodd, 1981) and provide avenues for the movement of thin
films of liquid water, which facilitate the mobilization of
chemical constituents (Gooding, 1986b). Due to frictional
heating during entry into the Earth's atmosphere, meteorites
develop a glassy fusion crust a millimeter or so thick on
their exterior surfaces. .A fractured fusion crust is far
more permeable to air and liquid water than one which
remains intact after the fall. It follows that a meteorite
with a highly fractured fusion crust will also be more
susceptible to weathering than one with a crust still
intact. The presence or absence of fractures in a fusion
crust may therefore become helpful in determining whether a
given alteration product within a specimen is terrestrial or
pre-terrestrial in origin (Gooding, 1986a). Secondary
products within the interior of meteorites may be pre-
terrestrial, whereas alteration products that crosscut or
are superimposed upon fusion crusts are terrestrial in

origin.

16

1.3 Antarctic Meteorites

1.3.1 Reconnaissance and Recovery

Since 1969, over 12,000 meteorites have been collected from
blue ice sheets in Antarctica by Japanese and American
expeditions funded by the National Institute of Polar
Research and the National Science Foundation, Division of
Polar Programs, respectively (Koeberl and Cassidy, 1991).
Meteorites are concentrated at stranding surfaces in
ablation zones where ice sheet movement has a substantial
vertical component (Delisle and Sievers, 1989; Lipschutz and
Cassidy, 1986). The horizontal motion of the ice sheet is
usually arrested in these areas by the presence of a

mountain range or line of nunataks.

Upon discovery, Antarctic meteorites are photographed and
assigned a number according to their location of recovery,
field season, year found, and number collected; i.e.,
ALHA85002, where ALH stands for Allan Hills in this example,
A indicates the first field season, ‘85 is the year, and 002
indicates the meteorite to be the second found during the
collecting season at this site. The nineteen Antarctic
meteorites used in this project were recovered from either
the Allan Hills or Lewis Cliff (LEW) recovery sites (see

Figure 1.5).

17

Bo.— 5.5m

5:32 @338 295m .4.
“cocscoo 0:88.}. .m._ 2:3..—

 

18

Maximum terrestrial age estimates for Antarctic meteorites
range from tens of thousands to one million years
(Nishiizumi, 1986; Nishiizumi et al., 1989). As a
consequence of so long an exposure to Earth conditions, all
Antarctic finds can be presumed to have weathered to some
extent. Stony meteorites in temperate climates weather
rapidly in as little as two hundred years, with a typical
half-life approaching 3600 years (Boeckl, 1972), while those
in Antarctica persist much longer due to the low prevailing
temperatures. Air temperatures range from.-70W: to +20%:
annually for the.Allan Hills area (Campbell and Claridge,

1987), where most of the study samples were recovered.

1.3.2 Antarctic Meteorite Weathering

Each Antarctic meteorite specimen has clearly undergone
chemical weathering to varying degrees as evidenced by iron
oxide staining visible in hand specimen (Gooding, 1981).
Chemical alterations are a concern to any cosmochemical
analysis of Antarctic samples because terrestrial weathering
reactions are capable of mobilizing and redistributing major
and trace elements in significant amounts. Examples of such
elemental redistribution were illustrated in 1988 by Velbel,
where it was determined that rubidium, cobalt, iodine, and
calcium may be removed from the interiors of Antarctic
chondrites to be concentrated at their surfaces during the

production of various evaporite minerals, thereafter to be

19

removed completely by wind erosion. The same was found to
be true for carbon (Hartmetz et al., 1989). Depending on

the study, the chemical analysis of a weathered meteorite

could therefore lead to uncertainties in pre-terrestrial

history interpretations.

The most obvious indication of Antarctic meteorite
weathering is the presence of iron oxide staining (Gooding,
1981). Preliminary classification of weathering intensities
is currently made by the Antarctic Meteoritic Curatorial
staff, who assign categories to each specimen recovered on
the basis of the degree of rustiness observed in hand
specimen (Marvin and Macpherson, 1992), and on the presence
or absence of evaporite mineral efflorescences (Velbel,
1988). Category labels range from A (least weathered),
through A/B, B, and B/C, to C (most weathered), with a lower
case "e" indicating the presence of evaporite
efflorescences. This is referred to as the ABCe system, and
is defined in the Antarctic Meteorite Newsletter (AMN) and
the Smithsonian Contributions to the Earth Sciences (SCES)

publications as follows:

A = minor; metal flakes have inconspicuous rust
haloes, oxide stain along cracks is minor.
B = moderate; metal flecks show large rust haloes,

internal cracks show extensive oxide stain.

20

C = severe; specimen is uniformly stained brown,

no metal survives.

e = evaporite deposits visible to the naked eye.

Antarctic meteorite finds are also assigned a fracturing
index to aid in the pairing of stones that may have been
separated from the original mass. Highly fractured stones
are more likely to be fragments of an initial larger mass
than stones of high competence. This fracturing index is

defined in the AMN and SCES publications as follows:

A = slight; specimen has few or no cracks and none
penetrate the entire specimen.

B = moderate; several cracks extend across the
specimen, which can be readily broken along the
fractures.

C = severe; specimen has many extensive cracks and

readily crumbles.

Despite the importance to meteorite research, very little
work has focused on the terrestrial weathering of Antarctic
meteorites, and the processes remain to be identified and
quantified (Gooding, 1989). The recognized secondary
products in Antarctic meteorites include hydrous and
anhydrous iron oxides (Gooding, 1981), carbonates and
sulfates (Marvin, 1980; Jull et al., 1988 and Velbel, 1988),

and amorphous mineraloids with smectitic compositions

21

(Gooding, 1986a,b, 1989). Gooding (1986a) recognized that
thin films of liquid water can exist at subzero temperatures
that facilitate the elemental migration necessary for
typical weathering reactions. In addition, it was
empirically determined that Antarctic meteorites exposed to
sunlight may have internal temperatures (at depths up to 2.0
cm in some meteorites) that rise as high as 5°C on wind-free
days (even when air temperatures remain below 0°C), enabling
capillary waters to promote reactions (Schultz, 1986b).
Moreover, some oxidation reactions have been found to occur
in the solid state at low relative humidities (Buchwald and

Clarke, 1989) (see Section 4.2.1.1).

1.3.3 Significance of Extraterrestrial Aqueous Alteration for Antarctic

Meteorites

Because of the low prevailing temperatures in Antarctica,
meteorite weathering is not expected to have progressed to
the degree of aqueous alteration found among the CI and CM
carbonaceous chondrites. However, the discovery made in
1986 by Gooding of amorphous clay mineraloids in moderately
weathered ordinary chondrites suggests that well
crystallized phyllosilicates may yet exist in the most
extensively weathered Antarctic samples. If so, they imply
by analogy that the alteration processes which occurred in
Semarkona and Bishunpur may have taken place at subzero

temperatures. The work of Alexander et al. (1989) indicates

22

smectite formation temperatures in these meteorites of less
than 260WC,'while Barber (1985) cites recent results that
suggest alterations of the CI and CM carbonaceous chondrites

occurred at temperatures as low as 0°C.

Popular thinking regards CI and CM meteorite low temperature
phases as having formed under hydrothermal metamorphic
conditions (McSween, 1979). Therefore, if hydrous
phyllosilicate minerals of the types found pre-terrestrially
in Semarkona, Bishunpur and CI and CM carbonaceous
chondrites can also be found as alteration products in
weathered Antarctic meteorites, the discovery could have
important implications for parent body formation and thermal

evolution models.

1.3.4 The Antarctic and non-Antarctic Meteorite Source Controversy

The observation that meteorites found within Antarctic ice
sheets differ in many ways from those found elsewhere has
been examined extensively in recent years and is used by
some to suggest the two populations (Antarctic and non-
Antarctic) have different extraterrestrial sources
(Dennison, 1986). The variables include terrestrial age,
mass, oxygen, mercury and carbon isotopic compositions, type
frequency distributions, chemical differences, and
petrologic differences (among carbonaceous chondrites; none

have been found in ordinary chondrites; e.g., Koeberl and

23

Cassidy, 1991). Petrographic differences were also observed
by Takeda (1991) between Antarctic and non-Antarctic
eucrites (one of the achondrites). Opponents to this idea
feel the differences can be attributed entirely to
terrestrial effects (weathering within or upon the ice
sheets). Others feel a combination of terrestrial and
extraterrestrial effects account for the observed
differences, (e.g., Begemann, 1989). All agree, however,

that the differences themselves are real.

The statistical problem of assessing exactly how many
individual falls are represented in the Antarctic population
is to be found in any discussion of this controversy. The
amount of fragmentation or its opposite “pairing” among
Antarctic stones directly relates to the level of
significance to be placed on any given difference. Of the
12,000 meteorites collected in Antarctica, an estimated 2000

to 4000 distinct falls are represented (Scott, 1984)3.

The following is a discussion on the nature and importance

of these differences.

 

3 By contrast, the number of non-Antarctic meteorite falls is in the neighborhood of 2500. (e. g., Koeberl
and Cassidy, 1991).

24

1.3.4.1 Age Difl'erences

As stated in Section 1.3.1, Antarctic meteorites are, on
average, much older than meteorites found elsewhere on
Earth. Terrestrial age dates based on cosmogenic
radionuclides are found to range up to 1 million years
(Nishiizumi et al. 1989), with the majority having
terrestrial ages of less than 0.5 million years (e.g.,
Koeberl and Cassidy, 1991). Terrestrial ages also range
noticeably from place to place within the Antarctic
environment. For example, meteorites from.Allan Hills are
typically older than those of the Yamato Mountains. It must
be noted, however, that terrestrial age measurements are
difficult to obtain and yield values with various margins of

uncertainty, depending on the method employed.

By contrast to the Antarctic situation, the half-life of a
stony meteorite in more moderate climatic regions of Earth
is estimated at 3600 years (Boeckl, 1972). This is a direct
result of greater temperatures and greater availability of
liquid water. If the Antarctic and non-Antarctic samples of
the source regions represent two well-separated time
intervals, and the samples differ significantly, it follows
the source regions differ significantly. This age
difference is therefore elementary to the argument that

source regions within the asteroid belt change with time.

25

Unfortunately, high residence times in any natural
terrestrial environment increase the chances for undesirable
weathering effects (Gooding, 1986). The importance of
establishing the character and measure of each type of
terrestrial alteration in each type of Antarctic meteorite

thus arises again.

1.3.4.2 Type Differences

Crucial to the problem of pairing is the occurrence among
the Antarctic population of uncommon or previously
undiscovered meteorites. The apparent meteorite-type
frequencies can become severely modified by the
fragmentation of a single stone if the total count for its
group numbers in the dozens or less (as is the case for many
meteorite groups). Such Antarctic finds include previously
unencountered varieties of ureilites (shock, or
hypervelocity impact-melted slag meteorites), Shergottites
(SNC or Martian meteorites), lunar meteorites (never
encountered outside Antarctica), rare iron meteorites, and
unusual chondrites. On the other hand, the greater
proportion of these meteorites may simply be due to the

larger total number of Antarctic meteorites.

26

13Ai3bg§sDflfizgmms

It has been noticed, nearly since the inception of the
Antarctic meteorite recovery program, that Antarctic
meteorites are, on average, smaller than non-Antarctic
meteorites. The total mass of the non-Antarctic population
(falls and finds) is “orders of magnitude larger” than that
of the Antarctic population (Koeberl and Cassidy, 1991).
This can be explained in part by the bias built into the
method of recovery. Nearly every dark-colored rock found on
the blue ice sheet will be a meteorite by default, since no
mechanism exits to transport terrestrial rocks to these
locations. The high color contrast between rock and ice
allows much smaller specimens to be found than could be
found in non-Arctic and non-Antarctic terrains. Meteorites
weighing less than 10 grams are believed to be removed from
the ice surface by the strong Antarctic winds (Harvey and

Cassidy, 1986).

The preponderance of small meteorites may simply be due to
breakage while decelerating upon entry in the atmosphere.
This is a common phenomenon, resulting in what has come to
be known as a shower fall (e.g., Hutchison, 1983). Such
shower falls typically produce strewn fields with thousands
of individual stones. A lack of proper pairing to
reconstitute the original mass would produce the effect of

greater type frequency for a given shower fall type.

 

 

 

 

 

11:114.

27

An oddly high H/L ratio is observed among the masses of
Allan Hills Main ice field ordinary chondrites at a >99
percent confidence level (Huss, 1991). Huss suggests an H5
shower fall took place to explain the nearly five times H5/L
ratio in the Allan Hills Main and Near Western ice fields.
Dennison et al. (1986) observed an H/L ratio three times the
value for non-Antarctic falls as for Antarctic finds. This
thesis will present the hypothesis of preferential H
chondrite fragmentation while within or on the ice sheet as
another possible explanation for the observed difference

(see Section 4.2.5.2,A,I).

13AL4CXgmmmlDflfinmnxs

1.3.4.4a Major Elements

Only one study on major rock-forming elements has been
conducted specifically to address the Antarctic/non-
Antarctic controversy. Jarosewich (1990) found that non-
Antarctic finds are lower in iron, sodium, and sulfur, and
higher in water than Antarctic meteorites. He concluded
this was due to weathering, since non-Antarctic falls had
values somewhat intermediate between the other two
populations. He also found the abundance of iron oxide to

be highest in Antarctic meteorites.

28

ISAAanmeEkmmus

Volatile and mobile trace elements in ordinary and
carbonaceous chondrites, and some achondrites, vary somewhat
between Antarctic and non-Antarctic meteorites. The
controversy was, in fact, initiated by the identification of
statistically significant differences for Antarctic and non—
Antarctic H5 chondrites (Dennison et al. (1986) and Lipshutz
(1986). Specifically, these elements include indium,
antimony, selenium, zinc, rubidium, bismuth, thallium, and
cadmium in H5 chondrites, with similar differences for H4
and H6 chondrites. Antarctic eucrites were found to have
higher abundances of gold, bismuth, cadmium, indium,
rubidium, selenium, thallium, and uranium than non-Antarctic
eucrites (Paul and Lipschutz, 1987). In 1989, Paul and
Lipschutz discovered that Cl and C2 chondrites had different
siderophile and mobile element concentrations. Kaczaral et
al. (1989) found L4—6 Antarctic chondrites to have generally
lower mobile trace elements than non-Antarctic chondrites.
Lipschutz and Samuels (1991) identified shock-dependent
trace element abundances among non-Antarctic L4—6
chondrites, but not for Antarctic meteorites of the same

types.

29

1.3.4.5 Cause and Implication of Difi‘erem

The major question concerning these and other differences
between Antarctic and non-Antarctic meteorites is whether
they are terrestrial or extra-terrestrial in origin. Chief
among the variables for confusing this issue is terrestrial
residence time. With the effects of weathering in the
cryogenic Antarctic environment still unknown, and perhaps,
unknowable, only a small number of differences can be
assigned an origin with any confidence. Much research

remains to be performed to resolve this problem.

1.3.4.5a Terrestrial Causes (Weathering and Contamination)

The effects resulting from differences in terrestrial
history are not known in detail for the Antarctic situation.
There is a consensus among researchers that the current ABCe
system (see Section 1.4 below) of indexing weathering
intensity among Antarctic meteorites is not sophisticated
enough to handle the broad range of weathering effects, or
even to quantify the intensity of oxidation (Sears, 1989)“.
For example, although Dennison and Lipschutz (1987) report
finding leaching of trace elements in weathering category C
meteorites, no correlations have been found between

weathering category and evaporites (Velbel, 1988), bulk

 

" The only benefit that was acknowledged for the ABC system was that it “...does constitute a reasonable
rewmhndmcmmmmfdwawfimmbmwdmnmeMmdqndmmmffimmSamekw”.

3O

chemistry (Jarosewich, 1989), or thermoluminescence

sensitivities (Sears, 1989).

It has already been noted that the degree of fragmentation
effects the statistical significance of meteorite type
frequencies. Other known terrestrial effects include
chemical contamination. Dennison and Lipschutz (1987) note
that trace elements are often enriched and not depleted in
Antarctic meteorites. However, terrestrial contamination
(enrichment) of selenium was found to occur consistently in
Antarctic eucrites (Mittlefehldt and Lindstrom, 1991). The
source of selenium was most likely volcanic tephra dust
(Koeberl, 1989; Mittlefehldt and Lindstrom, 1991). Delisle
and Sievers (1989) describe uranium contamination in
Antarctic chondrites due to close proximity to terrestrial
granites in glacial moraines. Differences in carbon
isotopic data for CM chondrites were found to be due to the
presence of terrestrial hydrous carbonates in the Antarctic

samples (Grady et al., 1991).

1.3.4.5!» Extraterrestrial Causes (Meteoroid Streams)

Although it is accepted with very few exceptions that the
vast majority of meteorites originate within the asteroid
belt, non-terrestrial Antarctic/non-Antarctic differences
suggest that Antarctic finds may represent a source region

that differs (in location within the asteroid belt) from

31

that of modern falls. This difference in source regions
implies the existence of meteoroid streams, defined as a
group of meteoroids with similar orbital elements (e.g.,
Oberst, 1989). Such meteoroid streams, if real, would
necessitate the alteration of dynamic sampling models. The
current models assume that the source region had been
thoroughly homogenized early in the evolution of the
asteroid belt, and should not, therefore, produce
compositional alterations in the flux of the meteoroids
reaching the earth (Wetherill, 1987 and 1989). Wetherill
(1986) found that changes in the source region (collision
between two asteroids) on time-scales of < 0.1 million years
are necessary to account for the meteoroid stream hypothesis
as used to explain the Antarctic/non-Antarctic differences.
Cosmic ray exposure ages are greater than 1 million years
for most meteorites (Schultz et al., 1991). Therefore, if

Wetherill is correct, meteoroid streams should not exist.

However, it was postulated by Oberst (1989) that the high
density, high energy cluster of impacts recorded on the Moon
between June 20 and 30, 1975 by the Apollo seismic network,
represented the encounter of the Earth-Moon system with a
meteoroid stream. It was further suggested the Farmington
meteorite fall (June 25, 1890) may have originated in the
same meteoroid stream, as this L5 chondrite had similar
orbital parameters and a very young (25,000 years) exposure

age.

32

1 .4 The Weathering Index

Central to the problem of assessing the extent to which
alteration has progressed on a per-sample basis is the need
for a quantitative weathering index or "weatherometer."
Campbell and Claridge stated in 1987 that even among
terrestrial rocks, ”The effects of chemical weathering
processes are immediately apparent to the observer in
Antarctica, although it may be difficult to gauge their
extent." A reliable weathering index is therefore vital to
assessing the magnitude of weathering-induced chemical
modifications in Antarctic meteorites. This need for
accurate indexing is amplified by the strong tendency for
meteorites to weather rapidly, due to their generally mafic

to ultramafic compositions.

In 1991, Koeberl and Cassidy stated, "For future
investigations it will be important to pay more attention to
the weathering status of samples and to develop a more
reliable and quantitative weathering index." This assertion
was made regarding the current Antarctic and non-Antarctic
meteorite source controversy, in which weathering is a
principal issue, but could apply to any cosmochemical
studies involving Antarctic meteorites. Thus, the
popularity of Antarctic meteorites in the research community
demands a weathering index based on a reliable analytical

method that could be put to immediate use.

33

The best system for measuring weathering intensity would
involve a number of simultaneous weatherometers, each of
which would focus on a separate "microparagenesis" of
terrestrial secondary phase production within the sample
(Velbel et al., 1991). Prepared with this information,
investigators could then select the samples best suited
chemically for a specific project, depending on which

terrestrial alterations are considered significant.

A weathering index is a set of values intended to measure
weathering intensity. However, due to the potential for
taxonomic ambiguity in the special case of meteorite
weathering, a precise meaning for "weathering index" must be
agreed upon before proceeding further. The gross definition
of weathering intensity used in this study, unless otherwise
specified (as in the case of iron oxidation), will be as

follows:

The degree to which pre—terrestrial bulk meteoroid
chemistries have been cumulatively altered by

terrestrial effects.

Note that this definition is specific for chemical
alterations and ignores physical weathering. .Also, this
definition does not refer to specific alterations, but

addresses only their collective consequences.

34

Like the ABCe index, the weatherometers examined in this
study will focus on relative abundances of iron oxide
formation; hence the term "oxidometer" to describe these
indices. The principle objective of this investigation is
to test a viable oxidometer which could function as a
complementary alternative to the existing.ABCe system for
choosing research-quality meteorite samples frcm the
Antarctic collection. Any index considered suitable for the
situation in Antarctica could also find applicability in the
newly discovered Australian meteorite fields (Bevan and
Binns, 1989). The approach used here is to compare two
independent measuring scales, both of which are designed to
quantify the severity of iron-oxidation, and to evaluate any
resulting relationships. If both indices accurately and
precisely represent real variations in the weathering
intensities of reduced iron in respective meteorite samples,
then samples should plot on an X-Y graph, with each index
represented by one axis, as a linear trend with a non-zero
slope. Another criterion for the usefulness of such an

index would be some measure of its reproducibility.

The two experimental oxidometers chosen for this project are
(1) a simple petrographic modal analysis of
micropetrographic weathering features, and (2) the negative
loss on ignition method (-LOI) modified for meteorites by

Ann Yates of.Arizona State University (Yates, 1988). The

35

-LOI technique compares weight gains (negative loss) from
laboratory-induced oxidation (accelerated weathering) of
ferrous and metal iron in a weathered sample of an
equilibrated meteorite, with weight gains predicted for an
unweathered "fresh" sample of the same taxonomic group. An
index value is assigned according to the difference of the
two weight gains, thereby indicating the relative amount of

rust produced by weathering for that sample.

In the case of Antarctic meteorites, any practical
weathering index must be easily and quickly employed for
large numbers of samples at a time. While point counting is
a time consuming task and therefore not practical for
assessing weathering indices for thousands of meteorite
samples, -LOI is relatively quick and uncomplicated, and

many samples can be assessed simultaneously.

1.5 Scope of Experimentation

The goals of this research are to (1) determine if clay
minerals are present in highly weathered ordinary
chondrites; (2) examine the potential for applicability of
the -LOI experimental oxidometer to the further
quantification of Antarctic meteorite weathering
intensities; and, as the understanding of meteorite

weathering is still in its infancy, (3) investigate any

36

additional weathering mechanisms or processes which may be

revealed during analytical procedures.

19 highly weathered (category C) ordinary chondrites from
Allan Hills and Lewis Cliff, Antarctica were obtained from
the Johnson Space Center in Houston, Texas through the NASA
Antarctic Meteorite Working Group in the form of E 5.0 gram
bulk samples and corresponding polished petrographic thin-
section microprobe mounts. The chondrites were selected to
range in petrologic type from 3 to 6. In addition to modal
analysis and negative loss on ignition (-LOI) studies, the
samples were examined petrographically for general
characterization and photodocumentation (see Appendices A
and B). Additional chips from each meteorite were examined
petrographically at the Smithsonian Institution National
Museum of Natural History, Division of Meteorites, in
Washington, D.C. Moreover, three samples were examined by
oriented powder mount X-ray diffraction (XRD) analysis for
identification of visible and microscopic oxides, as well as
a test for the presence of clay minerals for the reasons
outlined in Section 1.3.3. XRD analysis, -LOI and
petrographic analysis were also performed on the highly
weathered non-Antarctic ordinary chondrite Carichic (H5) for

comparison with the less weathered Antarctic samples.

2 EXPERIMENTAL METHODS

2.1 Photomicrography

All photomicrographs were taken through a Leitz petrographic
microscope with 100 ISO Ektachromen'slide film.

Photographic subjects were chosen according to their
relative significance to general and specific weathering
problems. An index to photomicrographic transparencies is
presented for reference as Appendix B. This index should be
examined thoroughly and regarded as a fundamental component

of this thesis.

2.2 Modal Analysis

Two separate modal analyses were performed on each of the 19
samples. The first used a petrographic count of 500 points
per sample to differentiate opaque, stained and unstained
phase volumetric abundances. In this count, stained areas
included all types of rust visible in thin section. The
unstained category simply referred to areas unaffected by
product deposition and included matrix, chondrules and free

crystal grains. Total stain values are regarded as

37

38

contributing significantly to visually determined ABCe
weathering taxonomy. The second count used 500 points per
sample to differentiate OPL from non-OPL phases, which were

distinguished as will be discussed in Section 3.1.

There were several advantages in point counting OPL material
as representative of iron oxide weathering. The material is
denser and easier to recognize than most other phases, even
when thin. Some structural organization (possibly incipient
or complete crystallization) is usually visible in both
plane and polarized light, making identification

straightforward.

All point counts utilized medium power objectives and the
condenser apparatus. The first modal count was performed by
making manual adjustments of the mechanical stage, while the
second employed an automatic stage-advancing unit. .All thin
sectioned samples were stored at room temperature in plastic
containers with screw-on lids, which in turn were sealed in

zip-lock bags.

2.2.1 Difficulties in Modal Analysis

Some difficulties in modal analysis vary with weathering

intensity. These problems are described below.

39

Some samples, such as ALHA85025,10 had very smooth color
transitions from light yellow to deep red among the oxide
products, making differentiation of OPL and NNL phases
difficult. It was necessary to rely on morphological
distinctions to identify OPL in such cases where color was

not a discriminating characteristic.

The classification under the crosshair of a given secondary
feature within the elected set of modal categories becomes
increasingly difficult with increasing thin section opacity,
particularly with regard to non-crystalline phases. Thus,
less weathered samples may be more accurately characterized
under the microscope than more weathered samples. This bias

will be discussed more fully in Section 3.2.

It was observed in some samples (e.g., ALHA77230,48) that
OPL material is heterogeneous in its local distribution,
making averaging throughout the meteorite volume very
approximate. Because of this intrasample heterogeneity
among secondary products, it is clearly necessary for
microprobe chips to be sampled from areas on the meteorite
that are close to the source from which the 5.0 gram bulk
samples were selected in order for -LOI data to yield the
best match with petrographic modal data. This information
was not provided by NASA when the samples were shipped.
However, it is probable that where the chip numbers of

microprobe mounts and corresponding 5 gram bulk samples are

 

40

consecutive, the chips were located adjacently on the parent

rock (refer to Table 4.1).

Due to the tendency for stained areas to grade to high
transparency, it is sometimes difficult to distinguish NNL
from unstained areas. However, for reasons that will be
outlined in Section 4.1.3, the volume percentage of NNL
material is anticipated to be negligible, compared to that
of the much denser OPL phases. This factor is therefore not
considered to be a significant problem for this project.
Variations in mount thicknesses were taken into account with

regard to stain intensities.

The deepest red OPL outlines were accepted to represent
former boundaries of opaque grains (i.e., OPL is
pseudomorphous after Opaques), and so may be counted in
addition to Opaques to represent former, pre-weathered
opaque volumes. This, when compared with -LOI values should
theoretically produce a near linear trend because both are
designed to measure the same degree of iron oxidation.
However, in cases where unaltered metal particles appeared
to grade into OPL products, an approximate interface between
OPL and metal would often be identified by secondary

reflectance from the metal surface of ambient room light.

41

2.3 X-ray Diffraction

Samples ALHA77182, ALHA77271 and ALHA84075 were chosen for
X-ray diffraction analysis because they are representative
of the suite variability of modal total oxide percentages
(refer to Table 3.1). X-ray diffraction was performed both
to identify secondary mineralogy and as a specific test for

the presence of clay minerals.

2.3.1 Sample Preparation and General Pretreatment

Samples were hand crushed with a porcelain mortar and pestle
to a maximum estimated particle diameter of 2.0 millimeters.
The mortar was carefully scraped with a clean spatula to
obtain as much residue as possible. The powder was then
suspended in 250 ml of distilled water in 400 m1 beakers,
and 5.0 ml of Calgon solution was added to inhibit particle
flocculation. Each beaker was then transferred to an
ultrasound unit and agitated for 10 minutes to further
disaggregate the larger particles. After removal from the
ultrasound bath, the slurry was first stirred to bring all
particles into suspension, then quieted as much as possible
with the stirring rod to minimize any horizontal movement.
The particles were allowed to settle undisturbed for a pre-
calculated amount of time until the undesirable size
fractions had descended to a depth of 5.0 cm into the

solution. Settling times were 1.7 minutes for 20 um, and

42

205.0 minutes for 2 pm size fractions. The liquid above
this depth was recovered with a clean pipette inserted to
the depth where the maximum desired particle size was
determined to be suspended. Approximately 100 ml of this

suspension was recovered for each specimen preparation.

A total of 17 preferentially oriented powder mounts was
created from the decantates by forcing the settling of
suspended material by negative pressure onto 0.20 pm
Millipore filters. During the suction process, samples were
saturated with glycerol and/or magnesium chloride, as the
experiment demanded. These pre-treatments were used in
anticipation of detecting and identifying clay minerals,
primarily in Carichic, and are not significant for the

identification of oxides.

Microsc0pic metal and silicate particles from the chondrite
matrix precluded transfer of the sediments to glass slides.
This is usually accomplished by pressing the still-moist
mount against the clean glass surface of a slide, and then
carefully pealing back the filter. The filters were
therefore trimmed and mounted to the glass slides directly
(sample-side out) using small pieces of double sided tape.
The tape pieces were located strategically along the
periphery of the filter to ensure a flat filter surface

parallel to the surface of the slide.

43

All diffractograms were generated at 35 kilovolts, 25
milliamps, a chart speed of 1 centimeter per minute, a scan
rate of 1°26 per minute, using 1*é/.3/2 filters at 1K or 2K
counts per second full scale and a time constant of 2

seconds with CuKa radiation (wavelength = 1.54091A).

2.3.2 Citrate Dithionite Pretreatment

The citrate dithioninte pretreatment dissolves and removes
crystalline iron oxides and oxyhydroxides by the reduction
of iron from the ferric to the ferrous state. It was
performed on subject samples in the hopes of reducing
background noise on diffractograms. Ideally, this enhances
reflections of clay minerals. While the attempt was largely
unsuccessful in removing iron (background levels remained
high), it was found that smaller size fractions produced
lower amounts of noise following the pre-treatment relative
to higher size fractions. Also the treatment had the effect
of revealing the possible presence of akaganéite in the

Carichic sample.

The following method is borrowed from Jackson (1967).
Approximately 1 gram of sample of the Carichic meteorite was
used to test the citrate dithonite treatment. The sample
was ground to a powder and added to 40 ml of 0.3 molar
sodium citrate solution and 5.0 ml of 1 molar sodium

carbonate solution. The mixture was brought to a

temperature of 79W: in a water bath before adding 1 gram of
Nagho4. The solution was then stirred continually for one
minute and then for alternating intervals of five minutes.
.A second 1 gram measure of Na28511was then added and stirred
as before, followed by a third 1 gram measure and additional
stirring for five minutes. 10 milliliters of saturated
sodium chloride solution and 10 milliliters of acetone were
then added to the slurry to promote flocculation, followed
by thorough mixing with continued heating. The suspension
was then centrifuged for 5.0 minutes at 1600-2200
revolutions per minute and the supernatant decanted to
remove the dissolved iron. XRD mounts were then prepared
from the sediment by adding distilled water and stirring to
re—suspend the desired size fractions before allowing them

to settle as described above.

2.4 Negative Loss on Ignition

The following procedure is based on that developed by Yates
(1988, 1989). Twelve k-ounce porcelain crucibles were
numbered with a permanent marking pen and heated in an oven
at llOWC'under negative pressure for twenty-three hours.
Each crucible was transferred from the low temperature oven
to a digital balance while hot and weighed to four decimal
places. The crucibles were then allowed to cool to room

temperature over a 30 minute period and re-weighed. This

45

procedure was performed to determine the weight percentage
of absorption by the crucibles of ambient volatile

constituents during cooling to room temperature.

It was found that a 0.05 to 0.07 weight percentage increase
occurred in each of the twelve crucibles during cooling (see
Table 2.1), presumably as a result of water vapor absorption
within the porous ceramic structure of the crucibles. As
this is a significant increase, it was decided to weigh each
crucible while hot during all loss on ignition procedures.
Such action would preclude unwanted adjustments of crucible

and sample weight values by water vapor.

A high temperature furnace was preheated to 1040W3. Ten
crucibles were placed in this environment for four hours,
and transferred to the 110%: oven immediately following,
before cooling to water vapor absorption temperatures. Here
they remained for 5 to 20 minutes before being tare weighed.
Immediately prior to weighing, each crucible was delivered a
single, brief blast of compressed air along the base to
drive away any clinging particulate matter inherited from
the furnace or the oven. It was reasoned that the crucibles
would be too hot to take on any significant additional
weight from the highly volatile constituents in the
compressed air. It was then noticed that the numbers made

with the permanent marker were no longer discernible. All

Table 2.1. Crucible weights before and after cooling

 

 

Crucible Hot

number weight
1 4.3977

2 4.3755

3 4.1739

4 4.3626

5 4.2253

6 4.1898

7 4.3268

8 4.1905

9 4.2462

10 4.1963

1 1 4.3239
12 4.3266

All weight values in grams

 

Cool
weight
4.3997
4.3776
4.1760
4.3651
4.2275
4.1917
4.3297
4.1934
4.2491
4.1989
4.3269
4.3293

 

 

% weight
increase
0.05
0.05
0.05
0.06
0.05
0.05
0.07
0.07
0.07
0.06
0.07
0.06

 

 

 

 

 

 

47

tracking of samples was therefore accomplished by carefully

recording their positions in the furnace and oven.

NASA-prepared 5 gram bulk samples arrived in vials sealed
within airtight packaging, and are further sealed within zip
lock plastic bags. Some vials are plastic, while others are
stainless steel. .Airtight packages are cut with a pair of
scissors and cannot be resealed. Approximately 1 to 1%
grams each of 19 bulk meteorite samples was crushed in a
ball mill for 2.5 to 3.0 minutes. Large pieces were
selected for the 1990 analysis as preferable to smaller
pieces and dust to minimize the effects of laboratory
weathering when comparing with point count-determined
volumes. Time duration in the ball mill for each sample
depended on subjective impressions of relative friability
before crushing and on visual inspections of the samples
during crushing. In this way a qualitative constancy of
maximum grain sizes for all samples was achieved. Between
uses, the ball mill was completely disassembled, washed
first with hot tap water, then cool distilled water, dried
with Kimwipem tissues, and allowed to air dry for five
minutes in the lltWC oven. Samples were deposited
separately in the tare—weighed crucibles and weighed to four
decimal places. Crucibles with samples were then
transferred to the high temperature furnace, which was
preheated to 1040WC. .After baking for 4 hours, the samples

were removed and re-weighed in the manner outlined above.

48

Reused crucibles were first cleaned of encrusted meteorite
material with a spatula, then rinsed and dried thoroughly.
They were then replaced into the high temperature furnace at
104093 overnight and re-weighed while hot to four decimal
places as before. Thus any material incorporated into the
ceramic structure from previous samples would be accounted

for in the following —LOI procedures.

The -LOI procedure was repeated for all 19 samples and the
non-Antarctic meteorite Carichic at a furnace temperature of
1040W: to obtain a second set of results. This second
analysis was performed 16 months after the first as a test
for reproducibility of weight gains upon ignition and to
monitor any addition curatorial weathering effects. The
first sample run was performed in November of 1990, and the
second in March of 1992. The samples were prepared by NASA
in June, July, August, and September of 1988. It was
decided to include small chips and dust in the 1992 -LOI
analysis as well as larger chips to better detect any
shifting in -LOI values over time, since more surface area
would be exposed. Thus shifting covers the time period from
1988 to 1992 for this second analysis, not merely a 16-month

interval.

3 ANALYTICAL RE§L|LTS

3.1 Petrographic Characterization of Oxides

3.1.1 Antarctic Samples

The most readily identifiable alteration products in the 19
polished thin—sections/microprobe mounts are limonites (iron
oxides), which form the basis for the current ABCe
weathering index. As is common among terrestrial rocks,
such oxides occur in Antarctic meteorites in the form of
rust stains, which are usually extinct between crossed
nicols, but also exist in other diversified phases, some of
which display pronounced birefringence between crossed
nicols. Many of these products can be recognized (although
not as easily identified) petrographically, and some
varieties can be identified mineralogically using X-ray
diffraction (XRD) techniques (see Sections 3.4 and 4.2.1.2).
This section defines the diagnostic features of the two most

common petrographically distinguishable types of rust.

The most widespread oxide visible in thin section is a stain

varying in color from very light yellow to deep yellow,

49

50

orange, and brown in plane polarized light (Figures 3.1 and
3.2). This material is most often isotropic, modifying
birefringence colors where it overlaps grain surfaces, but
crystallizes locally, as revealed by unique birefringence
colors, internal reflections and extinction characteristics
under crossed polarizers (Figures 3.3, 3.4 and 3.5). The
neoformation precipitation mechanism, that of constituent
chemical species being transported in solution to
precipitate at some distance from their source (e.g., Yatsu,
1988), appears to be the dominant formation process for this
phase. The phase does not seem to replace primary phases.
This is evident by its lack of a preferred association with
any primary phase, and by the near ubiquitous occurrence of
the stain throughout each specimen observed. Stain
locations appear to be governed more by the availability of
free-space avenues, than by the location of primary
minerals. The color variations are commonly thickness-
dependent, as indicated when the material is found near thin
edges of certain thin sections. At these places the colors
display a sequence of decreasing opacity in plane polarized

light from the interior toward the outer edge of the sample.

This neoformed, non-pseudomorphic limonite (NNL) appears as
ribbon-like strands, filling grain fractures, cleavage
planes and grain interstices throughout much of most
specimens. This is consistent with the petrographic

observations of terrestrial Antarctic rocks by Cambell and

 

- 4.9

“‘vl‘((((/li//////I /l{(({(/l(/l/l((((((ll(lrlll(/i/i

If /

 

 
 

.34" .,
. ‘ Marta

Figure 3.]. iron oxidc slain in chondmlc. ’lhis barred olivine chondrule (outlined in plane light by a disconnected ring of opaque metal
grains) demonstrates the pcn'asivc nature of iron oxide stain material. which fills fractures that continue beyond the chondrule
boundaries. In Al.ll;\77288,34. Frame dimensions are 1.0 by 1.5 mm,

5 '2.
.:.ft’

-s\_“t'v 3'. '~ ‘ i

a,

’ . . a ' '
A \ 7 .; I - ' . . , . ‘5. . . _ .
Flgure 3.2. iron oxide stain in cleavage planes. This pyroxene chondrule ti’om LEWRGO] 5,10 clearly shows how stain distrilution is
controlled by available migration pathways such as cleavage plant‘s. grain intcrsticcs and tinctures. Frame dimensiom are 400 by 600
microns,

 

 

 

‘ .

 

52

  

o ’ x _ ' '
. i ‘. V
l A . ’ ' l N' M - ' — . t ' 0
Figure 33. Iron oxide stain crystallinity. frame #1. A ribbon of iron oxide stain material within an olivine gain is shown here in plane
light. Note subconcentric structural banding. In ALHA7728834 Frame dimensions are 400 by 600 microns.

I)

, I

z .

i ‘\ i . V...
F' 3.4. Iron oxide stain crystallinity, frame #2. Under crossed polarizers, some incipient crystallization can be observed (second-
order yellow birefringent crystallites), Notice the optical continuity.

     

 

 

53

 

\
Figure 3. 5. Iron oxide stain crystallinity; frame #3. Grain has been rotated clockwise 47° to present the effect of sum-imposed olivine
crystal on oxide stain coloration Note well-crystallized red and orange region to the lower right of the time.

if)” l'. L ..
M, a . We. ,.

4"e._ \. "' w ..‘
“by... ~42: ’e - ‘
“37:19.” 41% "‘

. .. i"
‘ $9

 

‘ 4th.. .-
Figure 3. 6. Opaque—pseudomorphic limonite haloes. Typical example of opaque-pseudomorphic limonite (deep red) replacing metal
grains (black) Sample is ALHA84075, 7. Frame dimensions are 400 by 600 microns.

54

Claridge (1987). These strands are very uniform in tone and
completeness where they occur, filling the finest fractures
with little apparent interruption, and occupying fractures
that traverse the entire thickness of the thin-section, as
evident by "far" and "near" focusing of the microscope.
Under high magnification, a stippled or bubble-like texture
to NNL-filled grain fractures is often noticeable, possibly
indicating a circumventing migration path of NNL-bearing
fluids around points of contact between two fracture
surfaces. If such is the case, these residual points of
contact between separating crystal surfaces would indicate
that separation had not progressed very far prior to fluid

migration through the avenue.

The second common oxide phase often has a deep red color in
plane polarized light, displays high birefringence colors
between crossed polarizers and possesses well-defined
outlines (Figures 3.6, 3.7 and 3.8). This opaque-
pseudomorphic limonite (OPL) phase occurs most frequently as
haloes and partial haloes surrounding opaque grains, but
occasionally is found expanding away from opaque phases into
free spaces (Figures 3.9, 3.10, 3.11, and 3.12). Evidence
for in-situ alteration of opaque grains is that opaque/OPL
interfaces are frequently embayed or sutured, in contrast to
the smooth boundaries of opaque minerals not associated with
OPL. Although deep red is the color most often encountered,

many examples of OPL exist which have orange or yellow hues.

  
   

 

I \‘A- . ' v - J“- ‘ . l‘ d -‘\' J-N"
Figure 3.7. Voidspaoepanially filledwhhanunorphous oxide malevial,andsurrotmdedbyanopaque~pseudotnorphiclirnmitjc material

(OPL). mmmmmwmwmmdammmmmmmmmmww
tbeOPL Fm ALHA81027,26. anedhnensicnsm400by600nfim.

   

   
 
  

. x

‘f ,

   

. (a -

A ‘ ‘ n...‘ A. eight - - '
Figure 3.8. Above View in erased polarized light Note opacity of amorphous material, in oommsl to the well-aystnllizcd OPL material.

 

 

 

 

 

‘ .

(‘(‘\(’I‘((I I (I (I‘
'(

 

 

56

   
     
   

“.4 . 1 '
a.‘;, ‘ 'r

- 3C

 

., ;4

 

  

no

Figure3.9. Awell-defined OPL depositlappearstobeexpanding into the fi'eeqaaoebeyondthemeteorite ssurfaceand'separatingfiom
the inferred primary opaque metal gains Note embaymcms into metal gains, coalescence of material beyond the primary gains and
fractures along lamellae which are subparallel with opaque grain surface. From LEW86015 10. Frame dimensions are 400 by 600

'- A ‘3?" ”I”

r!

 

Figure 3.10. Above view increased polarized light showing crystallinity ofsubstanoe.

 

57

 

FigurISHll AnodierexampleofOPLmaterialatasample boardereiqiandingim othefi'eespacebeyondmdimonenrhyfi'am Note
deepernba ymentswrthin primaryopaquemaierial. FromLEW85332,ll. Framedimensionsare400by600microns.

 

, ‘ , ’ L .’ . . ._ .
Figure 3.12. Ahovc View in crossed polarized light showing crystallinity of substance.

58

The color variations are proportional to microprobe mount
thickness, and can be observed both in plane polarized and
cross polarized light (Figures 3.11 and 3.12). Figure 3.13
shows an opaque grain at the ALHA81027,26 thin section
boundary which appears to be altering and disintegrating
systemmatically, apparently facilitated by the expansion of
growing OPL alteration product visible within. This image
appears strikingly similar to the textbook description of

soil formation on a bedrock surface in cross-section.

OPL and NNL are the two most common oxide types observed in
thin section. They are usually easy to distinguish from
each other. The OPL material appears to have strong relief,
while the NNL material has low relief with color variations
appearing simply to be various intensities of surface

staining.

Other, less common phases and phase habits include the
following: (1) Transparent limonite fracture fillings
(TLFF), birefringent deposits appearing similar to OPL
deposits but filling fractures, and having their locations
farther from likely opaque source minerals than OPL phases,
suggesting higher mobility of their constituents than
typical OPL minerals (Figures 3.14 and 3.15); (2) Opaque
limonite fracture and void space fillings (OLFF), similar to
TLFF but appearing opaque in plane and polarized light, and

lining the walls of both fractures and void spaces (example

 

 

 

Ill" (

 

59

 

accompany
withinthe gain. From AUiA81027,26. ” L ' :32 b, ""‘

 

 

 

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Figur;3.l4. Transpmliinonitefiadm'efillmg Wellayndlizedweflunegated. Oidrkappearstotakeadvmageofexistingshodt-
fractures. OiddefillingarewidePZSmicrons), 'fiommeiraigimlwddthwhichpresnmblywas
ofsubmieronthickness FrornALHA85025,10. Frarnedimensronsare400by600miuons.

 

Figure 3.15. Above view in crossed polarized light. Material is wellerysullized

_ 1 -A n mfim "

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61

 

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difi‘minfoarsamongthevariouswotiudinghilbs. FromAlJiA77271,28.Frainedimuiaionsare160by2-10mim

 

 

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detaminewbethersuiaearegenaicinoriginorthereauhofauwblade. 'Ihepattansmayberelatedtotheprimarycrystalstrumn’e
primarytoreplacemeut. lnAUiATl004,22.Frarnediinenaimsare160by240mim

 

Figure 3.18. Above view in crossed polarized lign. Again, the material is well-aystallized.

 

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figure3.19. Sharpoontactplnse. Anaheremipleofaslurpooinaet phase. “ ‘wsisunithhigilysmured
edges Noteooneeutricinterual structme. FroinAIJ-IA77271,28. FramedimensiasareléObyNOmierons.

y r M
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Figure 3.20. Above view in crossed polarized light. Minerals display sweeping extinction

 

 

r.--‘ 1".“
“‘
4'
“:\.‘:‘Y

 

Figure3.21. Carichic "l'hinsectionofCarichicinrefleetedlighi. Brightfledrsarekamaeiteand-taenitegams Dendriticfraetim

unreadiespedmmaidalwuotabledark-stahndvemsdevoidofmaflgahnm
approximatelylbyP/aindies

 

65

visible in Figures 3.7 and 3.8). This material can also
have a structurally weak, "fluffy" appearance when found
within a void space; (3) Botryoidal void space fillings
(BVF), intricate, often orange colored deposits with bulbous
or botryoidal morphologies that partially fill some void
spaces (Figure 3.16); (4) Sharp contact phases (SCP), well—
defined bodies of oxide having very clean contacts with
silicate phases, often showing well—developed laminar or
concentric internal structures, conspicuous by the lack of
nearby opaque material (Figures 3.17, 3.18, 3.19, and 3.20).
These minerals may simply represent the complete replacement
of opaque grains by OPL material. It is important to note
that these morphologic differences do not necessarily
reflect mineralogical differences. For example, TLFF, BVF
and SCP may be the same mineral, but identified at different

stages of development or in different habits.

3.1.2 Carichic

Carichic is a 17 kilogram H5 1983 find from Chihuahua,
Mexico and is catalogued as extremely weathered (Graham et
al., 1984). The density of oxide product was too high to
permit study of oxides under transmitted light. However,
Figure 3.21 is an off-axis photograph of a Carichic thin
section in refelcted light. Immediately apparent are the
highly reflective residual taenite and kamacite grains

(opaque in transmitted light), which appear well preserved

66

in regions removed (approximately 2 mm distant) from
fractures that crosscut the section, and entirely absent in
regions close to fractures. Also, the metal-vacant area is
remarkably symmetric about the fractures. Closely
paralleling this pattern are zones of lighter and darker

oxide staining.

3.2 Modal Analysis

Petrographic modal analysis data are presented in Table 3.1.
The materials comprise three main categories including (1)
total counted opaque phases, (2) total unstained silicates
(identified as clear areas in plane light), and (3) total
identifiable oxides. The oxides are then further broken
down into OPL and non-OPL categories. Non-OPL includes,

obviously, all forms of rust not classified as OPL.

Table 3.2 presents the average normative mineralogy of H-
and L-group ordinary chondrites. All modal opaque values
obtained for the study samples fall consistently near
average normative values for chromite, ilmenite, troilite
and nickel-iron (opaque mineral) volume total abundances.
Normative mineral abundances for ordinary chondrites are
nearly identical to their modal proportions, particularly in

types 4 and higher (Dodd, 1981); volumetric abundances will

67

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68

Table 3.2. Modal opaque mineral percentages

 

 

 

 

 

 

 

 

Meteorite Class Mogal % opaque Reference
Fe-Ni Troilrte Chromite Total
L Chondrites
Junction, Texas L5d 4 8 0.7 12.7 Ehlmann 8. Keil, 1987
Qidong, China L5d 4.7 4.3 nr 9 Wang & Ruben, 1987
Bayard L5 6.4 nr nr 6.4 Struempler & Kaolin, 1987
Reliegos L5 6.1 nr nr 6.1 McCoy et al., 1990
Roosevelt County 025 L53 1.3 6.2 nr 7.5 Pen et al., 1990
Roosevelt County 028 L5a 3.9 6.5 nr 10.4 Pen et al., 1990
Roosevelt County 066 L5a 0 1.6 nr 1.6 Pen et al., 1990
Roosevelt County 067 L5a 1.3 4.6 nr 5.9 Pen et al., 1990
Roosevelt County 068 L5a 0.4 3 nr 3.4 Pen et al., 1990
Roosevelt County 069 L53 0.4 3.5 nr 3.9 Pen et al., 1990
Roosevelt County 074 L5(S3) Trace 1.5 nr 1.5 McCoy et al., 1993
Roosevelt County 076 L4(S3) 3.7 4.6 nr 8.3 McCoy at al., 1993
Roosevelt County 077 L4(S2) Trace 6.6 nr 6.6 McCoy et al., 1993
Roosevelt County 078 L4(82) 2 6.2 nr 8.2 Games 8 Keil, 1980
Normative % oparme
Average values 7.5 6.1 0.6 14.4“ Dodd, 1981
H Chondrites
Olton, Texas H 11.2 4 nr 15.2 Keil et al., 1990
Dalhart, Texas H5a 1 3 nr 4 Ehlman & Keil, 1987
Rosebud, Texas H50 8 4 nr 12 Ehlman 8. Keil, 1987
Cranfills Gap, Texas H6c 1 4 nr 5 Ehlman & Keil, 1987
lngella Station H5a 13.6 5.4 nr 19 McCoy 8 Keri, 1988
Molina H5 21.1 7 0.7 28.8 McCoy et al., 1990
Guarena H6 20.7 4.8 1.1 26.6 McCoy et al., 1990
Olmedilla de Alaron H5 20.9 nr nr 20.9 McCoy et al., 1990
Springer, Oklahoma H5(83) 15 5.3 nr 20.3 Ehlman & Keil, 1992
Roosevelt County 073 H5(SZ) 1.9 4.6 nr 6.5 McCoy el al., 1993
Anton, Texas H4b 19 8 nr 27 Ehlman 8. Keil, 1987
Venus, Texas H4d 15 8 nr 23 Ehlman & Keil, 1987
Hashima H4 8.2 5.8 nr 14 Hohino & Suea, 1992
Allen, Texas H4(82) 8 oxides nr 8 Ehlman & Keil, 1992
May Day, Kansas H4(82) 14.5 4.7 nr 19.2 Ehlman & Keil, 1992
Pony Creek, Texas H4(S3) 8.1 5.4 nr 13.5 Ehlman & Keil, 1992
Normative % opaque
Me values 18.6 5.3 0.6 24.7' Dodd, 1981
Fe-Ni = Iron-nickel metal
nr = Not reported

Many of the low values are low because of weathering

' includes ilmenite

69

be discussed more thoroughly in Section 4.2.2.3. This is
therefore accepted as evidence for the accuracy of the point

count.

An iron oxide weatherometer derived from petrographic modal
analysis cannot simply be a measure of the total rust point
counted in thin section. This is because the volume of
secondary species, which can be defined as mineral thickness
per unit cross-sectional area of microscope stage, is not a
constant. NNL films seldom span the perpendicular cross-
sectional distance from the top to the bottom of the thin
section, while other materials, such as OPL, apparently do.
Thus varying relative abundances of these different
materials will yield modal indices that are out of
proportion to actual rust product volumes. The geometry of

this phenomenon will be illustrated in Section 4.1.3.

In addition, H ordinary chondrites have greater reduced
metal fractions than L ordinary chondrites. In this study
the volume of NNL staining is taken to be negligible
compared to OPL volumes for the reason stated above. As OPL
material is considered an in-situ, pseudomorphic replacement
of reduced opaque phases, the weathering index for the point
count results will represent the volume percentage of such
replacement, i.e., OPL/(OPL + Opaques) x 100. The rationale
for assuming isovolumetric replacement is clarified in

Section 4.2.2.1. This calculation produces a metal

70

oxidation index that ranges from O to 100 percent (Figure

3.22).

 

 

9?

 

07. N 257.

v @

 

 

 

 

 

D

Figure 3.22. Theoretical opaque metal grain (black) replacement by
pseudomorphic limonitic material (OPL) (white within outlines).

3.3 X-ray Diffraction

X-ray diffraction data is presented as Appendix C. The non-
Antarctic ordinary chondrite Carichic (an H5 find) was
recovered in 1983 from the humid to semi-arid climate region
of Chihuahua, Mexico. It is in an advanced stage of
weathering (Graham et al., 1984) which probably exceeds by
far that of any Antarctic specimen. It was investigated for
comparison with the Antarctic samples as a possible
indicator of mineral assemblages toward which chemical
weathering of ordinary chondrites may be progressing in the
Antarctic environment; and to assist in assessing the long

term evolution of weathering meteorites in general.

71

Oxide mineral identification is particularly difficult in
the case of ordinary chondrites. X—ray backscatter from the
metal particles in the matrices of ordinary chondrites is a
significant problem. Some metal particles are small enough
that they are not separated from clay-size silicates during
sample preparation, but remain suspended within the sampled
water column along with secondary phase particles, and

therefore become included in the oriented powder mount.

With the exception of the citrate dithionite-treated
Carichic samples, the $20 um size fractions produced lower
background intensities than the $2 um size fraction. It
may be that particle size distributions for ordinary
chondrite matrices, which are essentially poorly sorted
sediments, are skewed toward the smaller size fractions, as
with some terrestrial diamictites (e.g., Flint, 1970). We
therefore find much more primary material incorporated
within the powder mount than experience with weathered
terrestrial rocks might lead us to expect. .Also, larger
size fractions contain larger crystals, which produce more
intense peaks. It was thus found that the s 2 um size
fraction produced correspondingly lower peak amplitudes and
poorer peak definition than the s 20 um fraction. The s 20
um particle size was therefore sampled most frequently for

the XRD analysis.

72

The natural superimposition of oxide peaks with each other
and with those of the primary phases also exacerbates the
task of mineral identification. This problem has been
encountered before with the identification of secondary
minerals in meteorites (Barber, 1985; Zolenski and McSween,
1988; Yates, 1988). Examples include the 2.51A olivine
peak, which overlaps 2.53A, 2.51A, 2.51A, and 2.49A.peaks
for magnetite, maghemite, hematite, and goethite
respectively. The 2.69A goethite peak overlaps the 2.69A
hematite peak, and the 2.95A.maghemite peak overlaps the
2.92-2.95A plagioclase peak. Because of this, oxide mineral
detection and identification are often reduced to the
identification of single diagnostic peaks. These peaks are
considered diagnostic only if at or close to the maximum

(100 percent) relative peak height.

Another probem is that incomplete isomorphous substitution
of ions within the crystal structure can affect line
positioning in iron oxides and oxyhydroxides (Dixon and
Weed, 1982). This uncertainty increases the potential for

confusing minerals with similar d-spacings.

3.4 Negative Loss on Ignition

Results from the loss on ignition measurements are presented

in Tables 3.3a and 3.3b. Meteorites are represented by

73

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75

their identification numbers in order of increasing class
and petrologic type designations. In the tables, column C
presents the mass in grams of each tare-weighed crucible.
Columns D through G present the mass in grams of each
sample, separately and together with its respective
crucible, prior to and immediately following ignition.
Column H then gives the percent difference between the pre-
and post-ignition sample masses. Column I is the oxidation

index, defined as;

01 = 100- 1x100
4.4

for L chondrites, where H is the value given in column H and
4.4 is the average percent weight gain for unweathered L

chondrites as determined by Yates, (1988); and;

H
01 100 - —- x 100
8.4

for H chondrites, where 8.4 is the average percent weight
gain for unweathered H chondrites as determined by Yates,
(1988). Thus, a scale of O to 100 percent will represent
the possible range of values resulting from a suite of
samples with metal and sulfide grains that are 0 to 100

percent oxidized.

76

The first 19 tests were conducted in November of 1990, and
the last 20 during March of 1992. The second -LOI data set
was obtained in an attempt to monitor any additional
weathering that may have occurred in the samples while in
storage. Carichic is too heavily oxidized (containing too

much opaque material) for petrographic modal analysis.

4 DISCUSSION

4.1 Graphical Representation of Data

4.1.1 Modal Analysis

Figure 4.1 presents the results of the modal analysis using
the three most basic petrographic components; (1) primary
opaque material, (2) unstained silicates, and (3) total
oxides. Immediately apparent is that even among the most
weathered Antarctic meteorites, some opaque material has
remained. Also to be noted is the narrow range of these
residual opaque mineral abundances. Notice too the spread
in values perpendicular to the “Opaques” end-member.
.Although the data points cluster near the “total oxides”
end-member, where one would expect to find weathering
category C ordinary chondrites, there is considerable

overlap into “less stained” areas of the diagram.

The ABCe oxidometer has five categories to encompass the
entire range of iron oxide production observed so far in
Antarctic meteorites. The intent behind these categories is

to measure the fraction of exterior surface area that is

77

78

Unstained Silicates

 

 

OxidesT Opaques

Fig. 4.1. Modal analytical data plot in
terms of three most basic components.

79

visibly (in hand specimen) discolored by rust. The five
categories can be defined precisely by allowing each to
represent one fifth of the total possible range from O to
100 percent stained. Weathering category A would then
represent 0 to 20 percent; A/B, 20 to 40; B, 40 to 60; B/C,
60 to 80; and C, 80 to 100 percent stained (Figure 4.2). In
actuality, the most intensely weathered Antarctic meteorite
is probably far less than 100 percent oxidized (all metal
grains consumed by oxidation). The category ranges in
Figure 4.2 will therefore serve as an upper limit to the

ranges possible for the Antarctic ABCe index.

In an attempt to identify any trends in the production of
secondary minerals (particularly OPL), the non-OPL rust
component of the "total oxides" end-member was separated
from the OPL fraction and included with the unstained
silicates fraction (Figure 4.3). This produces the effect
of keeping constant the sum of (1) all formerly clear areas,
whether stained or unstained, and regardless of the stain
intensity, and (2) all less abundant varieties of rust.
Hence the non-OPL and unstained silicates categories
together represent by definition all components visible in

thin section other than OPL and Opaques.

In Figure 4.3 the opaque abundances decrease from roughly 19
to 10 percent as the OPL abundances decrease from roughly 26

to 2 percent (excluding the obvious outlier). Meanwhile, a

80

Unstained Silicates

 

 

OxidesT Opaques

Fig. 4.2. Theoretical distribution of current
Antarctic meteorite weathering categories.

81

non—OPL
+
Unstained Silicates

 

 

OPL Opaques

Fig. 4.3. Redistribution of modal analytical data
with non-OPL separated from OPL.

82

corresponding increase from approximately 63 to 86 percent
occurs in the unstained and non-OPL fraction. .All points
originally begin at positions along the ternary boarder
perpendicular to the “OPL” end-member as unweathered
meteorites. If OPL were replacing opaque material
isovolumetrically, each point would be translated from right
to left along a horizontal line, ending up on the ternary
boarder perpendicular to the “Opaques” end-member, and
exactly opposite its original position, after all opaque
material had been consumed. Having weathered to category C
intensities, which we now see are somewhere intermediate
between “fresh” and “completely weathered” end—members, the
modal data points have somehow become aligned in Figure

4.3.1

4.1.2 Negative Loss on Ignition

Figure 4.4 represents a test of the index value
reproducibility for negative loss on ignition as an
oxidometer, using the two sample analyses performed in
November of 1990, and in March of 1992. Perfect
reproducibility would yield data points that fall on a line
with a 1:1 slope, as indicated in the figure. Only 18 of

the 19 samples are represented in this and all following

 

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1992 —LO| Index Results

83

 

100 I I I I I l f I I

80L . .

60f . _

50~ ’. —

30r- —

10» -

 

 

0 l l l l l l l l l

O 10 20 30 4O 50 60 7O 80 90100
1990 —L0l Index Results

 

Figure 4.4. Reproducibility of -LOI results.

—LOI charts because the L3 chondrite ALHA81031 underwent a
true loss on ignition during both sample runs, and so would
not produce a positive value when the -LOI index was
calculated (see Tables 3.3a and 3.3b). Notice that most
sample points plot near this line, with 13 plotting on or
above it. Figure 4.5 is another representation of these
data, but with sample-specific taxonomic class and
petrologic types indicated. Note that the data can be
divided into two dissimilar zones, one centered on the 1:1
line and consisting of H5 and H6 ordinary chondrites, the
other forming a somewhat smaller cluster above the 1:1 line,
and consisting entirely of L and H4 chondrites. Figure 4.6
shows these data once again, but with the fracture indices
represented. Notice that while no continuous trend exists,
the least fractured (A category) samples can be easily
separated from the most fractured (C category) samples.
Notice also that most of the H4 chondrites belong to the C

fracture category.

4.1.3 Experimental Oxidometer Contrast and Comparison Diagrams

The NNL stain material tends to occur in fractures and
cleavage planes which are thin compared to the thin section
thickness. In the modal analysis, volume percentages for
total rust, as determined from the point counting of light
stains plus denser oxides, are exaggerated by a factor that

is inversely proportional to the angle between a given NNL-

1992 —-L0| Index Results

85

 

100 , r T , T 1 I j T
90 — H5 —
80 ~ . 5 -
7o — l
60 l- H4~H4 _
50 — . 6 .H6 -
4o - . -
so — —
20 -— -

10- —

 

 

0 l L J l l l A l l

0 10 20 30 40 50 60 7O 80 90100
1990 -LO| Index Results

 

Figure 4.5. Reproducibility graph with meteorite class
and petrologic type indicated. Data points can be
separated into two groups with L and H4 ordinary chon—
drites comprising one, and H5 and H6 ordinary chon-
drites comprising the second.

1992 -—LO| Index Results

100 l l l l l l l l T
90 C.
B/C "
80 B 0 . .
o - A
70
60 C‘C ‘3'"
A/B '
50 8.. . t 0A
c. C?" ~A/B
40 .'/C EN
30
20 A
10
0 l l l l l 4 l l l
10 20 30 4O 50 60 7O 80 90 100
1990 -LO| lndex Results
Figure 4.6. Reproducibility graph with fracture

index indicated.

86

 

 

 

from the most fractured (dashed).

Note that while no continuous
trend exists, a line could be drawn just above the
1:1 line which would separate the least fractured

 

87

filled fracture plane and the microscope stage (Figure 4.7).
The NNL therefore comprises a substantially smaller fraction
of total rust than indicated in the modal analysis. For
example, ALHA81027,25 has a total count of 78.8 percent for
overall rustiness because of extensive light staining.
Conversely, LEW85322,9 has a very low total of 18.0 percent
because of very little NNL staining. However, these two
stones differ less drastically when either their modal or

gravimetric index values are compared (Table 4.1).

Thin-sectional area (x) Angle (6) between fracture
and microscope stage

 

 

 

 

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Nmn- a.9... 60:00:38.2»: 60mm...» .2283 SE. #2. £25.23 29.0.. a m. Nnmmw>>m4

 

 

 

5.65.22. o..._ooo«-o_aEm« .6 88:55 .32 2%...

 

89

The apparent volume would thus be given by,

F1 1
V. =ylxdx 0c -
o 0

where y is the thin section thickness, x is the thin-
sectional area perceived through the microscope objective,
and FL is the fracture length. Any oxidometer developed
using the NNL point counted values will probably be
misleading. The NNL modal fraction will therefore not be

included in the index formula used in this study.

The modal oxidometer for this thesis is based on volumetric
opaque and OPL abundances. Figure 4.8 introduces the modal
index, and is a scatter plot showing the samples as
represented by both modal and gravimetric experimental
oxidometers. The modal index is defined as the volume
percentage fraction of OPL in partly replaced opaque phases.
The negative loss on ignition index (abscissa) is the
percentage weight gain out of the possible weight gain for a
fresh fall of the same group for each meteorite; i.e.,
corrected for the difference between L and H ordinary
chondrites (refer again to Tables 3.3a and 3.3b). Figure
4.8 displays —LOI data for November of 1990 only. Figure
4.9 is a similar graph, but presenting data point positions
based on -LOI data obtained in March of 1992. Figure 4.10
shows the two sets of results together, where the 1992

positions are represented by the filled circle, and the 1990

100

90

80

70

60

50

40

30

20

OPL/(OPL + opaquesT) x 100

10

 

 

 

 

l J l A l l 4 J l

0 10 20 30 4O 50 60 7O 80 90100K

1990 -LO|

Figure 4.8. 1990 gravimetric and modal
index values.

91

 

 

 

100 r T T l l l l r l
90 ~ .
8 80 - —
X 70 _ o -1
’1
8
a so - -
O.
3 so . . —
o _
+ 40 ~ - . _
2’ °'
8 30 ~ . ” -
\ ° 0
i .
O 20 - , —
10 r- —
0 l l l J l l L L J

 

0 10 20 30 4O 50 60 7O 80 90 100

1992 -LO|

Figure 4.9. 1992 gravimetric and modal
index values.

OPL/(OPL + opaquesT) x 100

100

90

80

70

60

50

40

3O

20

10

0

92

 

 

 

l l l J l g l l l

 

O

10 20 30 40 50 60 7O 80 90100

-LOl

Figure 4.10. Plot representing both 1990
and 1992 —LOl data. 1992 data are de-
picted as points, 1990 data as vertical lines.

93

positions are indicated by the short vertical lines.
Horizontal lines connecting the two data sets illustrate the
direction and magnitude of shifting between the two sample
runs. Right lateral shifts have occurred in all but seven
samples. Of these seven, two have remained essentially
stationary. Five have undertaken significant left lateral

shifts.

As with Figure 4.5, the Figure 4.10 scatter plot was
examined for patterns in group and petrologic type
distribution. Figure 4.11 shows the data points can again
be represented by two assemblages with different patterns of
behavior. The Ls and H4 meteorites occupy the lower portion
of the data point cluster and display substantial and
relatively consistent increases (right lateral shifts) in
-LOI index values between the two analyses. The H55 and H6
meteorites occupy the upper portion of the cluster and
exhibit erratic shifting behavior: some shift to the right,

some to the left, and some hardly at all.

4.1.4 Recovered Sample Mass Diagrams

When considering the possible influences on meteorite
weathering behavior in the Antarctic environment that could
effect the interpretation of oxidometer relationships, it
was recognized that some investigation of individual sample

mass may be appropriate. Table 4.1 presents the recovered

94

 

 

 

 

 

 

 

 

 

100 l T 7 l 7 l l I T
90 - —
Class and Metamorphic Grade
0 I Ls & H4:
0 80 c A H53 6: H58 -‘
x 70 — '—" -
A
'—
3 60
3 ' l-
0'
O
8 50 - ._. ~——-4 —
Ia
+ A I J
40 F I—-—--I I a
ELJ I 4M
9, 30 — +——- ~
\ fi—q
E 20 , .
O
|-—I
10 2 ~
0 l l i l l l l l l

0 10 20 30 4O 50 60 7O 80 90100

-LO|

Figure 4.11. Taxonomic group and petrologic type.

95

mass of each meteorite (data from SCES publication No. 28).
Figure 4.12 is a scatter plot of sample mass (abscissa)
versus the modal index (ordinate). Notice that smaller
samples tend to be (1) more weathered, and (2) more diverse
in their weathering intensity, than larger samples. In
Figure 4.13, which plots the 1990 negative loss on ignition
data against sample mass, we find the —LOI oxidometer
reflects a similar "mass versus weathering” relationship,
where the most weathered samples are again the least massive
samples, and the smallest samples again show the greatest
variability in the ybaxis. However, if we plot the 1992
data for -LOI against the sample mass (Figure 4.14), the
scatter widens and is shifted in a positive x direction for

larger mass specimens.

Figure 4.15 shows the Figure 4.4 graph again, but with the
sample points divided into two groups; those with recovered
masses greater than (>), and less than (<) the arbitrarily
selected value of two kilograms. Note that with the
exception of a single point, the two groups are identical to
those of Figure 4.5. We can now observe that the H4
chondrites, in addition to being among the most fractured,
are also the heaviest chondrites, together with the two L

chondrites.

Figure 4.16 is the scatter plot for 1990 data showing, in

addition, the individual recovered sample masses in grams.

96

 

 

 

 

‘00 I r r

90 - -
O
O
- 80 )- _
X
”mi. 70 - ° 1
“3’
0' 60 " ""
O
0.
° 50 L‘ ..
+ ..
a 40 l‘. o. . "
8 .0 0
\ 30 L O -l
—1 0
CL 0 . .
O 20)- , ..

10 *- . 2

0 J l l
0 5000 10000 15000 20000

Recovered sample mass (9)

Figure 4.12. Recovered sample mass verses
modal index.

1990 —LO| results

100

90

80

70

60

50

40

30

20

10

 

 

 

 

7 l l
r 1 —
l l J
0 5000 10000 15000

Recovered sample mass (9)

20000

Figure 4.13. Recovered sample mass verses

gravimetric index.

1992 -L0| results

100

90

80

70

60

50

40

30

20

10

98

 

 

1 l

 

 

0

5000 10000

15000

Recovered sample mass (9)

1992 gravimetric index.

20000

Figure 4.14. Recovered sample mass verses

1992 —LOI Index Results

 

 

 

 

100 I I T I I I T T I

90 ~ .< —

80 ”' o< _.

>. o<

70 — .

60 '- >~> _,

50 b i... o .< '4
>0 >. < °<

40 - .< < °< —

30 L .4
o<

20 — a

10 + . -

0 l l l I l l l I L

O 10 20 30 40 50 60 70 80 90100
1990 -LOI Index Results

Figure 4.15. Reproducibility graph indicating
all samples with a recovered mass in excess of
2 kilograms (>), and those under 2 kilograms (<).

OPL/(OPL + opaquesT) x 100

100
9°
80
70
60
50
4o
30
20
10

0

100

 

 

I I I I I I f I I
'- "l
1134
l— . -—
r- "'1
609 582
._ O 0 —
556
1880 . 0713
._ ° 505 _.
O
1733 153.23 . 780 ° 1208
4087 ° 0 2230 .
I" 788 —
8
3835 o 5878
r , -
6494
— 2473 -
l I J l l I l J l

 

 

0 10 20 30 40 50 60 70 80 90100

1990 -LOI

Figure 4.16. 1990 —LOI data points with
recovered sample masses indicated in grams.

101

The samples separate into two vertically overlapping groups
(segregated above and below the value of 2 kilograms),
represented by the ellipses in Figure 4.17. When the 1992
data are added to the diagram, in the class- and type-
specific manner of Figure 4.11, we observe an obvious right
lateral shifting and enlarging of the > 2 kilogram ellipse,
but very little change in the < 2 kilogram ellipse, Figure

4.18.

This pattern is re-illustrated in Figure 4.19 for which the
difference in value between the two sample runs is

calculated by,

A-LOI = 1992a—1990a

where the OI subscript denotes the use of oxidation index
values (as opposed to raw loss on ignition values). .Again,
while those samples with masses below two kilograms may have
positive, negative or very small A-LOI values, those above
two kilograms all have pronounced positive shifts. Also
notice the greater average magnitude of shifts in the
positive direction than negative. This difference will be

addressed quantitatively in Section 4.2.5.2.

OPL/(OPL + opaquesT) x 100

100

90

80

7O

50

50

4O

30

20

 

 

 

 

 

0 1O 20 30 4O 50 60 70 80 90100
1990 —L0|
Figure 4.17. Mass distribution of 1990 —LO|

data points as determined by 2 kilogram cut—
off.

OPL/(OPL + opaquesT) x 100

100

90

80

70

60

50

4o

30

20

103

 

 

 

 

 

I l l I l l l l |
_ < 2 kg _
Clan and Metamorphic Grad.
I Ls 8c H45
*— A H55 8c H68” -
— 1990 —
> 2kg
-— zone .4
> 2 kg
1 l l l l l I l

 

 

 

0 1O 20 30 4O 50 60 7O 80 90100
—LO|

Figure 4.18. Addition of the 1992 -LO| data

causes a shifting and expansion of the > 2kg
cluster, but produces very little change in the
< 2kg cluster due to erratic shifting behavior.

1992 — 1990 -LO| Index Results

104

40

 

 

 

 

 

 

 

I 1 *fi l
30 ~ 5 _
20 — E _
_20 1 E l l 1 1 L l r l 4 l . 1 .
0 4000 8000 12000 16000

Recovered Sample Mass (9)

Fig. 4.19. Delta -LO| vectors with recovered
sample mass. Dotted line marks 2 kg for
reference.

105

4.2 Interpretation of Data

4.2.1 Secondary Mineral Identification

4.2.1.1 Theoretical Secondgy Mineralogy

The formation of iron oxides results when liquid water is in
direct contact with reduced iron at non-equilibrium PTX
conditions, and in an oxidizing atmosphere. Though air
temperatures rarely rise above OW: in Antarctica,
contamination of glacial ice by salts can adjust the ice-
liquid stability curve and produce liquid water at sub-zero
temperatures (Campbell and Claridge, 1987). Also, some
zones within the ice sheet may be at the pressure-melting
point and can produce liquid water (Sugden and John, 1984).
Through experimentation with the non-Antarctic carbonaceous
chondrite.Allende, Schultz (1986b) determined Antarctic
meteorites exposed to solar radiation may have internal
temperatures (at depths up to 2.0 cm) rising as high as 5W3,
producing liquid water through insulation heating under
"normal" PTX conditions. This was on windéfree days when
air temperatures remain below OWE. IMoreover, the following
irreversible reaction, paraphrased from Buchwald and Clarke
(1989), represents the formation of akaganéite from kamacite

in the solid state within Antarctic chondrites and with

106

minimal volume changez, where chloride (molecular chlorine)

can be provided by the environment3.

30Fe° + 2Ni° + 470 + 4H+ + 2C12

kamacite

 

> 2[Fe,,Ni][On(OH)20]C12(OH)

akaganéite

irreversible

The authors attribute the apparent ease with which chloride
is sequestered to (l) the high ionic mobility of chlorine,
(2) the electrochemical nature of the reaction and (3) the
size of the chlorine ion. However, under prolonged exposure
to these conditions, and with increasing relative humidity
and temperature, chlorine ions may remobilize through site
exchange with OH‘ ions. The remaining metastable phase may
then decompose to maghemite and goethite according to the

irreversible reaction;

 

invalid:

2[FC15 Ni][012 (0H )20](OH )3 >
alt-pat:

77-13820: +NiO +16a-FCOOH +NiO +15H20
nub-it Indie

The authors admit that the proportions of maghemite and
goethite products may vary widely and will not necessarily

adhere to the above example.

 

2 Buchwald (1989) noted “...the structural relationships of the meteoritic minerals are preserved in the
imfidsmgsahhumnmmmammk”mfl“mflwpmamnmwbemausanmhqmnmmmxas
TRSMfimyflmongmflmnwmm?

3If true, the introduction ofopen system chlorine should be noted as possibly significant in estimating
terrestrial exposure ages for Antarctic meteorites using oosmic-ray—produced 36Cl (e.g., Nishiizmni et al.
1989).

107

Scanning electron microscopy (SEM analysis) was performed on
Carichic and three Antarctic samples in an effort to
identify secondary oxide minerals. However, this proved
unsuccessful due to the paucity of void spaces into which
such minerals could grow unconfined by surrounding material

(see Section 4.2.2.1; assumption 2).

Also, the oxide mineralogy is difficult to determine in thin
section due largely to the fine-grained, cryptocrystalline
habit of the few secondary minerals that display

birefringence.

4.2.1.2 X-ray dl'fi'action
Carichic

The Carichic diffractograms A, B, C, D, E, O, and P indicate
the presence of the expected primary silicate minerals
olivine, hypersthene, and plagioclase, as well as goethite
and possibly akaganéite and maghemite. The diagnostic 4.18
A goethite peak can be found in all but diffractogram P, in
addition to other goethite peaks. Three akaganéite peaks
can be found in both diffractograms D and E, which were run
following citrate dithionite chelation treatment. The

akaganéite peaks are well defined and separated from other

108

peaks. The largest peak of diffractograms D and E is an
unknown at 3.11 A. This peak is anomalous and probably
represents a synthetic precipitation, possibly jarosite
(KFe3(804)2(OH)6), produced as a result of the citrate
dithionite process. A weak peak at 5.10 A in diffractogram
D is consistent with the presence of jarosite. Maghemite
peaks are present in diffractogram A, C, D, and P at
approximately 2.089 A. Evidence for the primary iron—
titanium oxide ilmenite is present in diffractogram C with

peaks at 3.73 A, 2.76 A and 2.25 A.

ALHA84075,7

Diffractograms F, G and H were produced for.ALHA84075,7.
Plagioclase, olivine, and hypersthene were, of course,
identified in each diffractogram as primary silicates. Most
diagnostic peaks for lepidocrocite (2.76 A and 1.495 A)
overlap olivine peaks, and at similar relative intensities,
so it is impossible to say with certainty that lepidocrocite
is present. There is a peak at 6.33 A that could be
lepidocrocite, but it is very weak. Also, this peak could
be the 6.35 A maghemite peak. Other possible lepidocrocite
peaks are weak but discernible, but the most intense

candidates are probably olivine reflections.

109

As most maghemite peaks could be confused with olivine,
magnetite or ilmenite, including the most intense (100
percent) peak, the peak considered to be most diagnostic for
maghemite is the 2.089 A peak, which was found in all three

diffractograms. The 1.474 A maghemite peak is absent.

Five possible goethite peaks are visible in diffractogram F.
Diffractogram H is very rough and the peaks have low
amplitudes due to the fine particle size of the oriented
powder mount. Many peaks are difficult to distinguish from
the background. The primary silicate peaks in diffractogram

G are well pronounced.

ALHA77271,27

Diffractograms I, J, K, and L were run for ALHA77271,27.
Too much noise exists in diffractogram K for the positive
detection of any oxides. However the 4.17-4.18 A.goethite
peak is present in each of the remaining three, and there
are suggestions of lepidocrocite in diffractogram I, but
again, these would be difficult to differentiate from those
of olivine. Primary phases identified include olivine,
plagioclase and hypersthene. High background noise coupled
with the small size fraction results in few and indistinct

peaks in diffractogram L.

110

ALHA77182,21

Only two diffractograms, M and N, yielded usable patterns
for sample ALHA77182,21. Peak intensities are low in
diffractogram M, due to the 2K CPS setting. The 2.086 A
maghemite peak is present. The 7.30 A and 5.26.A akaganéite
peaks rise just above background, as does the 4.18.A
goethite peak. Diffractogram N is only slightly better and
includes a peak at 7.31 A.that seems to represent the
leveling of the background noise more than the reflection of
X-rays. This may also be the case for the 7.30 A.peak in
diffractogram M. The 4.17 A.and 1.720 A goethite peaks are
sharp and easily distinguished from the background, The

2.083 A maghemite peak is also easily discernible.

On the basis of the theoretical evidence together with the
XRD data, it is proposed the secondary oxide mineralogy of
the study samples consists primarily of goethite, with
possible lesser amounts of maghemite, akaganéite and/or

lepidocrocite.

111

4.2.2 Modal Analysis Ternary Diagrams and the OPL-based Index

4.2.2.1 Assumptions

Two OPL-specific assumptions and one NNL-specific assumption
are made regarding the interpretation of modal analytical

results:

(1) It is assumed that the majority of OPL is in-situ
altered opaque material. This is based on the observation
that OPL is almost invariably found in intimate association
with opaque phases, and fills what appear petrographically
to be former opaque boundary outlines. OPL apparently forms
from constituents less mobile than those forming NNL and
other rusts, suggesting that it forms pseudomorphically

after metal and perhaps sulfide grains.

(2) In considering the pseudomorphic replacement of opaque
phases, it is also assumed that no significant volume change
has occurred during OPL production, meaning the volume ratio

/V

react)

of product to reactant (V; is close to unity. This

rod
assumption of isovolumetric replacement follows from the
general lack of void spaces into which secondary minerals
may grow. Ordinary chondrites are dense rocks with few pre-
terrestrial void spaces, although some shock-process-formed

vugs exist (Olsen, 1981). .A few small vugs (or "plucked

zones" possibly formed during mounting operations) have been

112

found petrographically among the subject samples. These
zones have become partially filled in many cases with BVF
material (refer to Section 3.1), indicating that some net
volume increase is taking place (Figure 3.15); but these are
not common, and have not been found in all specimens. It
was this lack of abundant pore space that rendered the SEM
analysis ineffective in detecting identifiable secondary

minerals (Section 4.2.1.1).

With little or no void space for OPL products to grow into,
and in the absence of any wholesale, volume—reducing
silicate weathering, for which there is yet no direct
evidence, OPL replacement of reduced metal primary phases is
probably isovolumetric. Non-isovolumetric metal alteration
could occur only if fractures were enlarged, or new
fractures created to expose unweathered surfaces to the
environment. The work of Buchwald and Clarke (1989),
focusing on akaganéite production in Antarctic ordinary
chondrites, suggested that replacement of kamacite with this

mineral occurred isovolumetrically.

(3) NNL stain is negligible. Extensive staining on
terrestrial Antarctic rocks gives them "...a rich brown
color and makes [sic] them appear considerably more
weathered than they actually are [sic]," (Campbell and
Claridge, 1987). NNL stains are essentially two-

dimensional, and should not contribute significantly to the

113

volume of secondary products produced (see Section 4.1.3).
It is quite possible that greater or lesser amounts of NNL
stain or "varnish" are responsible for inappropriate

estimations of Antarctic meteorite weathering intensities.

4.2.2.2 ABCe In_dex Inadequacies

If it is allowed that oxide material visible in thin section
(modal abundance values) is identical in extent to that
observed in hand specimen (ABC designation), then the
inadequacy of the ABCe oxidometer is evident in Figures 4.1
and 4.2. Ideally, all meteorites stained to C intensities
should plot within (or close to) the lower fifth region
indicated in Figure 4.2, and they fail to do so. In all
probability, however, the total oxide values represented in
the modal analysis are somewhat higher than those observable
in hand specimen because of enhancement of limonite features
in transmitted light under magnification (versus reflected
light in hand specimen). Hence, a corresponding shift
toward the "unstained" end-member in the ternary diagram is
probably appropriate when comparing these data to the ABCe
system. Such a shift would produce a still greater number
of data points to be found in "non-C" regions of the
diagram, rendering the ABCe system even less suitable as an
indicator of weathering intensities, unless one wished to
increase the number of categories (e.g., extend ABC to

include D, E and F and redistribute accordingly).

114

4.2.2.3 Residual Metal Grains and Pressure Equilibrium

While some variability in opaque components is present, no
trend in the destruction of opaque material is obvious in
Figures 4.1 and 4.2. Indeed, the linear pattern appears to
be nearly perpendicular to the opaque end-member. One may
infer that original opaque abundances have not been
significantly lessened by alteration. It would therefore
appear that only a minor volume of reduced metal is required
to produce copious amounts of NNL stain. This may also
indicate that weathering intensity (as represented by iron
oxidation) is as much or more a funCtion of the availability
of liquid water and the mobility of iron, within a given
volume of rock, than of actual exposure duration to
weathering conditions. If residence time were an important
oxidation factor, then one might expect to see a greater
range in residual metal grain abundances (even among samples
of a common weathering category), the stones having been
exposed individually to terrestrial weathering conditions
for lengths of time that may differ by several orders of
magnitude (2 [ilj x 103 to 950 Li1001leO3 y.b.p. for Allan
Hills (Nishiizumi et al., 1989))4. More significantly, the

persistence of metal grains within highly weathered ordinary

 

4This assumes that weathering continues for meteorites entrained within the ice sheet as well as for those
mflmhxsmflne“mkhMBnmymbamsmammmulIHnMdwdHnflwnwmhumgmammmume
place only while the stone is exposed to the atmosphere and sunlight Ifthe latter is true, and an
immmmmnambemukflmhumkmxdmmmauflwkxsmfiammufammmdmnugmfimkmnmum
IbmdmnumflmnemmnmgmfinanAmunficmaummsJMmenmamewemmmmgammMmmnMMd

lxommnmbk.

115

chondrites may indicate a metastable cessation of the
oxidation process prior to complete destruction of the
parent material. As will be explained in the following
sections, this latter hypothesis is supported by modal data
and observations of the Carichic meteorite, and is
consistent with interpretations of negative loss an ignition

data.

In Figure 4.3, each meteorite has left its origin on the
ternary boarder perpendicular to the “OPL” end—member and
migrated some distance toward the 100 percent-weathered
boarder (perpendicular to the “Opaques” end-member). A
linear pattern has resulted which, if north is toward the
top of the page, runs NNE to SSW. The possible data point
migration scenarios on the Figure 4.3 ternary plot will now
be examined to arrive at the most likely interpretation of

this linear pattern.

Figure 4.20 summarizes the four feasible components of
movement, where each arrow represents a direction of
migration, for data points within the conteXt of this
analysis. Arrow A.indicates the limit in left- to right-
hand movement. No data point can cross, from left to right,
a line that parallels arrow A because to do so would mean
that reduced metal abundances had increased; an
impossibility. .Arrow B represents the case where opaque

minerals alter pseudomorphically to OPL products with zero

116

non—OPL
+
Unstained Silicates

L chondrites (14.4%)

'J
“g H chondrites (24.7%)

 

OPL Opaques

Figure 4.20. Possible scenarios of movement

for data points on Figure 4.3. Positions for
average opaque modal abundances are indicated
for unweathered L and H chondrites (data from

Dodd, 1981).

117

volume change. Any NNL stain produced during this process
is not represented on the graph due to its absorption within
the "non-OPL + unstained silicates" category, which remains
constant. Arrow C represents alteration directly to non-OPL
rusts, and arrow D suggests the possibility that OPL
products may themselves be unstable and weather to non-OPL
limonites. Obviously, OPL values must first increase before
they can hypothetically decrease by such a tertiary

alteration.

The linear data point trend in Figures 4.3 and 4.20 is not
parallel to any ternary boarder, and therefore cannot be
attributed to a simple variability in any single pair of
components. Given that crossing a line parallel to Arrow A
in Figure 4.20 is prohibited, the linearity also cannot
indicate a preferred weathering pattern in a direction from
near the “non-OPL + unstained silicates” end-member downward
toward the ternary boarder perpendicular to the “non-OPL
plus unstained silicates” end-member (NNE to SSW direction).
Neither is it likely that the samples have moved along the
trend in the opposite direction, imitating an "arrow D"
route; no petrographic evidence was found to suggest that
OPL has been altering to other types of rust. The most
likely path of migration for each data point is along a line
intermediate between arrows A and B (A') because of OPL
expansion near rock surfaces and fractures (refer to Figures

3.8, 3.9, 3.10, and 3.11 for some examples). Such an “A!”

118

path is supported by the petrographic evidence given on the
replacement of opaque minerals with OPL product (Section
3.1), and could represent a dilatation of the rock as a
whole. Thus, on a ternary diagram, where volume is
conservative, an “A!” path is the projection onto two
dimensions of a three-dimensional tetrahedron where the
fourth end-member is “volume change”. Hence, apparent
opaque volumes may be represented in a ternary diagram such
as Figure 4.20 at less than their original value based on
dilatation alone. Such a factor becomes important in the

remaining discussion of this Section.

Empirically determined modal abundances of opaque minerals
in non-Antarctic H and L ordinary chondrites have been
gathered from the literature and are given in Table 3.2.

The numbers are comparable, though slightly lower in some
cases, to the point counted sum totals of OPL and opaque
grain abundances in this modal analysis (refer to Table
3.1). Values from Dodd, 1981 for average modal abundances
of opaques in L and H ordinary chondrites are indicated in
both Table 3.2 and Figure 4.20. The fact Of slightly larger
“OPL plus opaques” values is again attributable to the

expansion of product near rock surfaces or fractures.

It must be emphasized that general patterns based on volume
percentages and reflected by ternary diagrams do not

differentiate chemical alteration processes, but reflect

.119

only the summations of static features recognizable in thin
section. However, by effectively isolating the reactant
(opaques) and product (OPL) minerals of an alteration
sequence, some clues on the behavior of OPL reactions might
be detectable. With this in mind, it is felt the trend in
Figure 4.3 may indicate a temporary state of pressure
equilibrium for OPL production in the Antarctic environment.
The diagonal linear pattern seems to represent a zone of
stabilization beyond which the meteorites have not
progressed, and the positions along which seems to be
determined by each meteorite's initial opaque mineral
abundance; that is, the greater the original opaque mineral
abundance, the greater the OPL mineral abundance. A.direct
relationship between metal abundances and OPL abundances is
predicted by the observation that (l) a greater surface area
for metal exists in meteorites for which there is a higher
initial modal abundance of this material, and provided that
(2) OPL replacement occurs to similar depths for similar
time intervals within individual metal grains in different

stones.

Table 4.2 is a look at mineral volume changes based on
published unit cell dimensions, and with the assumption that
kamacite and taenite are pure iron minerals and not iron-
nickel alloys. One can see that for goethite, the most
likely common secondary product based on XRD results

(Section 4.2.1.2), the volume increase is more than three

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121

fold, and even greater for some other possible secondary
minerals. It would appear that the growth of goethite at
the expense of reduced metal phases must eventually exert a
pressure on the surrounding groundmass, possibly forcing
iron oxide-bearing fluids into all available fractures,
cleavage planes and grain interstices, being thus
responsible for the pervasive nature of the NNL staining.
Clearly at some point, either (1) the rock must break,
similar to periglacial frost wedging (Grawe, 1936), or (2)
the growth process ceases. The linear pattern in Figure 4.3
could therefore represent a state of pressure equilibrium
within each meteorite interior where the growth stress of
the growing minerals are in balance with the tensile
strength of the rock itself. This metastable state of
affairs is adjusted each time the meteorite is broken until

a state of equilibrium can be reestablished.

The bulk samples and thin sections chosen and prepared by
NASA curatorial staff arrived in Michigan as largely
unbroken single chips and sections, respectively. This is
not surprising, given that some measure of hardness would be
a natural prerequisite to the sectionability of samples,
causing the sample suite to be somewhat biased in favor of
moderate to high rock strength thin sections. Highly
friable zones within a requested meteorite would be bypassed
by NASA curatorial staff during sample preparation in favor

of stronger lithic fragments, thereby ensuring that only

122

rock volumes likely to have been predisposed to sustainable
pressure equilibrium conditions will be represented in
Figure 4.3. More friable portions of rock may, however,
have been included within the 5.0 gram bulk chip sample
suite, and may have theoretically weathered further toward
the ternary boarder perpendicular to the “opaques” end-

member of the diagram, where no modal data are found.

4.2.3 Negative Loss on Ignition and the -LOI Index

The —LOI weathering index involves the measuring of a weight
difference, which theoretically accounts for all secondary
oxides produced by weathering. However, the presence of
bound water and hydroxyl ions in the pre-ignited specimen
can have a marked influence on this difference (Jarosewich,
1993). The L3 chondrite.ALHA8lO3l apparently underwent a
true loss on ignition, presumably due to the volatilization
of bound water within its unequilibrated matrix. More
equilibrated chondrites have less bound water, but
uncontrolled amounts of moisture, bound or unbound, may
still conceivably pose a problem for the uSe of -LOI as a
weathering index. It is hoped that by cooking the samples
at 110°C before ignition, any unbound water was volatilized

and driven from the system.

The measure of —LOI data reproducibility from one sample

volume to another within a given meteorite specimen is

123

indicated by the combined index value trend in Figure 4.4.
The agreement of the data point trend with the 1:1 line is
good, suggesting that -LOI may yet be considered as an
alternative to the ABCe classification system for the
oxidation intensity of equilibrated ordinary chondrites.
This is also indicated in Figure 4.19, which shows that
1992-1990 reproducibility is within 10 percent for all but
one of the samples. However, in the absence of more
qualitative information on the nature of the reactions and
the pre-ignition chemistry (bound water and other volatile
and semi-volatile parameters) within individual meteorites,
it is difficult to say whether the oxidometer behaves
precisely as theory would predict. If we use the same
criteria for assigning the five ranges in weathering
intensity as was used in Figure 4.2 (0-20, 20-40,
etceteras), then again it can be seen the values extend
outside the zone of a single category. Notice that most of
the data points cluster within a midrange section where one
should expect to find A/B and B weathering category stones.
It would be worthwhile to pursue further the possibilities
of -LOI using a greater number of samples and with samples

of other weathering categories.

4.2.4 Experimental Oxidometer Contrast and Comparison

Figure 4.10 plots the two experimental oxidometer values

mutually. If both indices accurately represent a uniform

124

metal alteration, the points would align along a 1:1 line.
No such relationship exists in the Figure. Ordinate
deviations from the 1:1 line may be due to one or more of
the following possibilities: (1) Greater weathering of bulk
chips than of sealed thin sections while in storage; (2)
Modal analytical measurement inaccuracies (see Section 2.1.1
for technical difficulties inherent in the point counting
process); (3) Non-represented sample-specific variations in
opaque chemical and mineralogical compositions; (4)
Variations in pre-terrestrial modal abundances of metal.

The weight taken on during oxidation of reduced metal phases
Should be proportional to the total relative abundances of
these phases in each meteorite sample. However, the pre-
terrestrial total amounts of metal is not known on a per
sample basis, but is rather assumed from published averages
for ordinary chondrites based on Yates (1988). (5) Iron
oxide heterogeneity. .Adherence to the 1:1 generality
depends on how representative each chip is of the overall
rustiness of its respective meteorite. .As an aid to
evaluating this likelihood, samples that possess fusion
crusts are identified in Table 4.1. The presence of a
fusion crust identifies the sample as having been cut from
the surface of the meteorite and allows the assessment of
its being representative, or simply part of a hypothetical
weathering rind. It is not known which of the bulk samples

have fusion crusts. No patterns were found which could

125

relate the occurrence of fusion crusts in thin section to

any trends identified in the data analysis.

By the observation that most data points lie below the 1:1
line in Figure 4.9, in all likelihood the assumption of non-
OPL rust abundances being totally negligible for the purpose
of establishing an index is in error (assumption number 3 of
Section 4.2.2.1), and the ordinate values have been
underestimated by a factor proportional to individual non—
OPL rust abundances. Identifying a means to correct for
this discrepancy might raise each point an appropriate
distance closer to the 1:1 line. This is an area for
further study, for it could help to establish the
reliability of the -LOI experimental index. Possibility
number 1 of this Section, that of differential weathering of
the bulk samples in storage, is also a likelihood, and will
be further addressed in the Section on curatorial weathering

(Section 4.2.5.2).

4.2.5 Non-index-related Anomalies

Beyond the question of oxidation indices, additional
patterns can be recognized in Figure 4.4 and other figures
that may lend some insight into other aspects of Antarctic
meteorite weathering. That most of the data points in

Figure 4.4 lie above the 1:1 line may be significant. Also,

126

Figures 4.5, 4.11, 4.17, and 4.18 show how the points can be
separated into two non-overlapping groups based on taxonomic
class, petrologic type and recovered sample mass. Moreover,
Figure 4.6 illustrates that extremes in macro scale (trans-
specimen) fracture density are well separated among the data
point positions. It is the intent of the following
discussions to show that these patterns are the results of a
complex set of circumstances and processes, some of which
are perhaps unique to the Antarctic environment, that
involve the simultaneous interplay of the following four
factors at a minimum: (1) the fracture-induced metal
alteration heterogeneity effect (FIMAHE), (2) macro scale
fracture density, (3) recovered sample mass (as an
indication of size and therefore surface area), or class and
petrologic type, or both, and (4) curatorial weathering.

Each of these factors is discussed below.

4.2.5.1 Carichic Fractures and the FIMAHE

 

The Carichic thin sections suggest that, while dark oxide
staining is ubiquitous throughout the specimen, the
locations of residual metal grains are tightly constrained
by the location of fractures (refer to Figure 3.21),
rendering an aspect of metal grain heterogeneity to the
meteorite. Wright and Grady stated in 1989 the “——it is
probable that fracturing takes place along pre-existing

cracks, which may have been inundated with terrestrial

127

fluids.” The following pictorial series proposes an
alteration sequence based on the observations of the
Carichic thin sections in reflected light. This sequence
demonstrates how two samples from different locations within
highly weathered meteorite interiors can yield different
oxidometer values, and are more likely to do so, all other
factors being equal, than two samples from within a less
weathered specimen. This counterintuitive situation of
increased sample heterogeneity with increased weathering
intensity will be referred to as the fracture-induced metal
alteration heterogeneity effect (FIMAHE). Magnification is

approximately 15x in each of these figures.

128

 

 

 

 

 

 

Figure4.21. Representafionofantmweathered “fresh'ordinarychondrite sampleinthinsection. Metalgrainswlack
blebs)standoutineontrasttosurroundingmatrix,chondrulesandcrystalgrains(repesentedtogethabythewhite
backgound). Abranchingfiactme(blackline)transectsthesample.

 

 

 

 

 

 

Figrn'e 4.22. Initial stage. Incipient pseudomorphic alteration has occmred where the fracture encormters metal grains
directly(whiteoontactzonesalongfiactures),andinotherlocationsclosetofractures. Somelightstainingbas
occurred (stippled pattern).

129

 

 

Figure4.23. Intermediate stage. Widespreadstaininghasoccrnred Psardomorphic alteration of metalgrains(white)
ismostprmormcednearthefiacmres. MostweathaingcategmyCAntarcficmeteoritesarepmbablyatthisstage.

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Figure 4.24. The Carichic sample (advanced stage). The sample is highly altered Extensive dark staining
mmughommespedmmmompmiescompletempheunmtofmemlgnmsclosemfiacum. Moderatedegreesot‘
alteration can be found away from fractures, leaving OPL haloes surrounding relict metal grains.

If pressure equilibration does occur for secondary oxides

within weathered ordinary chondrites, it could account for

the persistence of heterogeneity within Carichic, and

130

presumably, other ordinary chondrites. Those zones closest
to fractures would weather more intensely and more rapidly
than zones further removed, since they are closer to (1)
available water, and (2) void spaces into which the
expanding secondary products can grow. However, this
increase in growth would eventually stabilize as the
differential forces between the growing oxide and the
surrounding rock become equal, thereby maintaining

heterogeneity.

131

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Figure 4.25. Example of the Erastus-induced metal altaation heterogeneity effect or FMAHE, whereby different

aiddamduvalwsmobmineddependingmwhaeudminmemewmimmesmpleismkm.

tensity of the FIMAHE would be

in

the

Theoretically,

controlled in general by (l) the surface area exposed to

the macro scale fracture density

and (2)

weathering agents,

Fractures permit the

within a given volume of rock.

132

migration of fluids to meteorite interiors. A.greater
fracture density would increase homogeneity and decrease the
FIMAHE. Conceivably, samples that satisfy specific criteria
for various ranges in fracture density might present a
measure of residual metal heterogeneity that would change in
a non-linear manner as the sample weathered. For example,
fractures could introduce the FIMAHE locally, increasing
heterogeneity, and with increased exposure the separate
weathered zones would coalesce, decreasing heterogeneity.

By contrast, in samples of lesser fracture density, the
FIMAHE would produce a heterogeneity that would reach a
critical limit and then stabilize due to the crystal growth
pressure and tensile strength conditions discussed in
Section 4.2.2.3, thereby preserving that magnitude of
heterogeneity. A.greater exposure of a given stone volume
to weathering agents, either by high terrestrial age or,
more probably, by a high surface area to volume ratio,
coupled with a minimal fracture density, generates a more

pronounced FIMAHE.

To summarize, the FIMAHE suggests that iron oxide
distribution is proportional to the fracture density within
a given volume of highly weathered meteoritic material. It
is possible too that FIMAHE—like circumstances exist for
other types of weathering than simple oxidation reactions.
Much further research is necessary to learn the constraints

on these effects and countereffects.

133

42Ji2Ammxmudi“haflwnngthhmans

A. Sample-specific factors

Petrologic differences may be responsible for differences in
weathering behavior. Figure 4.11 shows that (1) all Ls and
H45 show pronounced right lateral (positive) shifting, and
(2) the behavior of H55 and H65 can be described as
inconsistent or erratic. Similarly, the pattern produced by
petrologic differences in Figure 4.5 is nearly identical to
that produced by mass differences in Figure 4.15. The
obvious temptation is to conclude a correlation exists
somehow between recovered sample mass and petrologic type
and meteorite class. However, no control is given over the
original sample mass of the fresh falls, as opposed to the
recovered sample mass measured after some 0.1 to 1.0 Ma
within the ice sheet. It is therefore impossible from the
evidence at hand to describe the order of the causal
sequence; that is, whether (1) an original small mass is
responsible for higher weathering (by greater surface area
to volume ratios), or (2) greater weathering susceptibility
to certain petrologic types generates smaller samples
through disintegration as the tensile strength of the rock

is overcome by internal crystal growth pressures.

134

The circumstance of nearly perfect superimposition of the
dichotomous pattern in Figure 4.5 with that of Figure 4.15
therefore has at least one of four explanations: (A)
Differences in weathering susceptibility due to class and/or
petrologic type, which facilitate the breakdown of
particular stones into smaller fragments. It may be useful
to consider that certain volumes within a given stone may be
well protected from attack irrespective of any FIMAHEs,
possibly differing among petrologic types because of
differences in textural integration; (B) Differences in
weathering susceptibility due to initial differences in mass
because of surface area to volume ratios; (C) coincidental
selection for samples with mass and petrologic type
difference correlations, as opposed to any intrinsic
differences in weathering susceptibility; and (D)
mass/class/type correlations may be based in pre-terrestrial
compositional properties and thus independent of weathering

effects.

lmsmm l'c ‘wa

It is recognized that 19 specimens are too few to constitute
a statistical sample of the Antarctic ordinary chondrite
population. It was therefore decided to investigate the
remaining population to determine if the trends discussed
have any relevance on a larger scale where mass/class/type

relationships are concerned.

135

Figure 4.26 is a logarithmic plot of the percent frequency
distribution of 2066 unpaired H and L Antarctic ordinary
chondrites representing each type of meteorite studied in
this thesis. .All Antarctic meteorites collected by the
United States field teams through the 1986 field season are
included with the exception of stones with masses in eXcess
of 10 kilograms (source: SCES publications). The symbol
locations represent bins defined in logarithmic space, and
are not discrete data points. It can be seen that a greater
proportion of stones in the l to 10 kilogram range is found
among the H4 and L6 categories than in any other category.
This is consistent with the data point positions in Figure
4.18. Harvey and Cassidy (1986) discuss the relationship of
mass frequency distribution curves for Antarctic Allan Hills
Main Icefield find to modern falls and conclude: (l) "The
most startling difference between modern falls and Antarctic
finds is the observed overabundance of H's in Allan Hills
Main Icefield finds.," and (2) "...the H chondrites show a
curve strongly skewed toward the smaller sizes." The
researchers feel these differences are intrinsic to the pre-
terrestrial meteoroid population sampled at the time these
meteorites fell. In considering the weathering effects on
the frequency distribution of terrestrial age estimates,
Nishiizumi et al., (1989) state that "The weathering
hypothesis is contradicted by the greater abundance of H

chondrites in Antarctic meteorite collections compared to

Percent frequency

40

35

30

25

20

15

10

0

136

 

 

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—1.0—0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

log (massg)

Figure 4.26. Mass frequency distributions among
Antarctic ordinary chondrites. Symbol locations
represent bins defined in logarithmic space,
and are not discrete data points.

137

those found as recent falls in non-Antarctic areas." The
"contradicted" weathering hypothesis in this case was the
suggestion that weathering may disrupt the concentrations of
cosmogenic radionuclides to skew the terrestrial age

frequency distribution toward recent years.

However, the two Harvey and Cassidy (1986) observations
taken together may suggest preferred H chondrite
fragmentation (explanation (A) above) as responsible for the
differences, thus indicating that physical as well as
chemical weathering mechanisms could be operating within
Antarctic meteorites. Huss (1991) felt that such meteorite
fragmentation was caused by freeze-thaw effects in
Antarctica. However, the body of this thesis suggests that
H chondrites may be more susceptible to fragmentation than L
and LL chondrites because they have, by definition, a
greater abundance of reduced iron. More reduced iron might
allow, during weathering, the required critical amounts of
internal crystal growth pressures per unit volume necessary
to overcome the tensile strength of the rocks and cause them
to break apart. The means to verify this idea would be at
hand if a database of all specimens with fusion crusts, and
percentages covered thereof, were available. If true, a
second theory would be needed to explain why some H45 appear

to behave similarly to Ls.

138

By this reasoning, one might also expect the H population of
Antarctic meteorites to yield a higher percentage of highly
weathered samples than the L population. .Again referring to
the SCES publications, out of 644 H chondrites, 21.27
percent belong to weathering category C; whereas the L5 have
only a 18.35 percent C weathering abundance out of 327
samples. It was noticed while making this count that 92
(61.67 percent) of all weathering category C L chondrites
are L3 stones. Only 6 (0.9 percent) of the H chondrites are
of petrologic type 3. Allowing that UOCs may be more
susceptible to weathering for as yet unknown reasons, we
might try subtracting these from our database to determine
the relative susceptibility of EOCs to oxidation in the
Antarctic environment. Subtracting all petrologic type 3
meteorites from both the H and L classes yields totals of
638 and 235 for H5 and Ls, respectively; and weathering
category C stone abundances of 21.47 and 9.79 percent,
respectively. Based on these data, it would appear that H
chondrites tend to weather to C intensities more readily
than L chondrites in Antarctica. This is consistent with
the above supposition of greater fragmentatiOn among H
chondrites over L chondrites due to their greater metal

grain content.

Although H4s are also among the most fractured specimens
within the study sample suite, no trend in the occurrence of

fracturing among Antarctic ordinary chondrites was found to

139

exist between any group, class or petrologic type in the
SCES publications. The preponderance of the C category
fracturing index among the H4 samples is therefore
considered to be coincidental.

The occurrence of fragmentation would render explanation
(B), that of weathering susceptibility because of sample
size, as a possible secondary mechanism by which these
differences could be intensified and accelerated.

Explanation (B) is examined below.

(3) Massiepgndent weathering

Figure 4.27 again separates the samples into groups above
and below two kilograms, but uses the ternary format of
Figure 4.3. The distribution of sample weights among the
data points reemphasizes the role of sample mass in
weathering behavior. The distribution can be explained
easily using the theory of pressure equilibration outlined
in Section 4.2.2.3, and dramatizes the implications of
Figures 4.12 and 4.13 —— that smaller stones are more
weathered than larger stones. The width of the linear trend
in Figure 4.3 indicates the range in equilibrium states
represented by the study sample suite. Theoretically, if a
meteorite on the right-hand side of the cluster was in
pressure equilibrium, and then exposed to additional
weathering forces, it may break or its existing fractures

may widen. The data point position for this stone on the

140

non-OPL
+
Unstained Silicotes

 

 

 

 

 

 

OPL Opaques

Fig. 4.27. Recovered mass distribution.

141

ternary would then be shifted further to the left as more
OPL was formed at the expense of opaques. Therefore, the
least weathered meteorites should plot in the "north—
northeast" quadrant of the cluster. Samples that plot
further "south—southwest" along the diagonal trend would
necessarily be more weathered since, being further "west"
along the trend, they are a greater fraction of the distance
along from the ternary boarder perpendicular to the “OPL”
end-member (0 percent weathered) to the boarder
perpendicular to the “opaques” end-member (100 percent

weathered).

Figure 4.27 shows the "north-northeast" quadrant to be
precisely where the largest meteorites plot on the diagram.
These theories allow less weathered interiors of large
Antarctic meteorites to be in pressure equilibrium while at
the same time allowing small, more weathered interiors to be

in pressure equilibrium.

ILChnmnhhmuMRfimg

Both Figures 4.12 and 4.13, each by an independent
experimental oxidometer, display trends that show smaller
stones tend to include the most weathered meteorites. Both
trends also indicate that smaller stones have a more diverse

range in weathering intensity than larger stones. This is

142

true for —LOI results obtained shortly after the samples

were removed from their airtight packages.

However, Figure 4.14 shows that 16 months after the
meteorites were first exposed to the South Central Lower
Michigan climate, the smaller meteorites still show the
greatest diversity and weathering intensity, but the splits
from originally larger H4 and L parent samples have
increased their weathering intensity, and the trend in
Figure 4.13 is shifted in the positive direction. These
diagrams indicate meteorite weathering during curatorial
storage in Michigan. Considering the following curatorial
history, it is not surprising to find evidence of weathering
in storage: .According to the SCES publications, each sample
has been (1) removed from the Antarctic environment (of
proposed metastable pressure equilibrium) preserved in
Teflon sample bags sealed with tape and enclosed within
padded metal boxes, all of which has been cleaned "to the
same specifications used in processing lunar samples," at
temperatures below 0%: to the Curatorial Facility of the
NASA Johnson Space Center, Lunar Curator's Division, (2)
sealed within a cold storage room at a temperature of -40%3
and (3) cut or chipped in a nitrogen cabinet. The samples
were then (4) sealed within airtight packaging, (5)
relocated to a temperate mid-latitude climate, (6) removed
from their packages, (7) broken to obtain 1990 —LOI samples,

and (8) allowed to re—equilibrate for 16 months in non-

143

airtight containers with broken seals. It may be

significant that two of the samples with the least amount of
curatorial -LOI shifting were sealed in metal capsules with
tight fitting lids while all others were in plastic capsules

with somewhat loose fitting lids (see Table 4.1).

Comparison of Figure 4.16 with 4.17 suggests that because
the Ls and H4 stones are larger and have less weathered
interiors, they weather curatorially in a more consistent
way than the smaller stones. If curatorial weathering has
taken place, then an explanation is required for why it has
apparently affected the larger samples more than the smaller
ones. Such an explanation is implicit in (l) the FIMAHE and
(2) differences in weathering susceptibility due to

freshness; both discussed below.

(1) Recall from Figures 4.12 and 4.13 that the smaller
samples tend to be more weathered than larger samples, using
an arbitrary cut-off of 2 kilograms. It follows that the
influence of the FIMAHE may prevail over any curatorial
weathering effects among samples where the heterogeneity
effect is magnified by intense weathering (as described in
Section 4.2.5.1). The erratic behavior in Figures 4.16 and
4.18 among smaller stones would then be produced by
selecting samples from within zones of differing oxidation
intensity, as in Figure 4.24. Note in Figure 4.18 that the

four samples with the highest left lateral shifts have

144

masses less than 800 grams. It appears that the lower the
sample mass, the greater the chance for producing a left
lateral shift. Left lateral shifting seems to depend less
on laboratory weathering than on residual metal grain
distribution, which is proportional to the FIMAHE. Whether
a left or right lateral shift occurs for a low mass
meteorite in this study therefore depends on the intensity
of the FIMAHE and on the temporal sequence of the two

analyses.

Different zones within a larger, "fresher" meteorite would
be much more alike unless acted upon by differential
curatorial weathering forces. Therefore, large samples are
more likely to produce right lateral shifts than smaller

samples.

(2) Initially less weathered meteorites would also be more
susceptible to curatorial weathering than more weathered
stones due to their freshness. As secondary weathering

/V

react

products are produced for which V

mmd is greater than

unity, it is possible the layer of product becomes
"protective," meaning the rate-determining mechanism is
diffusion of mobile reactant and/or product elements through
the layer itself (e.g., Velbel, 1993). .As protective layers
of oxide build up across metal grain surfaces, reaction
rates could therefore be expected to decrease. Also, the

filling of fractures, cleavage planes and grain interstices

145

with product could conceivably function to lower the
effective permeability of the rock. Therefore, the less
weathered a given chondrite is initially, the more it should

respond to curatorial weathering forces in storage.

One should be able to notice a preferred positive trend
among the A—LOI data if curatorial weathering forces are
present, since these would increase the weathering intensity
across the board and so be superimposed upon all previous
weathering effects. Such a trend would be recognizable as a
greater average magnitude for A-LOI values in the positive
direction than the average magnitude of the negative shifts,
which theoretically result from the influence of the FIMAHE
or other factors. Figure 4.19 illustrates the A-LOI vector
by direction and magnitude between the two sample runs and
presents the 2 kilogram cut-off. Notice how negative A-LOI

values decrease in magnitude as sample mass increases.

Table 4.3 performs some statistical analysis to establish a
qualified mean based on the elimination of outliers (where
coefficient of variance values greater than 0.5 indicate an

outlier). The variance is given by;

2

 

 

146

Table 4.3. Determination of delta -LOI means

 

 

Negative Positive
Outlier Valid Outlier Valid
2.50 7.72
0.47 10.23
9.40 10.71
6.66 11.19
0.71 1 1.91
9.41 4.29
9.16 10.59
32.14
6.55
9.29
1 .31
m.” 5.471 E51:—
Qualified mean 7.43 8.38
Variance 7.14 10.54
Standudduvlation 2.67 3.25
Coeflieient of m 0.36 0.39

 

 

147

where Xj is a sample, ; is the mean, and n is the number of

samples. The standard deviation is then given by;

sdzJC,

and the coefficient of variance by;

CV: 2.
x
Arithmetic mean values are greater by 92.7 percent in the
positive direction than the negative direction and the
qualified mean values are 12.8 percent greater. It can be
observed that even by these conservative statistical
criteria, -LOI values favor an increase over the 16 month
storage period between analysis, suggesting that a
significant amount of oxidation has occurred during this
period. .At a minimum, it is advisable to take precautions
against the possibility of curatorial weathering phenomena
by maintaining an airtight and moisture deficient atmosphere
for samples to be stored for long time periods. In
addressing the long known curatorial oxidation effects with
regard to nickel—iron meteorites, Buchwald and Clarke (1989)
point out that "Understanding this corrosion process has
obvious implications for developing strategies to terminate,
or at least minimize deterioration that continues when these
scientifically important materials are incorporated into

research and exhibit collections."

5 C NCLUSIONS

Petrographic modal analyses, designed to characterize the
overall appearances of "rustiness" which lead to
meteorite classification within the ABCe system, show a
wide variation within the assigned weathering category
(C) for Antarctic ordinary chondrites. This lack of
consistency reflects incongruous proportions of OPL and
NNL materials among different specimens (and even within
different areas of the same specimen). These lead to
classifications that are non—representative of actual
alteration intensities, but instead of the extent of low—
volume stain (NNL) materials. Thus, the ABCe system
presently employed is heavily biased by the thin,
volumetrically insignificant but visually dominant NNL
stains. Such wide variability observed within an
established weathering category is significant and calls

for a restructuring of the index.

. An experimental modal index, designed to represent the

volume of iron oxide produced through weathering, does

not correlate with the experimental index based on H- and

148

149

L-corrected negative losses on ignition. However, good
reproducibility of the -LOI data suggest that the -LOI
method might serve as an inexpensive and complimentary
alternative to the ABCe system for the iron oxide-based
classification of weathering intensity in highly
weathered Antarctic equilibrated ordinary chondrites.
Both experimental indices also show a substantial
variability which extends beyond that interpreted for a

single weathering category.

Small-scale metal grain and iron oxide heterogeneities
exist, particularly among severely weathered ordinary
chondrites, which can inhibit the reproducibility of

oxidometer values.

The distribution of relict metal grains is determined to

a large degree by the fracture distribution.

Iron oxide production continues within samples while in
storage in temperate climates with high average relative
humidities and temperatures. It appears that this
curatorial weathering may be mitigated or prevented by

maintaining a tight seal on samples while in storage.

Iron oxide production can be arrested by the tensile

strength of the surrounding rock mass.

150

.In the dynamic environment of the Antarctic ice sheet,
where meteorites have resided for long period of time, H
chondrites appear to be more susceptible to
fragmentation than L chondrites. This appears to be due
to their higher reduced metal content, which produces
internal pressures sufficiently high to weaken the rocks
along existing fractures during iron oxide production,

thus overcoming the tensile strength of the rock.

Consistent with conclusion number 7, the H chondrite
population was also found to contain a higher relative
abundance of highly weathered meteorites than does the L

chondrite population in Antarctica.

H4 chondrites tend to occur as larger stones, than H5 and

H6 chondrites.

APPENDIX A

PETROGRAPHIC DESCRIPTION OF SAMPLES

TAPPEHUDEXJA

PETROGRAPHIC DESCRIPTION OF SAMPLES

Introduction

 

The following is a set of petrographic descriptions of the
19 NASA.Antarctic meteorite samples and the non-Antarctic
meteorite Carichic. The descriptions have been generated
from the examination of 38 highly polished microprobe mounts
and two Carichic thin sections. For descriptive purposes
only, an arbitrary texture scale was created where “fine—
grained” is s 2.5 um; “medium-grained” is > 2.5 and s 25

um; and “coarse—grained” is > 25 and s 250 um in average

diameter.

The microprobe mounts were prepared at the Johnson Space
Center in Houston, Texas by the NASA.Antarctic Meteorite
Working Group. However, for the purpose of this research,
they were used as thin sections for the petrographic
characterization of iron oxide type and distribution, and
for photomicrographic documentation of notable features.
The microprobe mounts were not necessarily cut to 30pm

standard thin section thicknesses. Birefringence colors may

151

152

therefore deviate from those commonly associated with

observed minerals.

Additional chips cut from different areas on the 19
Antarctic specimens were examined petrographically at the
Smithsonian Institution's National Museum of Natural
History, Division of Meteorites in Washington D.C. The
Carichic thin sections were prepared at the Department of
Geological Sciences at Michigan State University in East

Lansing, Michigan.

Virtually all features mentioned in these descriptions have
been photodocumented, with the exception of those found in
the Smithsonian Institution samples. Attempts were made to
photograph these latter chips, but were unsuccessful due to
a camera malfunction. The catalogue of photomicrograph

transparencies can be referenced for additional descriptive

details (see Appendix B).

153

ALHA81031,19 L-3

 

Chondrule types in this specimen include microporphyritic
olivine with euhedral crystals, granular olivine,
cryptocrystalline excentroradial pyroxene, and barred
olivine varieties. Opaque material fills portions of spaces
between olivine plates in some of these latter chondrules.
The chondrules are distinct, closely packed and range in
size from 0.2 to 2 mm in diameter. There is no observable
fusion crust. The matrix is fine- to medium-grained with
considerable opaque material, and presents a definite
clastic texture, where the individual sedimentary components
are easily resolved. Some of the matrix can be seen filling
embayments on granular chondrules. Other chondrules are
dimpled. The chondrules are circular to slightly oblate in
outline, with the oblate variety most common among the
porphyritic and granular populations. Some microporphyritic
chondrules are rimmed by 1 to 2 fine-grained alternating
rings composed of a mixture of transparent and opaque
material. Opaque phases exist as rounded blebs in size
fractions from very fine to approximately 0.5 mm in
diameter, with a calculated modal abundance of 11 percent.
The fine grain size of this material gives the specimen a
sooty appearance typical of type 3 matrix (Brearly et al.,

1988).

154

Deposits of OPL material are present in direct contact with
opaque phases as haloes and partial haloes, comprising some
3.4 percent of the sample area. The rest of the specimen is
lightly stained (52.8 percent). The mount is first order
gray to first order yellow in color between crossed
polarizers, indicating a non-standard thin-section
thickness. One chondrule contains a "boxwork" of red rust,
while other olivine grains appear to contain large scale
etch pits. The in situ alterations are more red-orange in
color than ruby red, again possibly due to the non-standard

thin-section thickness.

The Smithsonian chip ALHA81031,4 was very similar in
appearance, presenting haloes and partial haloes of OPL with

light colored NNL stain.

155

ALHA77230,48 L-4

 

This specimen contains many readily delineated
microporphyritic olivine, excentroradial pyroxene, granular
olivine, and barred olivine chondrules set in a fine to
moderately coarse textured matrix of fine material, crystal
grains and chondrule fragments. Chondrules range from 0.2
to 4.0 millimeters in diameter. Some chondrules are rimmed
with opaque particles, and very fine shock fractures can be
detected here and there which each crosscut many chondrules
simultaneously. Opaque phases are highly variable in size,
and are subrounded to highly irregular in outline. The

opaque phases comprise some 13 percent of the sample area.

A thin fusion crust is intact over most of the specimen
length. OPL-filled fractures within the fusion crust are
present locally, setting the fusion crust apart in this
regard from most other areas in the specimen, which are
lacking in OPL. The calculated modal abundance of OPL in
this specimen is 2.0 percent, while that for NNL stain is
54.6 percent. NNL material is light brown in color and can
be found confined to intercrystal spaces both within and
between chondrules across the specimen. Very few obvious
fluid migration avenues not been stained by this material.
Some restricted regions close to the fusion crust have been

stained darker than elsewhere in the sample.

156

Among the notable features present are barred olivine
chondrule fragments with rust staining between platelets,
widespread clouding of chondrules and crystal grains and a
large pyroxene chondrule with an unusual strained extinction
pattern. Some olivine grains display a fine network of

intersecting cleavages lined with a well-crystallized oxide.

The Smithsonian chip ALHA77230,7 has a thin fusion crust
over most of its length. NNL stain is light brown within
and between chondrules, and confined to intercrystal spaces.
All obvious migration pathways have been stained by this
material. The sample contains many glass-containing
microporphyritic chondrules containing euhedral olivine
crystals. The glass in these chondrules is devitrified in

many cases.

157

ALHA81027,26 L-6

 

Of the few chondrules discernible in this specimen,
microporphyritic olivine and granular olivine varieties are
evident. Crystal grain sizes range from 0.2 to 2.0 mm in
diameter. A faint suggestion of textural lineation (fabric)
exists among populations of elongated opaque grains. The
opaque phase sizes and concentrations vary across the
specimen from large and sparse to small and abundant, and
comprise a calculated 12.4 modal percent of the area. The
matrix is totally recrystallized, there is no fusion crust,

and the specimen is moderately fractured.

The sample exhibits a nearly even, light yellow NNL staining
(74.6 percent), with localized OPL deposits (4.2 percent),
as well as a non-stained area in its interior. Some
fractures which penetrate into the interior are devoid of
secondary deposits. Other fractures have been filled along
portions of their lengths. A granular, amorphous oxide
appears within a ring of OPL material. Some rust haloes are
light yellow or gold in color but show the same internal
structure as OPL material found elsewhere. Some void spaces
are partially filled with secondary products. One opaque
grain near the edge of the sample displays a pattern of
alteration deposits which resembles that of a classic "soil
horizon" formation on bedrock (refer to Figure 3.13).

Fractures penetrate to depth within the grain, allowing

158

migration of reactive fluids to produce a product
distribution which increases in density outward toward the

grain exterior.

The Smithsonian chip ALHA81027,5 possesses a isolated fusion
crust. The NNL stain distribution is uneven, with a nearly

even light yellow NNL stain occupying the region nearest the
fusion crust, and a non-stained area occurring in the sample

interior.

159

ALHA77004,22 H-4

 

This specimen contains readily delineated microporphyritic
olivine and pyroxene chondrules, some granular olivine, and
a few barred olivine and pyroxene chondrules. The
surrounding matrix ranges in texture from fine to moderately
coarse grained, with the chondrules themselves ranging in
size from 0.2 to 1.0 mm in diameter. The specimen is
heavily fractured, with many fractures branching
dendritically and partially oxide-filled. Very little of
the fusion crust remains intact. Opaque phases are present

at a calculated modal abundance of 13.4 percent.

NNL material is extensive between crystals, and along
fractures and cleavages within crystals, comprising a total
calculated modal abundance of 55 percent within the
specimen. OPL deposits (calculated modal abundance 5.8
percent) appear opposite a fusion crust remnant and in
association with opaque phases as partial haloes. The
darkest oxide stains follow the large scale dendritic
fracture network across the specimen. The degree of
staining decreases toward the edge of the mount poSsibly due

to thinning of the specimen in this direction.

Among the notable features present are an olivine chondrule
containing a partially OPL-filled void space, an olivine

chondrule containing a microfault and possessing an

160

interesting platelet symmetry, preferential staining of
glassy areas, and clouded pyroxene chondrules, one of which

contains polygonal features resembling etch pits.

The Smithsonian chip ALHA77004,12 contains no fusion crust.
It presents light staining with opaque grains sparsely
affected by OPL replacement, appearing again as partial halo
deposits along eroded opaque edges. The OPL is orange in

color.

161

ALHA77208,29 H-4

 

Most chondrules in this specimen are readily delineated and
include microporphyritic and granular olivine, barred
olivine, excentroradial pyroxene, and cryptocrystalline
pyroxene varieties. Chondrule diameters range from
approximately 0.2 to 0.8 mm. Many olivine crystals within
and outside chondrules are euhedral primocrysts. The sample
is moderately fractured with most fractures rust-filled, and
the matrix is fine grained to moderately coarse. A large,
widened fracture zone contains copious amounts of rust which
appears to have moved freely into the space, uninhibited by
surrounding material. Opaque phases appear splotchy,
containing silicates and displaying a partly
disconnected/partly interconnected order. There is no

fusion crust present.

Some chondrules are surrounded by haloes of fine-grained
opaque material. The specimen is extensively stained in
three distinct but overlapping tones of rust; OPL blebs
associated with opaque phases, dark orange NNL staining, and
light orange NNL discolorations which coat grain surfaces.

A wide fracture zone contains copious amounts of rust which
appears to have moved freely into the space unencumbered by
surrounding material. One noteworthy chondrule is nearly
surrounded by opaque phases which are altering to OPL

lightly along the chondrule/opaque contact.

162

Other possibly important features include intercrystal oxide
nodules (OPL-appearing oxide deposits visible within crystal
grains), teardrop shaped features resembling pitting and
opaque phases which appear to have been altered from the

interior outward toward the exterior regions.

Smithsonian chip ALHA77208,4 contains red rust fracture
fillings and has a dark, soiled appearance caused by a heavy
NNL stain. There are many examples of OPL haloed opaque

grains. The sample had no visible fusion crust.

163

ALHA77225,18 H-4

 

The chondrule types in this undifferentiated meteorite
include granular olivine with euhedral to subhedral grains,
pyroxene in coarse and cryptocrystalline excentroradial
form, fine grained microporphyritic chondrules, and barred
olivine chondrule fragments. They range in approximate
diameter from 0.15 to 1.0 mm. The chondrules are readily
delineated from the surrounding fine-grained matrix. Opaque
phases are highly irregular in outline and partially
surround some chondrules. They are present at a calculated
modal abundance of 15.4 percent. There are very few visible
fractures, and the entire specimen transmits in the first
order gray in crossed polarized light, suggesting a non-
standard thin-section thickness. Examples of partially
devitrified glass can be found. There is trace of a fusion

crust in this chip.

The rust staining is present at 51.2 percent, but less
opaque than in other specimens. There are many examples of
cleavage plane fillings exhibiting optical continuity upon
stage rotation (see Appendix B). Some examples of ruby red
OPL (4.4 percent) can be found in association with opaque
materials. However, most OPL is dark orange in color rather
than ruby red, possibly due to thinner mount thickness in
these areas. There are also portions of OPL in the sample

which seem to pinch out, becoming orange in color and

164

allowing greater light transmission, both in plane and
polarized light. Many void spaces can be identified which
contain thick growths of oxide material in botryoidal
morphologies. There are many examples of cleavage plane
fillings exhibiting an optical continuity upon stage

rotation suggesting a transformation weathering mechanism.

The Smithsonian chip ALHA77225,10 contains devitrified
glass, partial OPL haloes around opaque grains, light NNL
staining, orange colored fracture fillings, and ruby red OPL
deposits which pinch out near sample boarders, becoming

lighter in color as they do so.

165

ALHA77226,37 H—4

 

The chondrules in this specimen range from 0.2 to 1.7 mm in
diameter and consist of microporphyritic olivine with
euhedral crystals, excentroradial pyroxene, and barred
olivine varieties, with the former most abundant. One large
(0.7 mm in diameter) euhedral olivine crystal is present.
Partly devitrified glass can be found between olivine
platelets in one barred olivine chondrule. This specimen is
heavily fractured and possesses moderate chondrule/matrix
integration. Fusion crust is fractured and discontinuous.
Some devitrification is present within the glass zones of
the fusion crust, and many additional features of interest
are visible in and around the area. Among these are oxide-
filled vesicles within the glass itself. Opaque grains

comprise some 15.2 percent of the sample.

Rust staining is dark, extensive (59.6 percent) and almost
opaque. Red oxide deposits fill most fractures. OPL
haloes are sparse (8.6 percent total OPL), and many appear
to be poorly crystallized. The fusion crust penetrates the
meteorite interior along one corner of the chip, perhaps as
a result of melted material permeating, collecting and
solidifying within a pre-existing fissure as it migrated
away from the friction-heated surface upon atmospheric
entry. Additional features include a clouded pyroxene

chondrule fragment, and a pyroxene chondrule with unusual

166

linearities suggesting removal of silicate material, and
which have subsequently been filled with oxide material.
Smithsonian chip ALHA77226,20 is different from chip ‘37 in
that the OPL is nearly absent, except near one edge, and NNL

stain is also sparse and light.

167

ALHA77232,16 H-4

 

The chondrule types in this specimen range from 0.33 to 1.03
mm in diameter, and include microporphyritic olivine and
microporphyritic olivine/pyroxene chondrules, with
excentroradial pyroxene and barred olivine chondrules in
lesser numbers. A.few of these latter chondrules exhibit a
sweeping extinction pattern, and some pyroxene chondrules
have slight convolutions in their radiating plates. Except
for the cases of pure pyroxene chondrules (2 found), the
chondrule/matrix integration is high, rendering chondrule
boundaries indistinct. Olivine grains are predominantly
euhedral, and opaque grains are highly irregular in outline,
while remaining roughly equidimensional in form. This
sample is moderately fractured, and presents first order
gray and yellow birefringence colors under crossed
polarizers, suggesting a non-standard thin-section
thickness. Opaque grains have a calculated modal abundance

of 13.2 percent in this sample.

NNL stain can be found throughout the sample (61.2 percent
calculated modal abundance). This material tends to avoid
staining chondrule interiors increasing the contrast between
chondrules and matrix in plane light. One chondrule is
partially surrounded by a dark orange variety of rust which
appears similar in structure to red OPL in other specimens.

OPL itself is present at an abundance of 3.0 percent. The

168

color difference in this case is attributed to the non-
standard mount thickness. Examples of devitrified glass
exist which show little sign of alteration, in contrast to
in situ alteration immediately adjacent. Similarly, glassy
phases in barred olivine chondrules do not show signs of
preferential attack. Additional features include examples
of clouded olivine crystals, what appear to be brecciated
pyroxene crystal fragments set within a devitrified glass
groundmass which also exhibits transverse clouding, and

examples of amorphous oxide material lining void spaces.

Smithsonian chip ALHA77232,10 displays light grayish orange,
homogeneously distributed NNL staining which avoids staining
chondrule interiors, as with the ‘16 chip. Other features

are similar to chip ‘16 as well.

169

ALHA77233,21 H-4

 

The chondrules in this specimen are moderately defined and
range from 0.25 to 0.83 mm in diameter. Those which can be
identified are largely of the microporphyritic and granular
varieties, with lesser amounts of excentroradial pyroxene
chondrules. One compound chondrule exists which contains
two fused individuals, both of the droplet pyroxene variety.
Also, one dark-zoned chondrule (Dodd and Van Schmus, 1971;
Dodd, 1981 p 116.) was found. The matrix crystallinity is
moderately coarse. Most of the finer details are obscured
by oxide staining (57.4 percent calculated modal abundance).
The meteorite is moderately fractured in this thin-section,
and possesses a fusion crust only along one small (1.38 mm)

section.

The specimen is heavily stained, but some areas, i.e.
interiors of individual crystal grains and chondrules, are
unaffected. The NNL stain is light orange-brown, and red
rust fracture fillings line most, if not all, fractures.

The zoned chondrule exhibits an alteration pattern in its
interior different (more cloudy) than that of its outer
portion. One microporphyritic chondrule contains olivine
crystals which show clouding features resembling etch
pitting under high magnification. The specimen also contains
a pyroxene chondrule with interlocking grains whose contacts

are smooth but irregular as though melted and

170

recrystallized. This chondrule is also etched with faceted
(prismatic) pits. Smithsonian chip ALHA77233,13 displays a
homogeneous light orange-brown NNL stain which, as in chip

‘21, avoids some chondrule and crystal grain interiors.

Other features are also similar to those of chip ‘21.

171

ALHA77182,36 H-S

 

The chondrules in this meteorite range in size from 0.2 to
1.3 mm in diameter and include mainly microporphyritic
olivine and granular olivine species in roughly equal
proportions, with the remainder comprised of barred olivine,
pyroxene, and dark-zoned varieties. One metal-bearing,
large and rounded olivine crystal chondrule was found. A
few large (z 0.5 mm diameter) olivine crystals exist which
range in form from euhedral to anhedral. There are many
chondrule fragments distributed throughout the specimen.
Chondrule outlines are delineated in many cases by opaque
blebs which are present in moderate abundance (10.4 percent
calculated modal abundance) and highly interconnected.

There is no fusion crust and no detectable fracturing of the

sample as a whole.

OPL predominates in abundance (26.4 percent) over NNL stain
(15.0 percent), with very nearly every opaque particle
having been altered to some degree. OPL can be found
aligned parallel to pyroxene crystal platelets in one
chondrule. Signs of clouding are easily found among the
olivine crystals. One olivine chondrule fragment presents
alteration products along devitrified glass plates. With
this exception, the chondrule interiors appear untouched by
alteration. Botryoidal growths line void spaces in several

locations. The void space deposits appear porous and grow

172

discontinuously, requiring refocusing of the microscope to
View the texture across their entire thickness. It should
be noted that nowhere else but within such void spaces has

the botryoidal morphology been observed.

Smithsonian chip ALHA77182,15 has a dark soiled black
appearance with a high amount of OPL and very little NNL

stain, as with chip ‘36.

173

ALHA79025,16 H-5

 

Most chondrules in this specimen are readily delineated and
range in size from approximately 0.3 to 3.0 mm in diameter.
These include microporphyritic chondrules containing
euhedral olivine crystals, which comprise the majority of
the chondrule population, and granular olivine and pyroxene
chondrules, which are in the minority. One barred olivine
chondrule fragment was identified. Minor portions of a

splendidly devitrified fusion crust remain.

Many chondrules are outlined by oxide material. The opaque
phases, which make up some 15.4 percent of the specimen,
range from semirounded to highly irregular in outline. The
largest bleb of this material resides within an oblate
granular olivine chondrule, which also contains an
additional small, oblate, barred and rimmed olivine
chondrule. The meteorite chip does not appear to have been

fractured.

The specimen is extensively stained (43.3 calculated modal
abundance), exhibiting a network of NNL strands together

with OPL blebs (7.4 percent) which tend to partially envelop
chondrules. Clouded areas within olivine grains show linear

patterns.

174

Smithsonian chip ALHA79025,5 contains a great deal of OPL
material with very little NNL stain. The chondrules are

typically haloed by OPL, as with chip ‘16.

175

ALHA79029,21 H-5

 

This specimen is composed of microporphyritic olivine,
excentroradial pyroxene and barred olivine chondrules and
chondrule fragments. One excentroradial pyroxene chondrule
contains subhedral olivine crystals. Most chondrules are
readily delineated within a fine-grained matrix, and range
in diameter between 0.2 and 1.0 mm approximately. The
fusion crust is thin and sparse, but some portions contain
crystals. A shallow but intact crystal-bearing fusion crust
is present. Opaque phases are highly irregular in outline,
heterogeneously distributed, and comprise some 11.6 percent
of the sample. The sample displays a moderate amount of
fracturing, with many of the fractures being wholly or
partially filled with oxide deposits. The fractures are

subparallel near one linear edge of the chip.

This specimen has a soiled appearance with extensive (62.0
percent) NNL staining. One portion of the fusion crust
appears to have been completely replaced (or displaced) by
OPL material. OPL rust is present at a 7.2 percent
calculated modal abundance. The smaller chondrules are
clear, appearing to be less affected by rust-producing
alteration processes than larger chondrules. Some examples
of OPL material are dissociated from opaque phases.
Additional notable features include patterned clouding in a

pyroxene chondrule fragment and in olivine chondrules, a

176

possible example of altered glass, and well pronounced
features resembling etch pits in pyroxene grains and

chondrule fragments.

Smithsonian chip ALHA79029,6 presents very light NNL
staining and very little OPL material, except near one edge
where it appears deep orange in color, indicating a non-

standard thin—section thickness.

177

ALHA84075,7 H-S

 

A moderate amount of fracturing is immediately apparent in
this specimen. There is one extended, straight and rust-
filled fracture crosscutting the sample. Some fractures
remain unfilled with rust. Others are partially filled with

opaque material.

Some chondrules are poorly defined. Others, including a few
droplet chondrules, are readily delineated and include
microporphyritic, excentroradial pyroxene and granular
olivine varieties ranging in size from approximately 0.2 to
1.0 mm in diameter. The matrix appears well recrystallized
but remains somewhat fine-grained and clastic in appearance.
A small portion of the fusion crust is still intact. Opaque
grains are present at a calculated modal abundance of 15.6

percent.

This meteorite is extensively stained (68.6 percent), and
appears light brown in color. NNL circumscribes some
chondrules while staining the interiors of others. Most of
the unstained chondrules are of the droplet variety. With
the exception of these unstained chondrules, the staining
appears evenly distributed. There are relatively few areas

of OPL rust (5.6 percent).

178

One small chondrule has a population of OPL blebs within its
boundaries, and another displays a rust growth pattern which
is parallel to its radiating pyroxene plates. Very little
matrix material remains unstained, while the pyroxene
chondrules seem relatively resistant to staining. Certain
crystal grains (commonly pyroxene) tend to show optical
continuity with crystallized rust zones filling their
fractures and cleavage planes. This optical phenomena may

be consistent with a transformation weathering process.

The Smithsonian sample (chip ALHA84075,0) presented few
differing characteristics. As with chip ‘7, it was heavily
stained, only appeared a dark brown color as opposed to
light brown. Like chip ‘7, chip ‘0 showed this stain to be

evenly distributed, and possessed very few OPL deposits.

179

ALHA85025,10 H-5

 

This specimen is heavily fractured with a very coarse
crystalline matrix surrounding microporphyritic, granular
olivine, recrystallized pyroxene and coarsely crystalline
excentroradial pyroxene chondrules. The chondrules range
from 0.3 to 1.7 mm in diameter, and are set within a coarse
textured matrix. One barred olivine chondrule was found.
These chondrules are poorly defined with few discernible
internal features. The opaque phases appear as rounded
blebs, and are homogeneously distributed, but of varying
sizes. They occur in a calculated modal abundance of 17.0
percent. NNL staining is manifest as an interconnected
network across the specimen (33.2 percent). OPL (12.8
percent) appears in isolated areas, some of which are
located in the vicinity of the fusion crust. The appearance
is similar to that of ALHA77288,34. The oxides in this
meteorite show very smooth color transitions from light
yellow to deep red, making it very difficult to point count

the OPL.

Among the more notable characteristics in this specimen is a
set of opaque, NNL-surrounded dendritic features within a
chondrule or crystal fragment, the rust-filled fracture
patterns near and within the fusion crust, a clouded region

which displays unique optical effects (internal

180

reflections), and examples of subrectangular features in

olivine grains resembling etch pits.

The Smithsonian sample had no chip number (presumably zero),
and presented no additional or differing features beyond
those described above. Very smooth color transitions were
observed among the oxides from light yellow to deep red, as

with chip ‘10.

181

ALHA77271,28 H-6

 

The main chondrule type identified in this sample is
microporphyritic olivine, including some chondrules with
larger than usual (1.25 mm) crystals. Only one each of the
cryptocrystalline pyroxene and excentroradial pyroxene
chondrule varieties were found, the latter being a chondrule
fragment. The chondrules have poorly defined boundaries and
are set within a well crystallized matrix. They range from
0.25 to 0.88 mm in diameter). The opaque phases are highly
irregular in outline and range in relative abundance from
approximately 12 to 20 percent in different parts of the
specimen. The average calculated modal abundance for the
opaque grains is 13.2 percent. .A small portion of fusion
crust remains partially intact. No crosscutting fractures
were detected. However, a fine network of rust-filled

fractures can be found near the fusion crust.

The oxide texture gives a splotchy or spattered appearance
rather than that of networked strands, as in some other
samples. Some OPL examples possess well-defined internal
structures and exhibit optically continuous sweeping
extinction. Orange colored rusts can be found lining and
partially filling void spaces in a bulbous or botryoidal
habit. Many examples of product-lined pore spaces exist.
Most oxides in this specimen possess very clean, sharp

contacts with their neighboring constituents. The OPL

182

deposits are clearly in the process of replacing opaque
phases, as evidenced by the sutured borders of remnant
opaque grains, and outlines of the OPL deposits, which are
consistent with those of the grains they are replacing.

Some olivine grains present a zoned, clouded appearance with

distinct lineations resembling etch pitting.

This sample definitely thins toward the edge, as evidenced
by diminishing birefringence colors from second order blue
to first order yellow on a single large olivine grain. This
meteorite also possesses a chondrule with a highly eroded
boundary, appearing to have been attacked, introducing the
possibility of silicate weathering. This meteorite contains
the best example found in the 19 specimens of a very well
crystallized oxide product of exceptional detail and color
variation (see 29;XN;40/.70 in Appendix B). Referred to in
the text (Figures 3.19 and 3.20) as a sharp contact phase,
this material is interconnected with other, similar but less
striking materials. Some remnant opaque material remains in
contact with this oxide, suggesting it represents a nearly

complete replacement of a former metal grain.

Smithsonian chip ALHA77271,6 possesses no fusion crust. Its
texture, including OPL and NNL abundance and distribution is

very similar to that of chip ‘34.

183

ALHA77288,34 H-6

 

This specimen exhibits a high degree of chondrule-matrix
integration. Chondrule relics include three or four
pyroxene chondrules and only one barred olivine chondrule
with most "bars" obliterated by secondary processing.
Chondrule diameters range from 0.5 to 1.0 millimeters. No
obvious fracturing is apparent on a section-wide scale.
However, some alteration products appear to follow a pattern
consistent with fracture fillings. The matrix is highly
recrystallized around evenly distributed opaque grains (16.0
percent) A few large (up to 1.0 mm in diameter) olivine

crystals are present. No surviving fusion crust is visible.

The limonite network is distributed consistently with light
and dark NNL predominating (46.6 percent) and OPL present
moderately (11.2 percent). Many opaque grains have thin
partial haloes of OPL. The NNL permeates every sector of
the sample to nearly constant densities, appearing
homogeneous on a section-wide scale. This is possibly due
to the highly granular texture of this petrologic type 6
sample, which might allow greater interconnectedness of
fluid-conducting avenues. Of the few OPL examples present,
some show a full range of color tones from yellow to red.
These tone variations occur in OPL material deposited along
the sample perimeter and are very likely thickness related.

It is therefore difficult to distinguish NNL from OPL in

184

this sample. The deepest red deposits seem to outline

former boundaries of opaque grains.

Additional notable features include clouded olivine crystals
showing some linearity in the cloud patterns, a pyroxene
chondrule with remarkable color diversity, and a region of

NNL which appears to display incipient crystallization.

The Smithsonian Institution sample chip ALHA77288,4
exhibited virtually identical features to chip ‘34,
including linear clouding patterns in an olivine crystal
group, and similar proportions and distributions of OPL and

NNL oxide varieties.

185

ALHA84082,9 H-6

 

This sample contains microporphyritic, granular olivine and
excentroradial pyroxene chondrules and chondrule fragments.
Some microporphyritic chondrules contain euhedral olivine
grains. The chondrules are readily delineated and are set
within a moderately coarse matrix. They range from 0.37 to
0.85 mm in diameter. Opaque phases exhibit rounded,
irregular outlines. Many pyroxene crystals are highly
birefringent, indicating a non-standard thin-section

thickness.

OPL and NNL phases can be found in both the fusion crust and
chip interior. However, there are differences in the
relative amounts of these materials between the two regions.
OPL deposits are sparse across the specimen as a whole, but
are present at a calculated modal abundance of 15.2 percent,
found mainly in the vicinity of the fusion crust, and are
clearly associated with opaque phases; while the orange-
brown NNL (38.6 percent) does not present a readily
detectable association with any specific primary phase. NNL
staining is absent at the opposite end of the sample from
the fusion crust. One chondrule has a cloudy center which
has been stained preferentially relative to the outer
portion. This central nucleus also presents a pitted
appearance under high magnification and differs in

extinction from surrounding portions of the crystal,

186

suggesting zoning. This pitting occurs in linear and
parallel rows within a crystal interior, and with
rectilinear or oblate to irregular outlines.

Smithsonian chip ALHA84082,0 displayed similar differences

in the relative amounts of OPL and NNL across the specimen.

187

LEW85322,11 H-6

 

This specimen is marked LEW85332,11 on the bottle label.
This is believed to be erroneous. Rubin and Kallemeyn
(1990) report petrologic and bulk chemical data for LEW85332
that show it is probably a carbonaceous chondrite of a
unique type. Microprobe data from Rubin and Kallemeyn
(1990), indicate olivine to have a wide composition range
for this meteorite (Fazav) with a mean of Fag; less abundant
pyroxene has a composition range FSLgo. This is unlikely

for a petrologic type 6 chondrite.

According to Rubin and Kallemeyn (1990), the thin-section
for LEW85332 shows “...an aggregate of small chondrules (up
to 1.2 mm across, but most are less than 0.5 mm), chondrule
fragments and irregular granular masses set in a translucent
yellow-brown matrix. Chondrules are mainly porphyritic
olivine and radial pyroxene/cryptocrystalline. Minor
amounts of nickel-iron and sulfide are present, as small
grains scattered through the matrix, and locally
concentrated around chondrule rims.” This texture is

petrologically dissimilar to that of a type 6 chondrite.

The thesis specimen is composed primarily of micro-
porphyritic chondrules with euhedral olivine crystals, and
granular olivine chondrules. Other chondrule types are

present in very small abundances only. Most chondrules are

188

poorly delineated. One cryptocrystalline pyroxene chondrule
has a border partially integrated with opaque material. The
opaque phases range in size from submillimeter dimensions to
very fine particles, and have irregular outlines. Most
olivine crystals are rounded and quite large, with one
crystal pair slightly greater than 1 mm in diameter. There
are few discernible fractures. However, rust-filled avenues
can be found which resemble fracture fillings with the only
difference being that of a tortuous, rather than straight,

pathway.

The rust patterns are unusual with numerous OPL deposits and
very little NNL staining. Curiously, the distribution of
the OPL material does not appear to be as dependent on
opaque phase distribution as in other samples. OPL deposits
may represent completely altered metal grains in this case.
Vacuolization patterns on select olivine grains resemble
etch pitting and seem to occur within specific zones. Some
oxide deposits protrude into void spaces in a bulbous or
botryoidal habit. NNL stain penetrates as far as fractures

extend into grains.

The Smithsonian sample was a chip from yet a third Antarctic

meteorite, LEW85323, and is not relevant to this thesis.

189

LEW86015,10 H-6

 

A fractured and devitrified fusion crust borders one side of
this specimen. Chondrule types include microporphyritic
olivine, excentroradial pyroxene, and barred olivine
varieties with the microporphyritic olivine making up the
majority. The chondrules are readily delineated, ranging
from 0.2 to 0.88 mm in diameter, and the matrix is coarse to
fine grained. The opaque phases in this moderately
fractured specimen are highly irregular in outline. Opaque
grains are present at a calculated modal abundance of 17.0

percent.

Most if not all fractures in the fusion crust are filled by
red rust deposits. Many other fractures within the sample
interior are rust-filled as well. NNL stain can be found
nearly everywhere (60.2 percent) with the exception of one
small unaffected area. One concentrated region of OPL
material, apparently a completely altered metal grain, is
connected by a rust-lined fracture to the exterior. The
sample contains many examples of well crystallized OPL
phases (9.4 percent calculated modal abundance),
characterized by their highly transparent interiors under

crossed nicols.

Other notable features include amorphous oxide deposits

(identified by their opaque, downy appearance), kink banding

190

in an olivine chondrule fragment, cleavage controlled stain
distributions in a pyroxene chondrule, and blackening near

the fusion crust.

The Smithsonian chip LEW86015,0 has very similar features,
including red rust deposits in the fusion crust, a high NNL
stain density (but having a dark orange-brown color), and

well crystallized OPL deposits.

191

CARICHIC H-S

 

Carichic is an example of a highly weathered non-Antarctic
meteorite. Most details are obscured by intense oxide
staining and alteration. Highly altered areas between
chondrules and crystals larger than matrix-sized particles
are nearly or entirely opaque and indistinguishable from
metal and metal sulfide particles. Also most every
transparent crystal and chondrule is saturated with

cloudiness.

The specimen is moderately fractured with most fractures
being filled with opaque oxides. Interiors of many
chondrules and chondrule fragments have been inundated by
orange-brown oxides. Others are altered parallel to radial
pyroxene or barred olivine plates. Chondrule varieties
recognized include granular olivine with subrounded grains,
1/3 to 1 mm in diameter, barred olivine, excentroradial
pyroxene 0.33 to 1.25 mm in diameter, microporphyritic
olivine with euhedral, randomly oriented olivine lozenges
set in a matrix of either (1) fine grained and small rounded
olivine, or (2) cryptocrystalline pyroxene with randomly
oriented pyroxene plates, 0.25 to 4.7 mm in diameter. One
olivine crystal is almost 2.0 mm in length. A.harred .
olivine chondrule fragment exhibits sweeping extinction.
Two subparallel shock veins penetrate and transect the

specimen. Euhedral olivine crystals in one large

192

microporphyritic chondrule appear embayed, suggesting
partial re-equilibration at lower temperatures.

Examining the Carichic thin sections in reflected light
reveals zones of darker staining within which reduced metal
grains cannot be found. Outside these zones, metal grains

of varying sizes can be seen in abundance (refer to Figure

3.21).

APPENDIX B

DESCRIPTIVE CATALOGUE OF COLOR TRANSPARENCIES

APPENDIX B

DESCRIPTIVE CATALOGUE OF COLOR TRANSPARENCIES

ALHA81031,19; L3

 

10;XN;2.5/.008
Wide field shot showing heterogeneous texture of this petrologic
type 3 UOC.

11;PL;2.5/.008
Same view in plane polarized light. Fine—grained dark matrix
outlines chondrules. Chondrule types are highly varied.

12;XN;6.3/.20
Pyroxene chondrule with well defined platelets and sharp outline.

13;PL;6.3/.20
Same view in plane polarized light. OPL visible in upper left
corner of frame.

14;XN;6.3/.20
Excentroradial pyroxene chondrule.

15;PL;6.3/.20
Same view in plane polarized light.

16;XN;6.3/.20
Plagioclase feldspar in chondrule pair.

17;PL;6.3/.20

Same view in plane polarized light. Feldspar grains appear
hexagonal in outline and are difficult to distinguish from
euhedral olivine grains in plane polarized light.

18;XN;6.3/.20
Cryptocrystalline excentroradial pyroxene chondrule is crosscut by
NNL-filled fractures.

19;PL;6.3/.20
Same View in plane polarized light.

20;XN;6.3/.20
Black-rimmed granular chondrule.

193

194

21;PL;6.3/.20
Plane polarized light view displays black rim more clearly than
polarized View.

22;XN;6.3/.20
Granular olivine chondrule.

23;PL;6.3/.20
Same view in plane polarized light.

24;XN;6.3/.20

Chondrule contains a grain which appears to have weathered in a
boxwork pattern, and also exhibits possible evidence for direct
(transformation) weathering of primary to secondary phases. To
be noted in this view is that all rust-filled fractures, which
crosscut the grain at subparallel angles, are simultaneously
highly light-transmissive.

25;PL;6.3/.20
Same view in plane polarized light. Boxwork pattern in grain at
lower right of frame is pronounced.

26;XN;l6/.45

This image is part of a 2—slide sequence intended to demonstrate
against the assertion of transformation in this case. This
view shows several pyroxene grains with rust fillings in
cleavage planes ”lighting up" in optical continuity, as would
be expected from the transformed weathering of different
regions in a single primary crystal.

27;PL;16/.45
Same view in plane polarized light.

28;XN;16/.45

Rotated 47°, most all of the previously light—transmissive
cleavage plane regions go extinct. .A transformation weathering
mechanism would explain this process only if occurring within
cleavage planes of a single crystal.

29;PL;16/.45
Same view in plane polarized light.

30;XN;40/.70
Highly magnified view of cleavage plane and fracture fillings.

31;PL;40/.70
Same view in plane polarized light.

32;XN;40/.70
Rotation of stage show simultaneous illumination of parallel,
crystalline cleavage plane fillings.

195

33;PL;40/.70
Same view in plane polarized light.

34;XN;16/.45
Two different cleavage sets "light up" within pyroxene crystal.

35;PL;16/.45
Same view in plane polarized light.

36;XN;6.3/.20
Rotated boxwork slide.

37;PL;6.3/.20
Same view in plane polarized light.

ALHA77230,48; L4

 

Ol;XN;2.5/.008
Wide field view showing general texture and weathering intensity.

03;PL;2.5/.008
Same view in plane polarized light.

04;XN;6.3/.20
Pyroxene chondrule with unusual fracture/strain pattern.
Extinction sweeps as stage rotates.

05;PL;6.3/.20
Same view in plane polarized light.

06;XN;2.5/.008
Microporphyritic chondrule with interchondrule microfault.

07;PL;2.5/.008
Same view in plane polarized light.

08;XN;6.3/.20
Higher magnification shows extent of microfault.

09;PL;6.3/.20
Same view in plane polarized light.

10;XN;l6/.45
This grain appears to contain large etch pits.

11;PL;16/.45
Possible pitting is more pronounced in plane polarized light.

12;XN;40/.70
Pits appear to be free of secondary deposits, are elongated at
nearly right angles to visible cleavage planes, and exhibit

196

consistent polygonal geometries which become more distinct with
increasing dimensions.

13;PL;40/.70

This plane polarized light view shows the high relief of the pit
areas, which occupy zones at different levels within the
crystal as evidenced by focusing adjustments.

14;XN;16/.45

Another example of possible etch pitting within the same
microporphyritic chondrule. Pits cover a large percentage of
the visible crystal surface.

15;PL;16/.45
Pits are more readily discernible in plane polarized light than in
polarized light.

l6;XN;40/.7O
Higher magnification reveals features with similar outlines to
those of the previous slide series.

17;PL;40/.70
Same view in plane polarized light.

18;XN;6.3/.20
Granular olivine chondrule with clouded crystals.

19;PL;6.3/.20
Same view in plane polarized light.

20;XN;16/.45
Higher magnification reveals extent of clouding.

21;PL;16/.45

Plane polarized light view. Clouding pattern is not homogeneous
but occupies preferential zones along grain boundaries and
seemingly random zones within grains.

22;XN;2.5/.008
Fusion crust contains a higher relative abundance of OPL oxide
than most areas within specimen.

23;PL;2.5/.008
Same view in plane polarized light.

24;XN;6.3/.20
Higher magnification of same area centered on fusion crust.

25;PL;6.3/.20
Same view in plane polarized light.

26;XN;2.5/.008
Beginning of zoom-in sequence on possible pitting within
cryptocrystalline pyroxene chondrule.

197

27;PL;2.5/.008
Same view in plane polarized light. Thinning of microprobe mount
toward edge of specimen is apparent.

28;XN;6.3/.20
Higher magnification of chondrule rim.

29;PL;6.3/.20
Same view in plane polarized light.

30;XN;16/.45
Still higher magnification shows linearity of pit feature
patterns.

33;PL;40/.70
Linear pit feature patterns at high magnification.

34;XN;100/l.32
Same features at very high magnification.

35;PL;100/1.32
Same view in plane polarized light.

36;XN;16/.45
Elongated pit features in pyroxene chondrule fragment.

37;XN;16/.45
Duplicate view, but slightly more transparent.

38;PL;l6/.45
Outline of chondrule fragment delineated by NNL deposits.

39;XN;40/.70
Higher magnification of same chondrule fragment showing linearity
of pit features.

40;PL;40/.70
Same View in plane polarized light.

41;XN;100/1.32
very high magnification of pitted crystal.

42;PL;100/1.32
Same view in plane polarized light.

43;XN;6.3/.20
Barred olivine chondrule fragment with oxide stain between
platelets.

44;PL;6.3/.20
Same view in plane polarized light.

45;XN;16/.45 \
Higher magnification centered on chondrule fragment.

198

46;PL;16/.45
Plane polarized light View shows distribution of oxides along
avenues between platelets.

47;XN;6.3/.20
Wide field view of large (> 3 mm) clouded cryptocrystalline
pyroxene chondrule.

48;PL;6.3/.20
Plane polarized light view shows pyroxene chondrule to be highly
clouded.

49;XN;16/.45
Another pyroxene chondrule fragment exhibits clouding in linear
patterns.

50;PL;16/.45
Same View in plane polarized light.

ALHA81027,26; L6

 

02;PL;6.3/.20
Void spaces partially filled with amorphous oxide phase. The rest
of the meteorite is highly stained with NNL.

03;XN;16/.45
Magnified view centered on oxide-filled void showing partial
crystallinity.

04;PL;16/.45
Same field in plane polarized light shows encroaching oxide
filling void space.

05;XN;16/.45

Another example at the same magnification. Notice linear trends
within crystallized zones paralleling grain boundaries.
Amorphous oxide can be seen from secondary reflected light
partially filling void space.

06;PL;16/.45
Same field in plane polarized light.

07;XN;6.3/.20
OPL near edge of specimen.

08;PL;6.3/.20
Same view in Plane polarized light. Material nearly surrounds one
opaque grain.

09;XN;16/.45
Magnified view centered on fractured opaque grain. Attack of
alteration proceeds similar to classical "soil horizon

199

formation on bedrock” scheme. This suggests a mechanical
breakdown of metal grains may accompany and assist chemical
alteration progression, or that oxidation is simply taking
advantage of an existing fracture situation.

10;PL;16/.45

Plane polarized light view of same field showing more clearly the
”edge toward interior" progression of weathering. Fractures
are parallel and perpendicular weathering "front”.

ALHA77004,22; H4

 

01;XN;2.5/.008

Wide field view showing general oxide stain intensity. Note:
Oxide stain intensity decreases toward microprobe mount edge
due to thinning of specimen.

03;PL;2.5/.008
Same view in plane polarized light.

04;XN;6.3/.20
vein fillings in highly fractured chondrule fragment.

05;PL;6.3/.20
Same view in plane polarized light.

06;XN;16/.45
Microfaulted barred olivine chondrule. Platelets exhibit
interesting symmetry.

O7;PL;16/.45
Same view in plane polarized light. Some preferential staining of
glass areas has occurred.

08;XN;6.3/.20
OPL zone surrounding void space.

09;PL;6.3/.20
OPL distribution and void space more visible in plane polarized
light than in polarized light.

lO;XN;16/.45

Higher magnification of same area centered on OPL oxide and void
space. Space appears to have once been occupied by opaque
grain which may have been plucked during or preceding mounting
process.

11;PL;l6/.45
Features are more visible in plane polarized light.

12;XN;6.3/.20
Irregular glass patterns in olivine chondrule.

200

13;PL;6.3/.20
Same view in plane polarized light. Another void space is visible
adjacent to chondrule.

l4;XN;l6/.45
Higher magnification of same area centered on chondrule.

15;PL;16/.45
Glass areas appear highly stained in plane polarized light,
suggesting glass as probable source for iron during weathering.

16;XN;40/.70

Preferential weathering of glass zones apparent at high
magnification. Some clouding of adjacent areas appears to have
taken place.

l7;PL;40/.70
Same View in plane polarized light.

18;XN;l6/.45
Clouded pyroxene chondrule.

19;PL;16/.45
Same view in plane polarized light.

20;XN;40/.70
Higher magnification shows linear parallel and subparallel
patterns in clouded areas.

21;PL;40/.70
Patterns are more strongly pronounced in plane polarized light.

22;XN;16/.45
Triangular features resembling indentations or pits in pyroxene
grain.

25;PL;40/.70
High magnification view of triangular features in plane polarized
light.

26;XN;16/.45
Barred olivine chondrule.

27;PL;16/.45
Same view in plane polarized light.

28;XN;16/.45
Oxides found not in association with any remaining opaque phases.

29;PL;16/.45
Plane polarized light view of same area shows clean contact with
silicate phases.

201

30;XN;40/.70
Higher magnification shows internal structure of oxide.

31;PL;40/.70
Patterns may be related to primary crystal structure prior to
replacement. Contact with silicate phases is irregular.

32;XN;6.3/.20
Microporphyritic olivine chondrule.

33;PL;6.3/.20
Same view in plane polarized light. Matrix is highly stained.

34;XN;40/.70
Stained and/or altered matrix appears glassy.

35;PL;40/.70
Same View in plane polarized light.

36;XN;40/.70

Another view of same chondrule shows linear clouding patterns in
silicate grain.

ALHA77208,29; H4

 

01;XN;16/.45
Intercrystal oxide nodules.

02;PL;16/.45

Plane polarized light view shows intense staining in vicinity of
this chondrule. Nodules may be superimpositions of material
over crystal due to manipulation of oxide fragments during
mount preparation.

03;XN;l6/.45
Teardrop shaped features resembling etch pits.

04;PL;16/.45
Features are more pronounced in plane polarized light. Features
line up at oblique angles to visible cleavage planes.

05;XN;16/.45
Oxide-filled fractures.

06;PL;16/.45
Plane polarized light reveals oxide to appear similar to OPL
material, but without being well crystallized.

07;XN;6.3/.20
Partially stained pyroxene chondrule fragment.

202

08;PL;6.3/.20
Plane polarized light view shows extensive oxide deposition
adjacent to chondrule fragment.

09;XN;16/.45

Higher magnification of area centered on chondrule fragment
showing relative clarity of interchondrule region as compared
with high staining of surrounding region.

10;PL;16/.45
Same view in plane polarized light.

11;XN;16/.45
Partially crystallized OPL material filling fractures throughout
highly fractured crystal assemblage.

l4;PL;16/.45

Detail of OPL from photomicrograph number 07. Opaque areas seem
to be weathering from the inside out in this plane of
visibility. Red zones exhibit sweeping extinction in polarized
light.

26;XN;2.5/.008
Wide field view showing general character of sample.

27;PL;2.5/.008
Same view in plane polarized light.

28;XN;2.5/.008
Widespread oxide deposition along boarder of large void space.

29;PL;2.5/.008
Plane polarized light view shows delicate nature of amorphous
oxide material.

30;XN;6.3/.20
Detail of OPL distribution along void space boarder. Variable
crystallinity characterizes the material in this area.

3l;PL;6.3/.20
Same view in plane polarized light.

32;XN;6.3/.20
Oxides completely surround but do not penetrate small, transparent
grains.

33;PL;6.3/.20

Same view in plane polarized light. Note fractures only partially
filled with oxide material, possibly suggesting recent, post-
oxide-deposition development of fracture widening followed by
further oxide formation and deposition.

34;XN;l6/.45
Same area under higher magnification.

203

35;PL;l6/.45
Same view in plane polarized light.

ALHA77225,18; H4

 

02;XN;2.5/.008
Wide field view showing general character of specimen.

03;PL;2.5/.008
Same view in plane polarized light. Many examples of OPL can be
recognized.

04;XN;6.3/.20

Granular olivine chondrule with euhedral crystals. Olivine
crystals can be identified by morphology. First order gray
birefringence colors suggest nonstandard thickness for
microprobe mount.

05;PL;6.3/.20
Same view in plane polarized light. Chondrule is partially
bordered by OPL blebs.

06;XN;6.3/.20
Pyroxene chondrule surrounded by altered opaque material.

07;PL;6.3/.20

Distribution of OPL is more clearly discerned in plane polarized
light. Note that OPL appears restricted to zones between
crystals and does not penetrate the chondrule boarder except
along one fracture.

08;XN;16/.45

Higher magnification of same area centered on chondrule. An
oxide-filled fracture with a curious ”zig-zag" pattern,
apparently corresponding to cleavage planes, can be seen; as
well as many well-crystallized OPL blebs in association with
opaque phases.

09;PL;16/.45
Same view in plane polarized light. Former outlines of opaque
grains can be seen.

10;XN;16/.45
Partially devitrified glass.

ll;PL;16/.45
Same view in plane polarized light.

12;XN;16/.45
Secondary oxide products in en echelon arrangement within strained
pyroxene grain.

204

13;PL;l6/.45
Same view in plane polarized light.

14;XN;16/.45

As the grain is rotated, fracture fillings go extinct
simultaneously, suggesting possibility of in situ
transformation of primary crystal to secondary products.

15;PL;16/.45
Same view in plane polarized light.

l6;XN;6.3/.20
Beginning of zoom—in sequence of detail of OPL alteration products
in association with opaque phases.

17;PL;6.3/.20
Same view in plane polarized light. Large open space exists as
result of separation along fracture during mounting operations.

18;XN;16/.45
Well crystallized OPL surrounding opaque grains.

19;PL;l6/.45
Same view in plane polarized light.

20;XN;40/.70
At high magnification, details of crystal structure can be
observed in the OPL material.

21;PL;40/.70

Plane polarized light view shows OPL oxides to exhibit botryoidal
crystal habit. Material appears to be expanding into vacant
spaces and may thus have formed following or just previous to
mount preparation.

22;XN;6.3/.20
Granular olivine chondrule outlined by oxide deposits.

23;PL;6.3/.20
Same view in plane polarized light.

24;XN;16/.45
Same chondrule under higher magnification.

25;PL;16/.45
Same view in plane polarized light.

ALHA77226,37; H4

 

01;XN;2.5/.008
Wide field view showing general rust intensity of specimen.

205

02;PL;2.5/.008
Same view in plane polarized light.

03;XN;2.5/.008
Fracture fillings.

O4;PL;2.5/.008
Plane polarized light view of same field showing extent and width
of fracture.

05;XN;6.3/.20
Higher magnification of fracture zone.

06;PL;6.3/.20
Fracture presumably widened during mount preparation. However,
very little if any post-mount deposition is visible.

07;XN;l6/.45
OPL along fusion crust. Very well crystallized.

08;PL;16/.45

Definite color differences between the opaque-associated OPL and
the fusion crust OPL can be seen in this plane polarized light
view of the same field.

09;XN;6.3/.20
Another example of OPL along fusion crust.

10;PL;6.3/.20
Same view in plane polarized light.

11;XN;2.5/.008
Well developed crystal structure in OPL material.

12;PL;2.5/.008
Same view in plane polarized light.

13;XN;6.3/.20
Same area under higher magnification. .A glass-filled fracture
zone is visible running diagonally across the frame.

14;PL;6.3/.20
Same view in plane polarized light.

15;XN;l6/.45
Higher magnification view centered on OPL-dominated region.

l6;PL;l6/.45
Glassy areas are visible in plane polarized light view.

l7;XN;16/.45

Many features and feature associations are visible in the area
dominated by the glass-filled fracture. Some devitrification
is present.

206

18;PL;16/.45
The plane polarized light view shows bubbles within the glass zone
which are filled with oxide weathering products.

19;XN;6.3/.20
Pyroxene chondrule with unusual linearities suggesting removal of
silicate material and subsequent filling with oxide products.

20;PL;6.3/.20
Same View in plane polarized light.

21;XN;6.3/.20
Interchondrule features resembling recrystallization within
tension fractures.

23;XN;l6/.45
Same chondrule under higher magnification.

24;PL;16/.45
Same view in plane polarized light.

25;XN;6.3/.20
Interchondrule devitrification.

26;PL;6.3/.20
Same view in plane polarized light.

27;XN;16/.45
Higher magnification of devitrification area.

28;PL;16/.45
Same view in plane polarized light.

29;XN;16/.45
Zoned inclusion within clouded pyroxene chondrule fragment.

30;PL;16/.45
Same view in plane polarized light.

31;XN;40/.70
Same chondrule under higher magnification.

34;PL;2.5/.008
Wide angle view of specimen centered on fusion crust.

ALHA77232,16; H4

 

18;PL;2.5/.008
Pyroxene chondrule rimmed with iron oxides.

207

19;XN;2.5/.008
Same view in polarized light. Many different chondrule types can
be identified in this view.

20;PL;16/.45

Higher magnification of chondrule rim showing oxides lining rim
boarder, and in association with adjacent opaque phases. A
smooth transition appears to be in progress from in situ
alteration of opaque material to OPL oxides, to neoformation
along nearby avenues of constituents in solution to NNL
material.

21;XN;16/.45

Under polarized light, the transition boundary from NNL to OPL
phases can be discerned. The OPL material is well crystallized
and close to the opaque phases. By contrast, the NNL material
is poorly crystallized and is found farther along the
matrix/chondrule contact from the opaque iron source.

22;PL;16/.4S
This appears to be brecciated pyroxene crystal fragments set
within a devitrified glass groundmass.

23;XN;16/.45
Under crossed nicols, devitrification appears far less extensive.

24;PL;40/.70

Higher magnification reveals groundmass to exhibit clouding in
parallel linear arrangement. This view is centered on a
crystal fragment which appears to be in the process of
alteration by chemical attack from.selected sides.

25;XN;40/.70
Same view in polarized light. Parallel arrangements are clearly
visible.

26;PL;40/.70
Another example in same area. Weathering "fronts" render crystal
boundaries irregular and indistinct.

27;XN;40/.70

In polarized light, however, it becomes difficult to assert
whether crystal boundaries are indeed being attacked, or are
simply partially overlapped by NNL.

28;PL;l6/.45
Pyroxene chondrule fragment appears to have suffered offset along
fracture.

29;XN;16/.45

Same view in polarized light. Offset more clearly pronounced, but
is brought into question by apparent continuations of platelets
across fracture.

208

30;PL;l6/.45
Example of amorphous oxide material surrounding void space.

31;XN;16/.45
The amorphous nature of the material is evident under crossed
nicols.

32;PL;16/.45
Glass region exhibiting what appears to be devitrification
followed by alteration in a striking pattern.

33;XN;16/.45
Pattern is more visible under crossed nicols.

34;PL;40/.70
Same region at greater magnification. Transverse patterns are
parallel to subparallel.

35;XN;40/.70
Patterns are well pronounced in polarized light.

ALHA77233,21; H4

 

02;XN;6.3/.20
Olivine crystal-centered chondrule. Olivine crystal is clouded
throughout a zone concentric with its perimeter.

03;PL;6.3/.20
Same view in plane polarized light. Transverse fractures crosscut
chondrule, penetrating olivine crystal with little refraction.

04;XN;16/.45

Higher magnification reveals penetration of oxide phases into
fractures. This process is apparently seen in an arrested
state, as some fractures are only partly filled. Linear
texture of clouded zone is evident.

05;PL;16/.4S
Same view in plane polarized light. Olivine crystal is relatively
stain—free, in contrast to highly stained surrounding area.

06;XN;l6/.45
Microporphyritic olivine chondrule containing crystals which
display features resembling etch pitting.

07;PL;16/.45
Plane polarized light view.

08;XN;40/.70

Magnified view of same chondrule centered on pitted grain.
Features are linear and parallel. High relief of features
suggests hollow pits.

209

09;PL;40/.70
Features are more discernible in plane polarized light than in
polarized light.

10;XN;16/.45
Opaque grains in association with partially crystallized OPL
material.

11;PL;l6/.45
Plane polarized light view. Full extent of OPL material is
visible.

12;PL;6.3/.20
Dark-zoned chondrule.

13;XN;6.3/.20
Dark-zoned chondrule in polarized light.

14;PL;16/.45

Mineral with 3 cleavage sets. Staining location is partially
centrolled by cleavage. Elongated features restricted to one
select region and bounded by cleavage plane.

15;XN;16/.45
Same view in polarized light.

16;PL;100/1.32

Highly magnified view of crystal, centered on elongation patterns.
Patterns are in subparallel linear arrangements, and exhibit
bubble-like appearance.

17;XN;100/1.32
Same view in polarized light.

ALHA77182,36; H5

 

11;XN;2.5/.008
Wide field view of sample showing large excentroradial pyroxene
chondrule with oxides occupying spaces between platelets.

12;PL;2.5/.008
Plane polarized light view of same field. Note that NNL is sparse
in this sample.

13;XN;16/.45

Magnified view centered on chondrule. Clearly visible are linear
deposits of red oxide which parallel pyroxene plates and
terminate at a transverse zone.

14;PL;16/.45
Plane polarized light view of same field. Opaque phases can be
found in association with the red trends, but it is difficult

210

to say with certainty whether these were the sole contributors
or whether some alteration of the pyroxene itself might not
have taken place.

15;XN;16/.45

Visible in these olivine chondrules are curious pits which line up
in rows and interfere with light transmission, giving a cloudy
appearance.

16;PL;16/.45
Plane polarized light view of same field. NNL fills cleavage
planes or striations in one grain.

17;XN;16/.45
Barred olivine chondrule fragment with OPL invading former glass
spaces.

18;PL;16/.45
Plane polarized light view of same field. OPL seems preferential
for special conditions, not occupying all available spaces.

l9;XN;16/.45

Another pyroxene chondrule with OPL taking advantage of crystal
morphology, though to a lesser extent than with slides 11
through 14.

20;PL;16/.45

Plane polarized light view of same field. Visible in plane
polarized light are void spaces which are partially filled with
oxides of a botryoidal habit.

21;XN;16/.45
Another region of botryoidal void fillings. Not much is visible
of these deposits under crossed polarizers.

22;PL;16/.45
Plane polarized light view of same field. Knobby, botryoidal
deposits are visible in profusion.

23;XN;6.3/.20
Wide field view showing distribution of these deposits.

24;PL;6.3/.20
Plane polarized light view of same field.

25;XN;16/.45

Detail of previous slide set. Vbid space is rimmed with well
crystallized phase, though apparently not associated with any
nearby opaque grains, suggesting a three step process for the
present appearance of these features: 1. OPL forms by in-situ
alteration of opaque grain. 2. Grain to grain integrity is
weakened by the softer oxide rim and the remaining opaque
Haterial is removed by plucking. 3. Additional residual
growth of botryoidal phases occurs into new vacancy.

211

26;PL;l6/.45
Plane polarized light view of same field.

27;XN;2.5/.008
Wide field view showing many large chondrules of various
morphology.

28;PL;2.5/.008
Plane polarized light view of same field.

29;XN;16/.45
Barred olivine chondrule similar to that of slide pair 17/18. OPL
can be seen entering vacancies between plates.

30;PL;16/.45

Plane polarized light view of same field. OPL has low mobility,
but is clearly not immobile.

ALHA79025,16; H5

 

1b-4b;PL/XN;2.8/.008
Wide field views of devitrified glass.

5b-10b;PL/XN;6.3/.20
Medium power views of devitrified glass. Glass appears to be associated
with (or part of) fusion crust.

llb;XN;16/.45
Devitrified glass appears moderately clouded.

12b;PL;16/.45
Same view in plane polarized light.

13b;PL;16/.45
Devitrified glass containing NNL stain.

l4b;XN;16/.45
Same view in polarized light.

15b-18b;PL/XN;16/.45
Additional details of devitrified glass under high power. Sharp contact
phases appear as coating superimposing glass in 17b and 18b.

19b;PL;40/.70
Very high power view of devitrified glass.

20b;XN;40/.70
Same view in polarized light.

21b;PL;l6/.45
Partially recrystallized grain?

212

22b;XN;16/.45
Same View in polarized light.

23b;PL;16/.45
OPL in association with opaque grains.

24b;XN;16/.45
Same view in polarized light.

31;XN;6.3/.20
Microporphyritic olivine chondrule with one large olivine grain
displaying linear clouding.

32;PL;6.3/.20
Same view in plane polarized light. Surrounding material stained.

33;XN;16/.45

High magnification of same chondrule centered on large grain.
Clouding tends to avoid grain boundaries and inclusions. Could
the chemistries of these areas have been altered by the
presence of inclusions and other grains, sufficient to reduce
susceptibility to attack by weathering agents?

34;PL;16/.45
Same image in plane polarized light.

35;XN;2.5/.008

Wide field view of meteorite. Visible are several chondrule
types. One barred olivine chondrule exists within larger,
opaque-centered granular chondrule.

36;PL;2.5/.008

Same field in plane polarized light. Notice distribution of
opaque phases associated with OPL and NNL phases.

ALHA79029,21; H5

 

01;XN;6.3/.20
Unusual clouded pattern in pyroxene chondrule fragment.

02;PL;6.3/.20
Same view in plane polarized light. Chondrule fragment is almost
totally free of staining, in contrast to surrounding regions.

03;XN;l6/.45

Symmetry of clouded areas becomes clear at higher magnification.
Area appears highly pitted, with pit features arranged in
linear parallel and subparallel arrangements along intermittent
transverse stripes across the fragment.

213

04;PL;16/.45
High relief of clouded verses non-clouded areas is more visible in
plane polarized light than in polarized light.

05;XN;6.3/.20
Microporphyritic olivine chondrule.

06;PL;6.3/.20
Same view in plane polarized light.

07;XN;l6/.45
Under higher magnification, some pit features can be discerned.

08;PL;l6/.45
A variety of pit features are present, from random clouding to
polygons in linear arrangements.

09;XN;6.3/.20
Clouded pyroxene chondrule fragment.

10;PL;6.3/.20

More features are visible in plane polarized light than in
polarized light. Fragment contains linear zones of higher
opaque grain concentration, which seem to be relatively cloud-
free, in contrast to surrounding regions of high clouding.

11;XN;l6/.45
Higher magnification shows features more clearly. Chondrule
fragment is shot through with oxide—filled fractures.

12;PL;16/.45
Same view in plane polarized light.

13;XN;6.3/.20
Pyroxene chondrule with pit features.

14;PL;6.3/.20
Same view in plane polarized light.

15;XN;2.5/.008
Wide field view showing general rustiness of specimen.

16;PL;2.5/.008
Same view in plane polarized light.

17;XN;2.5/.008

Wide field view of less rusty area. The two slide sets are
intended to illustrate the heterogeneity of rustiness
encountered within specimens.

18;PL;2.5/.008
Same view in plane polarized light.

214

22;PL;16/.45

Microporphyritic pyroxene chondrule with polygonal pit features of
varying size. Altered material (which could be glass) is
visible.

23;XN;40/.20
Detail of above. Note hint of stairstep pattern to pit outlines.

24;PL;40/.20
Same view in plane polarized light.

25;XN;40/.70
Well pronounced pit features of polygonal outline.

26;PL;40/.70
Same view in plane polarized light. Some pits seem to have been
filled with oxide phases.

27;XN;16/.45
Barred olivine chondrule.

28;PL;16/.45
Glassy zones appear to be partially weathered.

29;XN;40/.70

The cause for the “cloudy” appearances observed is revealed in
this sequence of slides as a bubble or pit which occurs
internally and not necessarily in association with a fracture.
Focus changes in this sequence will show that these features
are found at varying depths within a given crystal. Pit
features in focus at southwest side of “cloud”.

30;PL;40/.70
Same view in plane polarized light.

31;XN;40/.70
Pit features in focus toward the northeast side of “cloud”.

32;PL;40/.70
Same view in plane polarized light.

33;XN;l6/.45
Wider field of view showing surrounding cloudy regions.

34;PL;16/.45
Clouding is more visible in plane polarized light than in
polarized light.

35;PL;6.3/.20
Still wider field of view.

36;XN;6.3/.20
Same view in polarized light.

215

ALHA84075,7; H5

 

02;XN;16/.45

Pyroxene crystal with rust-filled fractures and cleavage planes.
Common illumination of these fractures will occur upon rotation
of stage, visible in following slide pair.

03;PL;l6/.45
Cleavage planes are visible in plane polarized light.

04;XN;l6/.45
Same crystal rotated 40.5° clockwise. Cleavage plane fillings now
transmit light, while fracture filling goes extinct.

05;PL;l6/.45
Same view in plane polarized light.

06;XN;6.3/.20
Same area at lower magnification, centered on crystal.

07;PL;6.3/.20
Same view in plane polarized light.

08;XN;6.3/.20
Same view rotated 80°.

09;PL;6.3/.20
Same view in plane polarized light.

10;XN;6.3/.20
OPL apparently not in association with opaques.

11;PL;6.3/.20
Same view in plane polarized light.

12;XN;16/.45
Higher magnification view centered on OPL material.

13;PL;l6/.45
Same view in plane polarized light.

14;XN;16/.45
Pyroxene grain with inclusions.

15;PL;16/.45
Same view in plane polarized light.

16;XN;16/.45
Same view rotated 43°.

17;PL;16/.45
Same view in plane polarized light.

18;XN;l6/.

216

45

Another example of possible transformation.

19;PL;l6/.

Same view
opaque

20;XN;16/.

Same View

21;PL;16/.

Same view

22;XN;16/.

45
in plane polarized light.
phases.

OPL in association with

45

rotated 43°. Cleavage and fracture zones both transmdt.

45
in plane polarized light.

45

Olivine grains show same phenomenon as pyroxene grains.

23;PL;16/
Same view

24;XN;16/.

Same view

25;PL;16/.

Same view

45
in plane polarized light.

45
after rotation.

45
in plane polarized light.

ALHA85025,10; H5

 

05;XN;2.5/.08
Wide field view showing regional texture and a rust—filled
fracture.

06;PL;2.5/.08

Same view

in plane polarized light. Increased contrast reveals

fracture fillings more clearly than in polarized view.
Pleochroic minerals visible in corners of view.

07;XN;6.3/.20

Magnified

view of same region, centered on fracture zone.

08;PL;6.3/.20

Same view

in plane polarized light showing dispersion of colors

from fracture zone.

O9;XN;9/.20

Dendritic

features in chondrule.

10;PL;9/.20

Same View in plane polarized light.

Many rust features visible in

this large field view.

217

ll;XN;16/.45
Magnified view centered on dendritic features. Features are
opaque and are apparently confined to single crystal.

12;PL;16/.45

Same view in plane polarized light. The dendrite-containing
crystal is stained orange throughout, except for central region
which remains clear.

l3;XN;6.3/.20
Association between fusion crust and rust features.

14;PL;6.3/.20

Plane polarized light view showing fusion crust altered throughout
its extremity. Interior fractured zone heavily inundated with
rust features.

15;XN;6.3/.20
Example of crystal with rust-filled fractures.

16;XN;6.3/.20
Identical to above.

19;PL;6.3/.20
Wide field view showing overall rust intensity.

20;XN;2.5/.08
Wide field view showing fusion crust weathering.

21;PL;2.5/.08

Same view in plane polarized light showing thickness of fusion
crust. Comparison with previous slide reveals locations of
glassy material in crust.

22;XN;l6/.45
Clouded or pitted crystal in association with iron oxide staining.
Note that only a portion of the crystal has been effected.

23;PL;16/.45
Same view in plane polarized light. Association with oxides may
be coincidental.

24;XN;16/.45
Well crystallized OPL in fractures.

25;PL;16/.45

Same view in plane polarized light. Texture is well integrated,
suggesting that oxide location is dependent on avenue
communication.

26;XN;l6/.45
Partially clouded grain.

218

27;PL;l6/.45
Same view in plane polarized light. Clouding is not extensive.

28;XN;16/.45

Same grain rotated to show internal optical property of clouded
region. Region is seen to become transmissive at certain
orientations between crossed polarizers.

29;PL;l6/.45
Effect is not visible in plane polarized light.

30;XN;40/.70

Bubbles with irregular outlines are common throughout all samples
examined. They concentrate along fracture planes and appear
associated with NNL.

31;PL;40/.70
Same view in plane polarized light.

32;XN;40/.70
Elongated and subrectangular features in olivine crystal.

33;PL;40/.70
Same view in plane polarized light. Features do not appear to be
associated with any rust phases.

34;XN;100/l.32
Highly magnified view of features.

35;PL;100/l.32
Same view in plane polarized light. Becke line visible.

ALHA77271,28; H6

 

02;PL;16/.45
Pronounced clouding in zone within olivine grain concentric with grain
boundary.

03;XN;6.3/.20
Group of 2 7 clouded olivine grains of irregular outline.

04;PL;6.3/.20
Group of 2 7 clouded olivine grains of irregular outline. Clouding is
more obvious in plane polarized light.

05;XN;16/.45 —

Group of clouded olivine grains of irregular outline (magnified view of
same grouping). Clouding seems to concentrate within grain
interiors.

219

06;PL;16/.45

Same view in plane light. Area is relatively free of rust
staining, though some staining and red rust is visible in
corner of frame.

O7;XN;40/.70

Group of clouded olivine grains of irregular outline (highly
magnified view of same grouping showing choice example).
Clouding clearly concentrated along linear trends.

08;PL;40/.70
Linear trends are more easily discernible in plane polarized
light.

09;XN;6.3/.20

Example of red rust in association with opaque grains, but removed
some distance from them. Rust lines and partially fills void
spaces.

10;PL;6.3/.20
Void spaces clearly defined in plane polarized light. Rust color
variations from deep red to light orange.

11;XN;16/.45
Example of red rust in association with opaque grains (magnified
view of same region). Details of red rust clearly defined.

12;PL;l6/.45
Same view in plane light. High relief of void space perimeters
apparent.

13;XN;6.3/.20
Rust deposits lining void spaces in botryoidal habit.
Heterogeneity of chondrite grain size distribution obvious.

14;PL;6.3/.20
Plane polarized light view.

15;XN;16/.45
Rust deposits lining void spaces in botryoidal habit. Magnified
view centered on void spaces.

16;PL;16/.45

Same view in plane light. Botryoidal product habit apparent. In
focus verses out of focus areas along void perimeter show depth
of field.

17;PL;40/.70

Highly magnified view of void space in plane polarized light
showing knobby, bubble-like or botryoidal habit and bringing
the idea of silicate replacement into question by virtue of
irregular (attacked?) silicate void space perimeter. More
likely the overlapping of product onto silicate material. View
has relatively large depth of field.

220

18;XN;40/.70
Cross polarized view of highly magnified void space showing
crystallinity of rust deposits.

l9;XN;6.3/.20
Red rust in association with opaque phases, possibly replacing
these phases.

20;PL;6.3/.20
Plane polarized light view of same area.

21;XN;l6/.45
Magnified view of same region centered on red rust/opaque phase
combination.

22;PL;16/.45
Same field in plane polarized light.

23;XN;40/.70

Highly magnified view showing eroded opaque boundaries and
"seepage" of secondary phase overtop opaque primary. Notice
silicate phases remain unaffected.

24;PL;40/.70
Plane polarized light view of same image.

25;XN;6.3/.20

Best example found of well crystallized alteration products, near
fusion crust. Other, less impressive examples can be seen
nearby.

26;PL;6.3/.20
Plane polarized light view of same area. Visible are areas of
blackened fusion crust.

27;XN;16/.45

Magnified view centered on best crystallized product. Sweeping
extinction evident. Vivid color and textural variations.
Botryoidal habit. Seems to have completely or near completely
replaced primary phases.

28;PL;l6/.45

Other subtle features are revealed in this plane polarized light
view of the same field. Silicates appear unaffected. Opaques
in corner are being replaced. .Alteration begins at the edges
of opaque phase grains.

29;XN;40/.70

Highly magnified view centered on same association. "Weathering
fronts" can be seen clearly and are concentric with respect to
silicate and opaque grain boundaries. This image should be
studied for clues as to phase origin and materialization.

221

30;PL;40/.70

Plane polarized light view of same field. Some remnant opaque
material persists with highly sutured edges. Material appears
from these remnants to have almost completely replaced a former
opaque-occupied area with little or no net volume increase in
the plane of the image. Ghost outlines (pseudomorphs?) can be
discerned within the material.

31;PL;6.3/.20
Detail of weathering in fusion crust. Many color variations are
present among similar mineralogies.

32;XN;6.3/.20
Same field of view under crossed polarizers. variations in
birefringence colors are striking.

33;XN;16/.45

Higher magnification centered on area of greatest alteration
product concentration. Much dark material obscures details
within OPL phases.

34;PL;16/.45

Same field under plane polarized light. Color variations change
with thinning toward probe mount edge. Some botryoidal void
filling is in progress near edge of field.

35;PL;40/.70
Highly magnified view centered on fusion crust alteration. Are
these products of altered silicates, glass or metal?

36;XN;40/.70
Crossed polarized view of same field.

ALHA77288,34; H6

 

01;XN;2.5/.008

Wide field view showing cluster of clouded olivine crystals.
variations in birefringence due to thickness changes are
apparent in one fractured grain.

02;PL;2.5/.008
Same view in plane polarized light.

03;XN;6.3/.20

Higher magnification of same area centered on clouded crystal
group. Some linearity is visible at this scale in select
grains.

04;PL;6.3/.20
Same view in plane polarized light.

222

05;XN;16/.45
Higher magnification shows rows of lenticular shapes which
resemble etch pitting. Other clouding appears more random.

06;PL;l6/.45
Same view in plane polarized light.

07;XN;l6/.45
Pyroxene chondrule displaying striking color variations.

08;PL;l6/.45

Plane polarized light view shows some pleochroism. Dark material
between crystal plates does not seem to have been attacked
preferentially by weathering reactions.

26;XN;6.3/.20
Extinct grain contains highlights which reflect crystal structure
in NNL.

27;PL;6.3/.20
Concentric patterns suggestive of crystallinity are apparent in
plane polarized light.

28;XN;16/.45
Higher magnification better defines crystal pattern.

29;PL;l6/.45
Same view in plane polarized light.

30;XN;16/.45

Grain is rotated 47° to reveal effect of superimposed olivine
crystal on birefringence coloration of NNL crystals.
Crystallization appears incipient, in contrast to the sharply
defined, well crystallized red and orange region to the lower
right of the frame. Crystallization also appears to be
occurring in optical continuity throughout specimen.

31;PL;16/.4S
Same view in plane polarized light.

32;XN;6.3/.20
Chondrule containing platelets of opaque material.

33;PL;6.3/.20
Platelets do not seem to be attacked preferentially in plane
polarized light.

34;XN;6.3/.20
Moderately metamorphosed barred olivine chondrule.

223

35;PL;6.3/.20

Plane polarized light examination shows that NNL material has
entered the chondrule, but does not seem to have taken
advantage of intergrain avenues.

36;PL;2.5/.008
Wide field view showing overall rust intensity.

37;XN;2.5/.008
Same view in polarized light.

ALHA84082,9; H6

 

01;XN;6.3/.20
Granular chondrule with clouded and rust-stained center.

02;PL;6.3/.20
Plane polarized light view of same region.

03;XN;16/.45
Enlarged image showing textural and mineralogical differences
between chondrule center and outer rim.

04;PL;16/.45
Same view in plane polarized light showing preferential locations
both of clouding and of oxide staining.

05;XN;6.3/.20
Olivine chondrule displays preferred oxide deposition along plate
interstices. Wide field view.

06;PL;6.3/.20
Same view in plane polarized light.

O7;XN;16/.45
Enlarged view centered on olivine grain. Plates can clearly be
discerned.

08;PL;l6/.45
Same view in plane polarized light. Avenues of oxide deposition
can be identified.

09;XN;6.3/.20
NNL in fusion crust with OPL surrounding metal grains.

10;PL;6.3/.20
Same view in plane polarized light. NNL lines inside of void
spaces. Possibly altered glassy material.

11;XN;16/.45
Magnified view centered on OPL-lined void spaces. Well
cryptocrystallized.

224

l3;PL;l6/.45
NNL juxtaposed with OPL showing many interesting attributes of
each.

l4;XN;6.3/.20

Wider view of same area. variations in OPL color may be due to
density or thickness differences. Strong associative
tendencies of OPL to opaque grains is evident here.

15;PL;6.3/.20
Plane polarized light view of same field.

LEW85332,11; H6

 

16;XN;6.3/.20
Void fillings with botryoidal growth habit.

17;PL;6.3/.20
Same view in plane polarized light. Broken fragment attests to
pre-mount formation of fillings.

18;XN;16/.45
Higher magnification view centered on void spaces. High
crystallinity is evident under crossed nicols.

l9;PL;l6/.45

Plane polarized light view of same field. Difficult to say
whether isolated fragment is truly broken or simply a cross-
section of a protrusion from the third dimension.
Concentricity of growth "fronts” to outlines of silicate
substrates evident.

20;XN;40/.70
High magnification view of pyroxene crystal with parallel rows of
elongated or suborthogonal striations resembling etch pitting.

21;PL;40/.70
Same field in plane polarized light.

24;XN;16/.45
Granular chondrule surrounded by rust—lined void spaces.

25;PL;16/.45
Same view in plane polarized light.

26;XN;6.3/.20
NNL veins superimpose or cross-cut chondrule.

27;PL;6.3/.20

Same view in plane polarized light. Porosity of grain boundaries
for fluid migration and chemical transport is evident in rust
veins, which display a more tortuous or sutured formation

 

225

avenue than the often nearly straight formation habits of
fracture fillings.

28;XN;16/.45
Enlarged view centered on chondrule.

29;PL;16/.45
Same view in plane polarized light.

30;XN;2.5/.08
Unusual fracture filling pattern near fusion crust.

31;PL;2.5/.08
Same view in plane polarized light.

32;XN;6.3/.20
Higher magnification. Fracture filling is well crystallized.

33;PL;6.3/.20
Plane polarized light view of same field. Fracture has moderate
width.

34;XN;6.3/.20
Red rust grades to orange rust.

35;PL;6.3/.20
Plane polarized light view of same field.

36;XN;16/.45
Higher magnification of same field.

37;PL;16/.45
Plane polarized light field of same image.

01;XN;6.3/.20
Group of olivine crystals with clouding.

02;PL;6.3/.20
Plane polarized light view of same field.

03;XN;16/.45

Higher magnification centered on two large olivine grains.
"Clouding" is actually parallel rows of something like etch
pits.

04;PL;16/.45
Plane polarized light view of same field.

LEW86015,10; H6

 

Ol;XN;l6/.45
OPL in fusion crust.

226

02;PL;l6/.45
Same view in plane polarized light.

03;XN;6.3/.20
Another area of weathered fusion crust showing widespread
crystallization in OPL phases.

04;PL;6.3/.20
Same view in plane polarized light.

05;XN;l6/.45 -

Higher magnification of OPL zone. A well defined structure,
paralleling outlines of associated opaque phases in intimate
contact justify in situ formation and replacement conjecture.

06;PL;16/.45
Same view in plane polarized light.

07;XN;16/.45
Adjacent area at same magnification displaying similar features.

08;PL;l6/.45
Same view in plane polarized light. Note black zones in crystal
fractures near fusion crust.

O9;XN;l6/.45
Glass in a barred olivine chondrule fragment.

10;PL;16/.45
Same view in plane polarized light.

11;XN;2.5/.008
OPL material and associated fracture feed.

12;PL;2.5/.008
Same view in plane polarized light.

13;XN;6.3/.20
Same area under higher magnification, centered on opaque area.

14;PL;6.3/.20
Opaque area occupies void space in quasi-honeycomb pattern.
Opaque material is amorphous oxide deposit.

15;XN;16/.45
Staining is visibly heterogeneous in this wide field view.

16;PL;16/.45
Heterogeneity is more visible in plane polarized light.

17;XN;6.3/.20
Close up view of arrested staining front.

227

18;PL;6.3/.20
Same view in polarized light.

19;XN;6.3/.20
Kink bands in olivine chondrule fragment.

20;PL;6.3/.20
Same view in plane polarized light.

21;XN;6.3/.20
There is very little glass in this fusion crust, possible due to
weathering.

22;PL;6.3/.20
Same view in plane polarized light.

23;XN;16/.45
Unusual pattern of microvoid spaces in olivine chondrule, possibly
a good example of pitting.

24;PL;16/.45
Same view in plane polarized light.

25;XN;6.3/.20
Stained boxwork cleavage in pyroxene chondrule. The stain
distribution is cleavage controlled.

26;PL;6.3/.20
Stain control is more pronounced in plane polarized light.

27;XN;16/.45
Same crystal at higher magnification.

28;PL;16/.45
Same view in plane polarized light. Excellent example of NNL
migration habits.

29;XN;2.5/.008

Rust vein and fracture filling extends to edge of meteorite,
allowing direct avenue for fluid migration from surface to
interior.

30;PL;2.5/.008
Same view in plane polarized light.

31;XN;6.3/.20
Higher magnification of oxide-filled fractures.

32;PL;6.3/.20
Same view in plane polarized light.

33;XN;16/.45
Very thin, well crystallized OPL material deposited along fusion
crust surface.

228

34;PL;16/.45
Same view in plane polarized light.

35;XN;16/.45
Rectangular outline of well crystallized OPL as discrete zone
surrounded on both sides by amorphous oxide material.

36;PL;16/.45
Same view in plane polarized light.

lSB;XN;16/.45
Highly magnified view of amorphous oxides.

16B;PL;l6/.45
Same view in plane polarized light.

17B;XN;2.5/.008
Zoom in sequence on blackening near fusion crust.

18B;PL;2.5/.008
Same view in plane polarized light. Blackened area more visible.

19B;XN;6.3/.20
Higher magnification of same area.

208;PL;6.3/.20
Various zones within fusion crust are clear in plane polarized
light.

218;XN;6.3/.20
Iron oxide deposits near fusion crust.

ZZB;PL;6.3/.20
Same view in plane polarized light.

23B;XN;16/.45
Well crystallized to amorphous OPL material in association with
opaque phases.

24B;PL;16/.45
Same view in plane polarized light.

258;XN;16/.45
Another example of well crystallized and amorphous OPL material,
some of which is growing into a void space.

26B;PL;16/.45
Same view in plane polarized light. Color variations are visible
which are partially or entirely due to OPL phase thickness.

27B;XN;16/.45
Another example.

229

28B;PL;16/.45
Same view in plane polarized light.

29B;XN;16/.45

A wide variety of oxide details can be discerned in this view.
Principal among these are thick NNL deposits, parallel fracture
fillings, and well crystallized OPL material which has
apparently consumed primary opaque material completely.

308;PL;16/.45
Same view in plane polarized light.

3lB;XN;l6/.45
Another example opaque material in process of alteration.

3ZB;PL;l6/.45
Same view in plane polarized light.

3BB;XN;16/.45
Oblate massing of opaque particles.

34B;PL;l6/.45
Opaque particles are more easily visible in plane polarized light.

358;XN;40/.70
Amorphous oxide material in void space.

368;PL;40/.70

Same view in plane polarized light. Repetition of LEW86015,10/15B
and LEW86015,10/16B.

CARICHIC; H5

 

l;PL;2.5/.008
Wide field view showing weathering intensity plus attacked chondrules
and chondrule fragments.

2;XN;2.5/.008
Same view in polarized light.

3;PL;6.3/.20
Close up of chondrule fragment in same orientation. Note opacity of NNL
staining.

4;XN;6.3/.20
Same view in polarized light.

5;PL;6.3/.20
Another example at same magnification.

6;XN;6.3/.20
Same view in polarized light.

 

230

7;PL;6.3/.20
Another chondrule and portions of surrounding chondrules. Chondrules
show considerable clouding.

8;XL;6.3/.20
Same view in polarized light.

9;PL;2.5/.008
Another set of chondrules.

10;XN;2.5/.008
Same view in polarized light.

ll;PL;6.3/.20
Closeup of one chondrule.

12;XN;6.3/.20
Same view in polarized light. Color difference is due to sweeping
extinction.

13;PL;6.3/.20
Microporphyritic chondrule.

l4;XN;6.3/.20
Same view in polarized light.

15;PL;16/.45

This chondrules appears to have been either (1) attacked in a stair-step
fashion, or (2) fractured along cleavage planes to produce a stair-
step appearance which later became filled with rust.

16;XN;l6/.45
Same view in polarized light.

l7;PL;2.5/.008
Large chondrule containing what appears to be weathered, devitrified
glass.

18;XN;2.5/.008
Same view in polarized light.

19;PL;6.3/.20

Devitrified glass under higher magnification. While crystals appear
relatively untouched by weathering attack, some areas of clouding
occur as ribbon-like stripes which transect the grains. Note opacity
of fracture fillings.

20;XN;6.3/.20
Same view in polarized light.

21;PL;6.3/.20
Microporphyritic chondrule with very large euhedral olivine crystals.

231

22;XN;6.3/.20
Same view in polarized light.

23;PL;6.3/.20
Completely clouded pyroxene chondrule.

24;XN;6.3/.20
Same view in polarized light.

25;PL;6.3/.20
Another example with classic pyroxene cleavage. Cleavage planes are
filled with rust.

26;XN;6.3/.20
Same view in polarized light.

APPENDIX C

X-RAY DIFFRACTION DATA

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