MSU LIBRARIES m RETURNING MATERIAE§: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped be1ow. ._.. .4 a...“ _.p__-—..-._.__... SYNTHETIC DOLOMITIZATION : RATE EFFECTS OF VARIABLE MINERALOGY, SURFACE AREA. EXTERNAL CO32‘ AND CRYSTAL SEEDING By Timothy R. Bartlett A THESIS Submitted to Michigan State University in partial FuiFiIiment of the requirements For the degree of MASTER OF SCIENCE Department of Geologicai Sciences 1984 ABSTRACT Synthetic Dolomitization: Rate Effects of Variable Mineralogy Surface Area. External C032“ and Crystal Seeding by Timothy R. Bartlett The purpose of this study is to evaluate the contributions of several variables on replacement rates during synthetic dolomitization. The specific problem addressed concerns the origin of selective dolomite fabric produced during carbonate diagenesis. Dolomitization at 175°C resulted in the Following order of descending susceptibility to replacement: at equivalent surface areas. aragonite dolomitized more rapidly than Iceland spar calcite. which dolomitized faster than high-magnesian calcite (12 mole Z MgCO3). Reaction time generally decreased for finer size reactants (greater surface area). However, an inverse relation between reaction rates and surface area resulted for two calcite samples. The effects of contrasting reactant surface properties on reaction kinetics are invoked to account for this result. Dolomitization rates were accelerated by the addition of NaZCO3 to standard experimental conditions. Seeding reactions with dolomite crystals produced no rate Change. TABLE OF CONTENTS LIST OF TABLES . . . . . . . . . . . LIST OF FIGURES . . . . . . . . . . . . INTRODUCTION . . . . . . . . . . . . . . . PREVIOUS REPORTS OF SELECTIVE DOLOMITIZATION DOLOMITE SELECTIVITY-THEORY . . . . . . Potential Thermodynamic Controls of Selective Dolomitization . . . . Potential Kinetic Controls of Selective Dolomitization . . . . . . . EXPERIMENTAL DESIGN . . . . . . . . . . . Sample Preparation . . . . . . . . . Surface Area . . . . . . . . . . . . . Dolomitization Method . . . . . . . . . Analyses . . . . . . . . . . . . . . . RESULTS . . . . . . . . . . . . . . . . . . Reaction Sequence . . . . . . . . . . . Partial Dissolution and Intermediate Phases . . . . . . . Interval of Dolomite Precipitation Surface Area vs. Minimum Time for 100% Dolomitization . . . . . . . . Seeding and External Carbonate Ion . INTERPRETATION AND DISCUSSION OF EXPERIMENTAL RESULTS . . . . . . Nucleation of Dolomite Growth of Dolomite APPLICATION OF EXPERIMENTAL RESULTS . . . Rock Selectivity . . . . . . . Carbonate Ion and Dolomitization SUMMARY AND CONCLUSIONS APPENDIX BIBLIOGRAPHY 14 14 15 29 4O 41 43 44 48 49 49 SD LIST OF TABLES Table 1 Previous Experimental Work . . . . . . . Table 2 Thermodynamic Drive of Dolomitization Reactions . . . . . . . . . . . . . . . Table 3 Fixed Experimental Conditions . . . . . Table 4a BET Surface Area (SA) Results . . . . . Table 4b Geometric Comparison to BET Surface Areas Table 5 Dolomite Compositions Prior to 100% Dolomitization . . . . . . Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure O‘UlthH \I 13 I4 15 16 I7 18 I9 20 21 22 23 LIST OF FIGURES Reaction Progress. Reaction Progress. Reaction Progress, Reaction Progress, Synthetic Calcites Mg-calcite . . . . Ward’s Aragonite. . Reaction Progress, Aragonite . . . . . External Carbonate and Seeds . . . . . Dissolution etching on Iceland spar calcite . . . . . . . . . . . . . Unreacted Iceland spar calcite . . . . Dissolution etching of Ward's aragonite Unreacted Ward’s aragonite . . . . . . Partially dissolved Mg-calcite . . . . Unreacted HMC echinoid grains . . . . Earliest dolomite on HMC substrate . . Earliest dolomite on calcite substrate Early dolomite rhombs on aragonite . . Edge and corner dissolution rounding on Iceland spar calcite . . . . . . Final dolomite texture after Iceland spar calcite . . . Final dolomite texture after HMC . . . Final dolomite texture after aragonite Final dolomite texture after a reagent calcite . . . . . . . . . . . Minimum time for surface area . . . . . . . . . . . . . Extremely smooth calcite rhomb surfaces Stepped calcite surfaces . . . . . Iceland Spar Calcite 100% dolomitization vs. 16 I7 18 I9 20 21 24 24 26 26 28 28 32 32 34 37 37 39 39 47 47 INTRODUCTION Dolomitization of CaCO3 in nature can result in certain components of a sediment being selectively replaced while other components remain unaltered. This implies that the rate of dolomitization is not equal for all carbonate reactants within a diagenetic setting. The purpose of this study is to evaluate the influence of several variables on the rate of dolomitization with application to the origin of selective dolomite. The results of a series of experiments designed to test the effects of I) reactant mineralogy 2) reactant surface area 3) external C032‘ and 4) crystal seeding on dolomitization rates are reported and discussed. The underlying assumption is that the more rapidly a reactant is replaced by dolomite in these experiments. the more likely an analogous substrate would be replaced in nature. Previous Reports of Dolomitization Selectivity Selective replacement has been attributed to both reactant size and mineralogy. A commonly cited case of selective dolomitization is the replacement of lime mud by dolomite and non—replacement of adjacent allochems and calcite spar (Murray and Lucia. 1967; Schofield and Nelson, 1978; Armstrong et al, 1980). This observation is consistent with the hypothesis that crystal size (surface 2 area) of the substrate determines its susceptibility to dolomitization. There are many examples in nature where dolomitization has apparently occurred on a mineralogically selective basis. Kocurko (1979) reported the occurrence where aragonite allochems were dolomitized before high-magnesian calcite (HMC or Mg-calcite. approx. 10-30 mole % MgCO3) and low-magnesian calcite (LMC or calcite. <4 mole % MgCO3). Others have noted that HMC allochems were more readily dolomitized than allochems of LMC (Schlanger, 1957; Schmidt. 1965; Land. 1966; Land and Epstein. 1970; Buchbinder, 1979; Kocurko. 1979; Sibley. 1980 and 1982; and Seller. 1984). Variations in the rates of CaCO3 replacement during synthetic dolomitization can also be equated to dolomite selectivity. Reactant size and mineralogy may again be important. Mineralogical effects were noted by Gaines (1980). who reported that aragonite dolomitized more rapidly than HMC (30 mole % MgCO3) while dolomitization of LMC was relatively very slow. Studies by Katz and Matthews (1976) and Baker and Kastner (1981) also showed that aragonite dolomitized much faster than calcite. Dolomitization of Mg-calcites composed of greater than 29 mole % MgCO3 was reported by Land (1967) to have proceeded more rapidly than for aragonite. Mg—calcites having less MgCO3 content were progressively less reactive. 3 Evidence for crystal size control on dolomitization rates can be found in the work of Bullen (1983). Cryptocrystalline HMC red algae (17 mole % MgCO3). and cryptocrystalline LMC red algae artificially recrystallized from the HMC originals. each dolomitized within equal time intervals. This indicates that the mineralogical difference did not noticably effect the replacement rate. Perhaps dolomitization rates were equal because both reactants had nearly the same surface area. since no textural destruction occurred during conversion of HMC to LMC. Size effects were also apparent in the work of Grethen (1979). His results showed that finely crystalline calcite (approx. sum) was replaced in less than one-third of the time required for dolomitization of a coarsely crystalline calcite (75-150um). Dolomitization rates may also be influenced by the aCO32' and presence of dolomite seed crystals (Morrow. 1982; Lippmann. 1973; Gaines. 1980). Recognition of the rate effects of these variables may therefore provide clues to the origin of selective dolomite. An abridged summary of the experimental conditions from the cited studies is presented in Table 1. Data has been selected to further illustrate the contrasting reactivities of the carbonate reactants used under the Specified sets of conditions. The present study is the first to discriminate the contributions of surface area and mineralogical variation on experimentally determined replacement rates. 4 Tlatalee 1 Previous Experimental Work Calhg Temp. Time for 1001 dolo. Study Reactant Grain Size (soln) Conc. (’C) or I completion aragonTte pelecypod 3-5 on fraguents. > 247 hrs. nicrocrystalline LHC structure pelecypod ) 320 hrs. echinoid Bullen. 3.13 22 hrs. (1983) LHC to 20 250 red algae 3-5 ml fragments. 2.33 cryptocrystalline . structure foran 92 hrs. echinoid HHCID.17) 22 hrs. red algae k r and LIIC uder. 0.51 n2 4.7-6.1 figs-Inn. 9° ’9 0.17 =0.l 200 days (1981) aragonite no data < 2 days aragonite. . seeded 901 in 214 hrs. Gaines. no data 0.20 20 100 (1980) aragonite. unseeded 01 in 214 hrs. aragonite 0.25 ’TUU’hrs. Gaines. 'uniforn' I ZH 100 (1974) HHC10.321 0.20 400 hrs. Hfit10.ldl 1-15um crystals 0.14 14 days calcite Sun rhombs g 14 days , no Ca2+ calc1te lS-lSOuu rhombs ) 14 days Grethen. ZH 150 (1919) HHC(0.09) echinoid lS-lSOun grains 0 20 > 14 < 28 days aragonite 1-10um rods . < 8 days Katz and calcite 100 hrs. Matthews. no data 3.8 20 252 (1976) aragonite 24 hrs. aragonite coral 1001 in HHCIO.29) 26 hrs. red algae Land. skeletal fragments 1.0 0.10 300 (1961) HHCIU.III 661 in foram 26 hrs. calcite 74291 in 26 hrs. DOLOMITE SELECTIVITY - THEORY Selective dolomitization results from contrasting susceptibility to replacement among the variety of carbonate particle types found in nature. Within a particular diagenetic setting. selective replacement by dolomite may be influenced by the mineralogy and crystal size distribution of the pre-dolomitization assemblage. The mineralogy of the reactant will effect the free energy of the reaction. whereas the crystal size of the reactants may effect surface area controlled kinetics of dissolution and/or precipitation. Potential Thermodynamic Controls on Selective olomiti ation Reaction 1 is used to demonstrate how differences in reactant mineralogy could cause selective dolomitization. For aragonite reactants x=0. and for calcites. the fraction of Mg2+ to Ca2+ incorporated in the lattice is substituted for x. 2+ 2+ + -2 M = + 1-2 1 ZCal_ngxCO3 (l x) g CaMg(CO3)2 ( x)Ca ( ) The free energy changes for these reactions in solutions of differing MgZT/Ca2+ are summarized in Table 2. All solutions are supersaturated with dolomite. 5 Table 2 Thermodynamic Drive of Dolomitization Reactions AGdz = RT ln {K(IAP) / K(eq)) Environ Mineral KIAp Keq AGdz HMC(11)5 0.248 15.69 -2.46 Sea HMC(15) 0.286 30.33 -2.76 Water Arag 0.167 21.99 -2.89 LMC 0.167 10.29 -2.44 4 HMC(11) 0.794 15.69 -1.77 951 F.W. HMC(15) 0.813 30.33 —2.14 5% S.W. Arag 0.743 21.99 -2.01 LMC 0.743 10.29 -1.56 Laguna HMC(11) 0.112 15.69 -2.92 Madre HMC(15) 0.141 30.33 -3.18 Brine Arag 0.061 21.99 -3.49 LMC 0.061 10.29 ~3.04 1) KIAp for sea water and brine from Gudramovics. 1981; KIAp for mixing zone from Badiozamani, 1973. 2) Keq of mineral + dolomite is calculated from thermodynamic data in Carpenter. 1980; and Morse. 1983. 3) AGdz in kcal/mole. 4) 95% fresh water + 5% sea water. from Badiozamani. 1973. 5) HMC(11) and HMC(15) are high-Mg calcites with 11.0 and 15.0 mole % MgCO3. respectively. The order of decreasing AGdz per solution reflects increasing disequilibrium and therefore an increasing potential for dolomite to replace the reactant. Uncertainty in the dolomite and HMC solubility products limits strict application of these calculations; however. both the magnitude and order of decreasing AGdz are shown to change as a function of mineralogy and solution chemistry. For example. in the brine solution. the order of increasing - AGdz is: HMC(II) > LMC > HMC(IS) > aragonite. Using freshwater-seawater solutions. the order becomes: LMC > HMC(II) > aragonite > HMC(15). -Preferential replacement of one mineral over another suggests a rate effect on the selective process. The association of increasing reactant metastability with potentially rapid reaction rates may thus account for mineralogically selective dolomitization. Potential Kinetic Controls on Selective Dolomitization Substrate characteristics that promote dolomite nucleation may effect dolomite selectivity: for example. addition of dolomite seed crystals greatly accelerated the dolomitization rate of aragonite at 1000 C relative to unseeded experiments (Gaines. 1980). However. no rate effect was detected in seeded experiments at 2950C by Katz and Matthews (1976). The temperature difference between studies possibly accounts for the conflicting results because nucleation rates increase exponentially 8 with increasing temperature (Berner. 1980. p. 95. equation 5-15). Rates of heterogeneous nucleation are also predicted to increase as the degree of lattice similarity between substrate and product becomes greater (Nielsen. 1964. in Berner. 1980). The presence of dolomite seed crystals. at relatively low temperatures. should therefore reduce any lattice-related nucleation barrier to a minimum level and result in accelerated replacement rates. Analogously. selective dolomitization of Mg-calcites over calcite and aragonite substrates may be a result of the closer approximation of the Mg-calcite structure to the dolomite lattice. The physical nature of a reactant’s surface may effect its susceptibility to dolomitization. Steps. kinks and dislocations are, features of crystal surfaces which are energetically favored as sites for dissolution and/or heterogeneous nucleation. Relative to planar and unstrained surfaces. enhanced reactivity of such sites can speed surface reations (Berner. 1980: Morse. 1983). A rapidly dissolving reactant will supply ions to solution faster than a slowly dissolving one. As a result. the ion activity product of dolomite in the bulk solution. and possibly at the interface level. will increase at a relatively greater rate. The sensitivity of nucleation rates to the degree of oversaturation of the precipitating phase (Berner. 1980) could therefore cause 9 dolomite to nucleate sooner upon the rapidly dissolving substrate. Dolomite nucleation may also be promoted by the activity of the carbonate ion. independent of the substrate dissolution rate. Beyond its contribution to the IAP of dolomite. C032‘ ions may enhance the dehydration of Mg2+ ions (Lippmann. 1973). thereby increasing the rate of dolomite nucleation and growth. Precipitation experiments by Oomori et a1 (1983). Nechiporenko and Bondarenko (1984). and Lippmann (1973. Part 0) support this hypothesis. To summarize. a fine grained sediment will expose a greater surface area to a diagenetic solution than an equal mass of a coarser grained sediment and will therefore be favored for dolomitization. Reactants having high densities of reactive sites per unit surface area are also highly susceptible to replacement. Both reactant types will undergo relatively rapid dissolution and provide abundant sites for potential dolomite nucleation. EXPERIMENTAL DESIGN A standard set of experimental conditions was employed to determine the time required for complete replacement by dolomite of several calcium carbonate samples (Table 3). 10 Table 3 Fixed Experimental Conditions Temperature 1750C Mg/Ca. solution 4 Ionic strength 3 Sample mass 0.35 9 Solution volume 6.0 ml Reagents 1M CaCL2:2H20 plus 1M MgClZ-6HZO .Sample Preparation Three mineralogies were dolomitized: radial-fibrous aragonite (Ward’s Scientific). synthetic aragonite needles. reagent grade calcite (Ward’s Scientific and Mallinckrodt). synthetic calcite. optical grade Iceland spar calcite. and biogenic Mg-calcite (echinoderm. 11.8 mole 1 MgCO3). For the remainder of this paper. the Ward’s aragonite. Iceland spar calcite and biogenic Mg-calcite will be referred to as aragonite. calcite and HMC. respectively. unless otherwise stated. Aragonite. calcite and HMC were manually ground and sieved into 45-75 um and <45um size intervals. Multiple decantation with doubly distilled water removed the vast majority of the fine particles produced during crushing and homogenized the grain size distribution of the residuum. These suspensions were then retained as samples. with crystal diameters in the I-IDum range. Organic matter within the echinoderm skeleton was removed by soaking fragments in a Chlorox solution for 24 hours. 11 followed by thorough rinsing. All crushed samples (aragonite. calcite and HMC) were oven dried for 24 hours 0 at about 75 C after decantation and rinsing. Sggface Area Surface area of all samples were calculated by application of the BET equation (see Adamson. A.W.. 1982. Chap. XVI) to nitrogen adsorption data obtained from the Perkin-Elmer Model 2128 "Sorptometer". Results of the surface area determinations appear in Table 4a. The range of surface areas obtained was between 2.66 and 0.15 mz/g. corresponding to average crystal diameters of about Sum and 75um. respectively. The average percent difference for all samples run more than once is equal to $8.21; the average absolute difference for the same runs equals :0.107 square meters per gram. Several estimates of geometric surface area from microscopic examination are listed in Table 4b. The calculations are based on assuming cylindrical shapes for sample AF2 (aragonite needles). and cubes for samples C13 and CF2 (equant calcite rhombs). The consistency of the geometric to BET ratios suggests internal consistency of adsorption data and surface area calculations. Dolomitization Method Approximately one—hundred experiments were run to test for surface area and mineralogical influences on 12 Table 4a BET Surface Area (SA) Results Mineral SA. mZ/g # runs Source Symbol Aragonite coarse 0.17 t 0.017 2 Ward’s Sci. AC intermediate 1.06 2 " AI intermediate 0.29 1 " A12 fine 1.2 t 0.015 2 " AF fine 1.8 i 0.22 3 synthetic AF2 H.119 coarse 0.54 1 echinoderm HC intermediate 0.79 1 " H1 fine 1.85 i 0.05 2 " HF Calcite coarse 0.15 1 Iceland spar CC intermediate 0.73 1 " CI intermediate 0.25 1 reagent C12 intermediate 0.59 120.075 2 synthetic C13 fine 2.66 120.045 2 Iceland spar CF fine 1.87 ii0.34 3 reagent CF2 Average absolute difference equals multiply run samples. for all difference equals Table 4b Geometric Comparison to BET Surface Areas :8.21 i 0.107 m 2/g average 1 Sample Geometric Estimation. mZ/g Geometric/BET AF2 8.5 4.7 C13 2.52 4.3 7.5 4.0 CF2 13 dolomitization rates. The dolomitization method consisted of placing a known mass of a sample. plus solution. into stainless steel Stellate-type bombs. These were placed into a muffle furnace set at 175°C. Numerous runs were performed for each sample and many were quenched at various intervals prior to 1001 dolomitization. In this way. the effect of surface area variation on the rate of replacement could be ascertained per mineral: and. at a given surface area. the relative effect of reactant mineralogy on the replacement rates could be directly compared. In addition. twenty-one experiments were run to determine the effects of 1) an external source of the carbonate ion and 2) seeding experiments with dolomite. The affect of external C032“ was determined by adding 10 weight 1 (of the sample mass) Na2C03 to normal starting conditions just prior to bomb sealing. This amount was sufficient to cause initial supersaturation with calcite: however. no precipitate was observed from the bomb solution within twenty minutes after Na2C03 addition at room temperature and pressure. Several experiments seeded with synthetic dolomite (5 weight 1 of the sample mass) produced during previous runs were also run alongside the external CO32‘ runs. Seeds consisted of both disordered and well—ordered dolomite. Control runs lacking NaZCO3 and seeds were run simultaneously with the other runs . l4 Analyses X-ray diffraction analyses for phase identification. composition and ordering (Graf and Goldsmith. 1956: Gaines. 1974) were made after bomb quenching. product filtration. rinsing and air drying. Detailed petrographic descriptions are included in Appendix 1. Reactants and products were examined with an SEM. Percent dolomite was estimated petrographically with the aid of Alizaren red-S staining. This method was practical among the coarser grained reactants when dolomite appeared to be sparsely distributed on reactant surfaces. Dolomite content was also estimated by comparing product diffractograms to prepared calibration curves. where dolomite to reactant peak height ratios were plotted against their known weight proportions. This method was employed among the finer grained reactants and when the dolomite content visually exceeded about 10-20 percent. RESULTS Reaction Sequence Plots of percent dolomite with respect to time. per mineral and surface area category. are presented in Figures 1—6. The form of these plots is similar among the different reactants. Each figure consists of points 15 located along the x—axis representing runs containing no dolomite. followed in time by an abrupt change to points which correspond to runs containing 1001 dolomite. Few points appear in intermediate positions. The lines in Figures 1-6 were hand drawn for schematic purposes only. Microsc0pic and X-ray diffraction analyses revealed that two distinct reaction stages accounted for the characteristic shape: 1) partial reactant dissolution and precipitation of intermediate phases occurred in the runs located on the x-axis. and 2) dolomite precipitation and accelerated reactant dissolution occurred in the interval between the last point on the x-axis and the first appearance of 1001 dolomite. Partial dissolution and intermediate phases A small amount of substrate dissolution occurred preceding the crystallization of intermediate phases. Further reactant dissolution followed. Figures 7-12 typify the occurrence of intermediate phases and partial grain dissolution during the pre-dolomite stage of the reaction sequence. Calcite dissolution is marked by distinct lattice related etch features and retention of sharp edges (Figure 7. compare with unreacted Iceland spar calcite. Figure 8). Dissolution of aragonite however produced rounded corners and surface pits (Figure 9. compare to Figure 10. unreacted 16 100 um 00 0 El SIMPLE CF (9 sflflPLE CI PCT. DOLONITE~ 000 TIME (HOUR8) Figure la :00» E] m Inimuumszcc PCT. OOLOHITE 28’ Oi—B—WB—g—h—fi-Bh—fl—d O 40 1 180 240 200 320 300 TIME (HOURS) Figure lb Figure 1 Iceland Spar Calcite PCT. DOLOHITE PCT. OOLOHITE (00 78 100 E1 SRHPLE CF2 (D ORHPLE 812 1. SRHPLE C13 b l7 [D 010 ID i 2 1m m‘m TIHE (HOURS) Figure 2a (9 C) (D 2. mg. as. .3. a... 80 TIME (HOURS) Figure 2b Figure 2 Synthetic Calcite PCT . OOLOHI TE OOLOHITE PCT. 18 100 - O a!) El 1!] (D o snhPLE HF I!) SAMPLE H1 75 - E1 so > 0 u b o L L LL L L L L L L L L A L L L L A L j 0 so no I too too 210 no 270 TIME (HOURS) Figure 3a 100 - A an O 0 0 SRHPLE HC 75 . so - O 26 . o L L L A L L .L L 150 180 no 3 too 2% no 220 zoo 240 390 TII‘IE (HOURS) Figure 3b Figure 3 HMC PCT . OOLOH 1 TE PCT. OOLOHITE 100 100 78 25 19 El BRHPLE 9F O 10 { '0 0 SRHPLE RI 0 SIMPLE 912 /fD A L L L A I. L L A L L L L A to so TIME (HOURS) Figure 4a 0 O 001!) \ L 1 A L I!) L A L L 1 A J \I" 80 70 80 90 to so 40 so TIME (HOURS) Figure 4b rigure 4 0 Hard s Aragonite PCT. OOLOMITE OOLOMITE PCT. 100 100 78 25 20 . O O ananE 9F2 ‘0 b ' o L JHhH—‘i— L L L L A o I so 38 to II 30 3‘ so 6‘ so TIME (HOURS) Figure 5a Synthetic Aragonite Needles ' E) 1!) SIMPLE 90 E] 30 “60 U “1% no 120 70 00 TIME (HOURS) Figure 3b Ward's Aragonite PCT. OOLOMITE DOLOMITE PCT. 21 ‘mp .0 In H/CflRO out/assoc m '6 CONTROL is»~ I! so» O! a» / 0 In / /: 0o E 05 1s! a; a; so as so as so TIME (HOURS) Figure 6a Sample AI too Ion/cans (D H/OEEOS O CONTROL 76- so» as» 0 c ‘ ‘ “.36 130v 0 no to so so rs C..(C TIME (HOURS) Figure 6b Sample CF' Figure 6 ernal Carbonate and Seeds 22 Ward's aragonite). HMC dissolution occurred preferentially within certain regions of the grains to form internal voids (Figure 11. see Figure 12 for unreacted HMC). Inner surfaces of these features appeared ragged while the outer surfaces remained smooth and apparently non-etched. Aragonite dissolved more rapidly than calcite and HMC. The rate of HMC dissolution was the slowest among these reactants. Abundant etch pits were observed on aragonite (sample AC) after 37 hours of reaction. Over the same amount of time. calcite of comparable surface area (sample CC) had no etch features. Fine grained HMC (sample HF) showed no evidence of dissolution within 92 hours. whereas a coarser grained calcite (sample CI) was strongly etched within 80 hours. There was never an example of total dissolution of a primary substrate before dolomite appeared. The composition and mineralogy of the intermediates which precipitated varied with the reactant. Magnesite precipitated during LMC runs. ranging in composition between 6 and 15 mole 1 CaCO3 (identification and composition determined by the presence and location of the (104) peak; Grethen. 1979). The compositional range of magnesite formed in HMC runs was 9.4 to 12 mole 1 CaCO3. The earliest magnesite crystals were sometimes observed to have formed preferentially along calcite twin planes; otherwise. a preferred orientation did not result. Individual crystal sizes ranged between 1 and 10um. 23 Figure 7 Early dissolution of Iceland spar calcite. Grains are angular and show lattice—like etch features. 1000X. scale bar equals 20um. Figure 8 Unreacted Iceland Spar calcite. 700x. scale bar equals 28.6um. Figure 7 Figure 8 25 Figure 9 Dissolution etching of Ward's aragonite. Note the rounded corners and surface pits. 2000X. scale bar equals 10pm. Figure 10 Unreacted Ward’s aragonite. radial- fibrous texture is displayed. 2000X. scale bar equals 10um. 26 Figure 9 Figure 10 27 Figure 11 Partially dissolved Mg-calcite. Regions of preferential dissolution are indicated by the arrows. Bright colored material is magnesite. 3000X. scale bar equals 6.7pm. Figure 12 Unreacted HMC echinoid grains. Bright material is fine grained HMC adherred to large grain surfaces. 400x. scale bar equals SDum. 28 Figure 11 29 occurring as subhedral rhombs with highly stepped surfaces. Magnesite packing on substrate grains from a single run varied from dense coverings to nearly barren surfaces. The total content never exceeded approximately 5-101 of total solids present. Magnesite appears in Figs. 7 and 11 as the lighter toned. randomly distributed subhedral crystals. Experiments with aragonite reactants did not produce magnesite. Instead. an amorphous material formed during the Ward's aragonite runs. and HMC (15 mole 1 MgCO3) formed during runs using synthetic aragonite needles. Crystals of the HMC intermediate displayed a euhedral prismatic habit and attained long dimensions of 10-15pm. substantially coarser than the reactant needles (2-5um in length). The amorphous material appeared as clustered spherulites with diameters of approximately 2-5um. Interval of dolomite precipitation The second stage in the reaction sequence involved dolomite precipitation and accelerated substrate dissolution. The abrupt change from 01 to 1001 dolomite in Figures 1-6 indicates that precipitation of dolomite proceeded rapidly once initiated. The time span between the first appearance of dolomite and 1001 replacement was commonly about 15 to 20 hours. which is in general. much Shorter than the partial dissolution stage. Coarse grained HMC. for example. dolomitized in the interval between 30 190 and 210 hours (Figure 3b). Figures 13 and 14 show the distribution of early. minute dolomite crystals (0.25-Sum) on HMC and calcite hosts. respectively. Dolomite is distinguished from magnesite in the SEM images by its euhedral form and strong preferential alignment. Nucleation of dolomite on aragonite produced a non-preferentially aligned fabric (Figure 15). In all experiments. the dolomite nucleated directly on the primary substrate surface rather than upon intermediate phases. Compositions of dolomite detected prior to 1001 replacement of the reactant are listed in Table 5. All such occurrences were Ca-rich relative to ideal dolomite: however. a uniform composition common to each reactant mineralogy did not result. Instead. a distinct compositional range characterized each reactant (see Table 5). Reproducibility of the compositions from X—ray diffraction equals 1.2 mole 1 CaCO 3 . The dolomite was either poorly-ordered or disordered at this point in the reaction. Table 5 Dolomite Compositions Prior to 1001 Dolomitization Reactant Average composition (mole 1 CaCO3) No. runs HMC 52.3 4 Calcite 55.2 8 Aragonite 61.5 11 Figure Figure 13 14 31 Earliest dolomite detected on HMC substrate. The dolomite is euhedral and shows a strong preferential alignment. Other crystals are magnesite. The finest crystals of dolomite are about 0.25pm in diameter. 5000X. scale bar equals 4“”). Earliest dolomite on Iceland spar calcite. The dolomite shows a strong preferred orientation and forms directly on the calcite surface (Fig. 13 also). Note the rounded corners of the substrate. 3000X. scale bar equals 6.7um. 32 Figure 13 Figure 14 33 Figure 15 Early dolomite rhombs on Ward’s aragonite. The dolomite is euhedral and randomly oriented. 7000X. scale bar equals 2.9um. Figure 16 Edge and corner dissolution rounding on Iceland spar calcite at the onset of dolomite precipitation. 1000X. Scale bar equals 20um. 34 Figure 16 35 Concurrent with rapid crystallization of dolomite was rapid substrate dissolution. i.e.. substrate dissolution rates were accelerated during the dolomite precipitation stage. Calcite grains became distinctly rounded. in contrast to the sharp edges and corners produced during the earlier pre-dolomite stage (refer to Figs. 14 and 16). Aragonite and HMC dissolution textures remained unchanged. Magnesite dissolution was completed before LMC was entirely replaced by dolomite. However. it persisted during HMC runs for approximately 100 hours after all HMC had been dolomitized. HMC produced during synthetic aragonite runs existed with dolomite for about 24 hours after the aragonite had been completely replaced. Figures 17-20 are SEM images showing representative textures of final dolomite products. Replacement of individual crystals of coarse calcite and HMC resulted in highly oriented polycrystalline dolomite grains (Figs. 17 and 18). The dolomite preserved only a crude outline of the primary grain. Dolomite replaced HMC to produce rhombic shaped grains. despite originally anhedral shapes. Non-undulatory unit extinction was exhibited by the polycrystalline grains. Coarsely crystalline aragonite was replaced by randomly oriented dolomite rhombs. Virtually no precursor texture was preserved by the resulting interlocking mosaics (Figure 19). Finely crystalline reactants tended to be replaced by individual dolomite rhombs. Figure 20 exemplifies the Figure Figure 17 18 36 Final dolomite texture after Iceland spar calcite. The oriented crystals produce unit extinction of entire grains unden crossed polars. 700x. scale bar equals 28 . 6111'“ . Final dolomite texture after HMC. Grains have taken on rhombic shapes. contrasting with originally anhedral shapes. These grains typically Show unit extinction under crossed polars. 1000X. scale bar equals 20pm. 37 Figure 18 Figure 19 Figure 20 Final dolomite texture after Ward’s aragonite. Random orientation of the dolomite rhombs has totally destroyed the primary aragonite texture. A remnant of the reactant appears in the lower right corner. 1000X. scale bar equals 20pm. Final dolomite texture after a reagent calcite. Single crystal replacement contrasts to the polycrystalline product of Figs. 18 and 19. 3000X. scale bar equals 6.7um. 39 Figure 19 20 Figure 40 replacement dolomite texture after a reagent calcite powder (Sample CF2). Dolomitization of fine grained reactants also appeared to produce a slight grain coarsening. Ordering of the dolomite product was assigned on a relative scale based on the position and definition of the (006) and (015) superstructure peaks. Dolomite progressed with reaction time from an initially disordered phase to a well-ordered final product. achieving its maximum degree of ordering within approximately 25 hours after it was first detected. The final products of this study. however. never displayed superstructure peaks equal in intensity and definition to those of ancient dolomite samples. Dolomite composition progressed from an early Ca-rich phase (Table 5) to a nearly stoichiometric final product. This change occurred within the interval between the first detection of dolomite and the earliest occurrence of 1001 dolomite for that reactant. No trend in compositional variation was observed beyond this point. The final dolomite composition for each mineralogy was quite uniform. The average final dolomite composition for HMC was 52.2 mole 1 CaCO3. for calcite. 51.3 mole 1 CaCO3. and for aragonite. 51.7 mole 1 CaCO3. Surface Area vs. Time for 1001 Dolomitization Figure 21 is a plot of surface area (Table 3a) versus the minimum time required for complete dolomitization (from Figures 1—6). Negative slopes of the HMC. calcite and 41 aragonite curves imply that with increasing substrate surface area. less time was required for dolomitization. Synthetic calcites reacted oppositely. i.e.. complete replacement of the high surface area calcite required the most time and vice versa (positive slope. Fig. 21). The reaction time for synthetic aragonite needles also did not plot on the Ward’s aragonite curve. requiring as much time to dolomitize as a substantially coarser Ward’s sample. Seeding and External Carbonate Ion Samples CF and AI were used to compare the effects of dolomite crystal seeding and addition of NaZCO3 on the rates of dolomitization. The sequence of reaction events during these experiments replicated those described above for calcite and aragonite reactants. SEM indicated however that dissolution of the substrate was more pronounced in the seeded runs than external CO32‘ runs over equal periods of time. Seeding did not noticably accelerate the reaction rate relative to control runs for either calcite or aragonite reactants (Fig. 6). Addition of Na2C03 did however result in significantly accelerated rates (Fig. 6). The first appearance of dolomite was always detected much sooner in these runs. resulting in a reduction in the total time for replacement by about one-third. HOURS 02. TIME FOR 100 PCT. 240 210 180 150 120 90 30 ./+\ clams X toanimmo ‘TMMC +*CRLCITE A\ X SYN CRLCITE 13 X L L L L L A l A 1 A oa no 1J5 24) 25 SURFRCE HRER. SOURRE METERS PER GRRM 3.0 Figure 21 Minimum Time for 1002 Dolomitization vs. Reactant Surface Area INTERPRETATION AND DISCUSSION OF EXPERIMENTAL RESULTS An examination of Fig. 21 suggests that mineralogy is more important than surface area in determining the relative susceptibility to dolomitization between the aragonite. calcite and HMC reactants. The vertical separation between reactant curves typically accounts for more reaction time than is represented by the rise of the slope along an individual reactant curve. Superpositioning of these curves does not occur. nor do they intersect within the range of surface areas tested: such would be expected results if crystal size were hypothesized as the stronger rate determining variable. Surface area variation is obviously important per mineral however. because the curves for aragonite. Iceland spar calcite and HMC each slope negatively. Non-intersection of these curves for all surface areas. indicates that aragonite dolomitized more rapidly than Iceland spar calcite. which in turn dolomitized faster than HMC (12 mole 1 MgCO3). Neither mineralogy nor surface area emerged as the dominant rate controlling variable however. The coarse grained Iceland spar calcite and coarse synthetic calcite dolomitized at very different rates. although they were of identical composition and had approximately equal surface 43 44 areas. A strict mineralogical control did not therefore have a consistent effect on replacement rates. Furthermore. the oppositely sloping curve of the synthetic calcites (Fig. 21) indicates that surface area variation did not have a consistent effect on the replacement rates either. The following two sections address the processes that were recognized during dolomitization and their possible roles in determining reaction rates. Nucleation gf Dolomite Dolomite seeding had no apparent effect on reaction rates. In addition. the earliest nucleation of dolomite was detected on an aragonite substrate although its lattice structure is least similar to that of dolomite among the reactants tested. Nucleation of dolomite occurred latest on the HMC of equivalent surface area. Therefore. relative nucleation rates were not influenced by the lattice similarity between the substrate and dolomite. These results tend to support the hydration barrier hypothesis proposed by Lippmann (1973) as an obstacle to crystallization. The strong preferential alignment of dolomite crystallites on HMC and calcite. but random orientation on aragonite. suggest that the lattice similarity between substrate and dolomite determined the nucleation fabric. The fabric of the final product was subsequently controlled by that of the earliest crystals (refer to Figs. 18-20). 45 The observed order of dolomite nucleation rates may have been determined by the relative rates of substrate dissolution. Since C032” was derived solely from the reactant in the standard experimental runs. the aCO32“ and saturation state of dolomite would be directly effected by the dissolution rate. These factors should have consequently influenced the rate of dolomite nucleation. By this mechanism. nucleation. occurred sooner upon aragonite than other reactants because of the greater rates of C032“ production and increasing IAP of dolomite. via rapid dissolution. Results of the NaZCO3 runs support this interpretation. Dolomitization was accelerated by the addition of Na2C03. However. immediate nucleation did not occur and etching of calcite was observed prior to the detection of dolomite. The rate increase in the Na2C03 runs may have occurred because I) the substrate dissolution step was eliminated as the single source of C032‘ and 2) dolomite oversaturation was greater relative to standard runs. The inverse relation between surface area and dolomitization rates of the synthetic calcites (Fig. 21) is attributed to the effects of surface properties on dissolution and nucleation rates. SEM revealed the surfaces of the fine grained synthetic calcite to be extremely smooth and perfect (Fig. 22). By contrast. the more reactive but coarser grained sample had imperfect. 46 Figure 22 Sample CF2. Rhomb surfaces appear extremely smooth even at high magnification. 10.000X. scale bar equals Zum. Figure 23 Sample C12. Crystal surfaces are stepped and non-uniform relative to Sample CF2. 3000X. scale bar equals 6.7um. 47 Figure 22 23 Figure 48 highly stepped surfaces (Fig. 23). It is consistent with dissolution and nucleation rate modelling (Morse. 1983; Berner. 1980) that rapid dolomitization of the coarse calcite resulted from its rough surface topography. causing rapid dissolution and/or providing more potential nucleation sites. Conversely. the extreme surface smoothness of the finer calcite provided fewer reactive sites. thus accounting for its slower reaction rate. The compositions of dolomite produced early in the reactions may have also been related to substrate dissolution rates. The earliest dolomite on aragonite contained 61.5 mole 1 CaCO3 in contrast to a value of 52.3 mole 1 CaCO3 for HMC substrates. Rapid dissolution of aragonite may have reduced the MgZT/Ca2+ in solution. relative to the slowly dissolving HMC. directly effecting the degree of non-stoichiometry of the earliest dolomite crystals. By the time the reactants had been completely replaced however. the dolomite had attained a rather uniform. near-stoichiometric composition. The relationship between dolomite non-stoichiometry and substrate mineralogy therefore appeared to be a relatively short—lived feature of the overall process. Growth of Dolomite Overall replacement rates did not depend specifically on the growth rate of dolomite crystals. The growth rate 49 of dolomite appeared to be rapid and similar for all mineralogies. once precipitation was initiated. Variation in the replacement rates were consequently a result of the differing lengths of time required for dolomite to nucleate on the various reactants. A change in calcite dissolution textures accompanied the rapid growth of dolomite. It is suggested that the rate of calcite dissolution prior to dolomite precipitation was limited by a slow surface process. in accordance with the lattice-related etch features seen during that stage (see Fig. 7). As the concentration of the carbonate ion gradually increased via calcite dissolution. nucleation of dolomite ensued. A greater degree of disequilibrium with calcite could have then occurred locally as C032‘ and CaZT were quickly removed from solution by the growing dolomite. Dissolution of calcite could then proceed rapidly to produce rounded shapes. APPLICATION OF EXPERIMENTAL RESULTS Rock Selectivity Experimental results suggest the importance of surface area variation in determining a reactant’s susceptibility to dolomitization. despite the inverse relation exhibited 50 by the synthetic calcites (Fig. 21). Murray and Lucia (1967) present a case where dolomite of the Turner Valley Fm. (Mississippian) is concentrated in wackestone facies and occurs very sparsely in the grainstones. Dolomite within the wackestones shows a strong preference for selectivity of the mud matrix. It is consistent with the experimental results of the present study that this distribution. and others like it (for example. Armstrong et a1. 1980). could be a consequence of rapid dissolution in the mud supported sediments. Locally high aCO3z’ would then favor their replacement over the coarse grained fractions of the same rock. Sibley (1980. 1982) reports selective dolomitization that post-dated fresh water diagenesis of forereef deposits of the Seroe Domi Fm. (Pliocene). Red algae were inferred to have recrystallized from primary HMC to LMC during fresh water diagenesis and were later dolomitized. but LMC spar cement that precipitated during the same fresh water episode resisted dolomitization. In this example. LMC red algae dissolved and dolomitized more rapidly than sparry calcite because of the presumed higher surface area of these allochems. Carbonate Ion and Dolomitization Accelerated reaction rates which resulted from the addition of NaZCO3 indicates the potential importance of the aCO32_ to dolomitization in natural diagenetic 51 settings. Small quantities of protodolomite have been found in unconsolidated Recent sediments within the fresh water/brackish zones below tidal flat hammocks of Andros Island (Gebelein et al. 1980). A characteristic of this setting is high carbonate alkalinity. about five times that of normal marine water. The pCOZ is also about one-hundred times that of normal atmospheric conditions. Dolomite is not detected in the sediments exposed to normal sea water that surrounds the brackish zones. Rapid dissolution and precipitation of aragonite within the needle mud sediment occurs on a seasonal basis. The dolomite contains 56-62 mole 1 CaCO3. very similar in composition to the initial. rapidly precipitated dolomites produced in the aragonite experiments of this study. It does not occur as a replacement product however. but as a cement. High carbonate alkalinity and rapid reaction rates. associated with the spatial distribution of this example of Recent dolomite. suggest a natural analog to the experimental results of the present study. Holocene diagenesis of late Pleistocne rocks (Falmouth Fm.) in the mixing zone beneath Jamaica represents another setting where high aCO32‘ may promote early. near-surface dolomitization. Meteoric waters on Jamaica are grossly oversaturated with calcite due to extremely high pCOZ (Land. 1973a) and should therefore have high carbonate alkalinity. Mixtures of these waters with moderate amounts of sea water are oversaturated with 52 dolomite (Land. 1973a). Diagenesis in the Jamaican meteoric phreatic zone includes conversion of HMC to LMC. partial aragonite dissolution and precipitation of calcite cement. Mineralogical alteration of the primary sediment is absent in the marine phreatic zone. Partial dolomitization of HMC micrite and red algae is found to occur in the mixing zone between the meteoric and marine phreatic zones (Land. 1973b). The supply of abundant carbonate from meteoric waters and Mg2+ from sea water may create a solution particulatly favorable for dolomitization. SUMMARY AND CONCLUSIONS Important results pertaining to the origins of sedimentary dolomite are: 1) Selective dolomitization cannot be explained simply in terms of either surface area or mineralogical variation. Reactant surface properties (determined qualitatively from SEM and light microscopy) are suggested to have made a significant contribution to reaction rates. A fine grained. very smooth-surfaced calcite was far more resistant to dolomitization than both a coarser grained but rough-surfaced reagent calcite and a crushed (and presumably strained) Iceland spar calcite. also of lesser 53 surface area. Among samples sharing identical preparatory procedures. an apparent mineralogical effect on dolomitization susceptibility was recognized: aragonite dolomitized more rapidly than Iceland spar calcite which dolomitized more rapidly than HMC (12 mole 1 MgCO3). 2) An interval of partial substrate dissolution. as a source of CO32‘. preceded the first appearance of dolomite. This interval was typically much longer than the time between the first appearance of dolomite and 1001 replacement. Variation in the replacement rates arose as a function of the different lengths of time required for dolomite to nucleate on the various reactants. 3) Addition of sodium carbonate beyond calcite saturation accelerated the replacement rate by reducing the time interval preceding the first detection of dolomite. The activity of CO32‘ at the time of dolomite nucleation is unknown. 4) SEM revealed that the most rapidly dolomitized substrate (excluding external C032“ runs) also dissolved most rapidly prior to the first appearance of dolomite. and vice versa. It is suggested that aCO32‘ is important in the process of selective dolomite nucleation and may be directly related to the relative rates of substrate dissolution. 54 5) The final dolomite composition was slightly Ca-rich (average equals 51.7 mole 1 CaCO3) and uniform between reactants. More pronounced dolomite non-stoichiometry occurred very early on in the reaction sequence. Each reactant mineralogy was characterized by a distinct early-dolomite compositional range that appeared to be related to the composition and/or dissolution rate of the reactant. 6) Dolomite crystal seeding produced no rate change. APPENDIX APPENDIX Experimental Results Reactant Reagent calcite. Sample CF2 Runs 1-7 Reaction time 38-200 hrs. Products and description Dissolution of substrate is evident in later runs . otherwise unaltered from starting material. Run 8 Reaction time 224 hrs. Products and description 100% dolomite; 50.9 mole 1 CaCO3: euhedral rhombs: ordering is fair. Run 9 Reaction time 224 hrs. Products andggescription Approx. 501 calcite and 501 dolomite: dolomite is disordered with 57.7 mole 1 CaCO3; magnesite is present. 15 mole 1 CaCO3. 8.9.0.10 Reaction time 257.5 hrs. Products and description 1001 dolomite with 50.6 mole 1 CaCO3; strong. well defined ordering peaks: euhedral rhombs of both limpid and highly porous varieties (from SEM): no magnesite. 32211 Reaction time 264 hrs. Products and description 1001 dolomite with 51.0 mole 1 CaCO3;well defined order peaks: dolomite is slightly coarser than reactant grains; limpid and clouded varieties present: no magnesite. Bach? Reaction time 308 hrs. Products and description 1001 dolomite; 50.0 mole 1 CaCO3; well—ordered; euhedral. 54 55 Reactant Reagent calcite. Sample C12 Runs 1-3 Reaction time 30-100 hrs. Eggggggevand description No changes observed EMT. 4 Reaction time 120 hrs. Products and description 1001 dolomite with 51.5 mole 1 CaCO3: euhedral rhombs: well defined order peaks: no magnesite. Bee. 5 Eeection time 130 hrs. Products and description 1001 dolomite with 51.3 mole 1 CaCO3: well-ordered. Ban 6 Beection time 180 hrs. Products and description 1001 dolomite with 51 mole 1 CaCO3: unit extinguishing interiors of rhombs: rough surfaces: no magnesite: well-ordered. Reactant Synthetic calcite. Sample C13 Bu_n 1 Reection time 149 hrs. Products end description 100 1 dolomite with 50.6 mole 1 CaCO3: cloudy rhombs: sub-euhedral; ordering is fair. Run 2 Reaction time 135 hrs. Products and description Approx. 1001 calcite reactant: sparsely distributed magnesite. 12 mole 1 CaCO3. Reactant Iceland spar calcite. Sample CC Runs 1-2 Reaction time 15 and 37 hrs. Products and description No visible change with SEM. R_uo 3 Reaction time 65 hrs. Products and description Approx. 1001 calcite reactant; very fine (<2um) magnesite along cleavage traces and steps. 56 Runs 4 and 5 Reaction time 80 and 100 hrs. Products and description Approx. 100% calcite reactant; minor magnesite with 14.5 and 13 mole 1 CaCO3. Egg 6 and 7 Beection time 125 and 200 hrs. Products egg deecription Approx. 1001 reactant: magnesite is commonly aligned along twin planes on substrate. 322 8 Reaction time 240 hrs. Products and deecription 1001 dolomite with 51.4 mole 1 CaCO3: fairly well-ordered: unit extinguishing grain interiors: rough surface texture: crude pseudomorphs after primary grain. Run 9 Reaction time 220 hrs. Products end d9§Cription Approx. 1001 calcite reactant. Run 10 Reaction time 220 hrs. Products and description 1001 dolomite: texturally identical to Run 8: 52 mole 1 CaCO3. Run 11 Reaction time 239 hrs. Producte and description 951 calcite and 51 dolomite: dolomite has 52 mole 1 CaCO3: very diffuse. weak major peak: abundant magnesite. 11.7 mole 1 CaCO3. Run 12 Reaction time 240 hrs. Products 80d description Approx. 1001 calcite reactant with magnesite: obvious dissolution of substrate. Reg 13 and 14 Reaction time 269 hrs. Products and description 1001 dolomite with 50 mole 1 CaCO3; well-ordered: rough grain surfaces with unit extinguishing cores: no magnesite. Ben 15 Reaction time 360 hrs. Products and description 1001 dolomite: 51.2 mole 1 CaCO3: well-ordered; product grains are composed of an assembledge of uniformly aligned crystals; highly stepped surfaces and unit extinction are characteristic. S7 Reactant Iceland spar calcite. Sample CI Runs 1 and 2 Reaction time 80 hrs. Products and deepription Approx. 1001 calcite reactant: sparse magnesite with 9.4 mole 1 CaCO3: etching of substrate is pervasive. so. 3 Eeection time 130 hrs. Products and description 601 calcite reactant and 401 dolomite: dolomite is disordered with 54 mole 1 CaCO3: calcite grains are rounded by dissolution. Run 4 Reaction time 150 hrs. Producte and deecription 1001 dolomite: limpid. euhedral rhombs: 55 mole 1 CaCO3: ordering is fair. 39.0. 5 Beection time 80 hrs. Products pend description Approx. 1001 calcite reactant: etch features common: minor calcian magnesite with 13 mole To CaCO3. Reg 6. 0.035 g Na2C03 added Reaction time 80 hrs. Producteeand description Calcite just at X-ray detection limit: 951 dolomite with 50.9 mole 1 CaCO3: well-ordered: non-undulatory .unit extinction. Run 7 Reaction time 145 hrs. Products and description 1001 dolomite; well-ordered: 52.2 mole 1 CaCO3. Ban 8 Reaction time 160 hrs. Products and description 1001 dolomite: 52.2 mole 1 CaCO3: fair ordering; rough surfaces; unit extinguishing cores; no magnesite. Reactant Iceland spar calcite. sample CF Runs 1-4 Reaction time 21—44 hrs. Products and description Unchanged 58 sea 5 Reaction time 67.5 hrs. Products and description 1001 dolomite: 50.8 mole 1 CaCO3: sub-euhedral. individual rhombs: hollowing of coarser grains is common: no magnesite. Bus 6 Beection time 71 hrs. Products and description 1001 dolomite; 53 mole1 CaCO3: poorly-ordered. Reactant HMC. sample HC Rm 1 Reection time 176 hrs. Products and deecrigtion Approx. 1001 HMC reactant: minor magnesite: randomly oriented: 12 mole 1 CaCO3: minor dissolution evident. M. 2 Reaction time 190 hrs. PFOUUCtS and description Partial replacement by dolomite: approx. 301 dolomite: strong preferential alignment: 53.4 mole 1 CaCO3: minor magnesite with 11.4 mole 1 CaCO3. Run 3 Reaction time 200 hrs. Products end deecription Approx. 1001 HMC: abundant magnesite:11.1 mole 1 CaCO3. Bug 4 Reaction time 208.5 hrs. Products and description Approx. 1001 dolomite with 51.2 mole 1 CaCO3: poorly-ordered: rhombic outlines of originally rounded grains: unit extinction is observed; magnesite with 10.3 mole 1 CaCO3 present. Run 5 Reaction time 210.5 hrs. Products and description 1001 dolomite: 52.5 mole 1 CaCO3: poorly-ordered: texture same as Run 4. Bee 6 Reaction time 216 hrs. Products and description Approx. 1001 dolomite; 53.4 mole 1 CaCO3: poorly-ordered; trace magnesite. Run 7 Reaction time 224 hrs. Products and description 1001 dolomite; 53 mole 1 CaCO3; fair ordering. 59 Reactant HMC. sample HI M 1 Reaction time 190 hrs Products and description Approx. 1001 HMC reactant: sparse magnesite: 11.1 mole 1 CaCO3. Run 2 Reaction time 202 hrs. ELQQHE£§__§Dd dgegription Approx. 1001 dolomite: 55.3 mole *1 CaCO3: fair ordering: unit extinction is common excepting in cryptocrystalline areas. where the primary texture is inferred to have been cryptocrystalline. 31.4.12 3 Reaction time 212 hrs. Products and deecription Approx. 201 HMC reactant: 801 dolomite: 50.3 mole 1 CaCO3: poorly-ordered: minor magnesite: 10.8 mole 1 CaCO3 Run 4 Reaction time 218 hrs. Producte and description 1001 dolomite: 53.7 mole 1 CaCO3: fair to poor ordering: grain surfaces are rhombic shaped. Reactant HMC. sample HF uh I eaction time 77 hrs. Products and description Approx. 1001 HMC reactant: trace magnesite with 11.4 mole 1 CaCO3: rare dissolution I) un 2 Reaction time 92 hrs. Products and description No visible change with SEM: very rare magnesite present. Eye 3 Reaction time 125 hrs. Products and description Approx. 1001 HMC reactant: rare magnesite crystals. < 5pm: HMC composition is identical to initial reactant. 60 Run 4 Reaction time 160 hrs. Products and dsecription Approx. 1001 dolomite. 50.3 mole 1 CaCO3. well-ordered: coarse grains appear as polycrystalline assembledges. fines appear as monocrystalline grains. unit extinction prevalent: no distinct rhombic shapes with light microscope: minor magnesite. 10.8 mole 1 CaCO3. 8.9.9. 5 fleection time 169.5 hrs. Products and description Approx. 501 HMC reactant and 501 dolomite with 50 mole 1 CaCO3: disordered: faint rhombic overgrowths observed with light microscope: grains appear limpid: HMC composition same as original. Run 6 Reaction time 186 hrs. Products and description 1001 dolomite. 50.6 mole 1 CaC03. rather poor ordering. grains are subhedral and limpid with faint rhombic outlines: minor magnesite with 9.7 mole 1 CaCO3. .R_uo.7 Reaction time 190 hrs. Products pend description 1001 dolomite. 50.3 mole 1 CaCO3. fair ordering. limpid crystals very common. ems Reaction time 258.5 hrs. Producte and description 1001 dolomite with 51.8 mole 1 CaCO3. well-ordered. product retains original grain shapes with some addition of rhombic overgrowth. Reactant Ward’s Aragonite. Sample AC Run 1 Reaction time 37 hrs. Products and description 1001 reactant: dissolution etching observed. Run 2 and 3 Reaction time 75.5 and 84.5 hrs. Products and description Approx. 1001 reactant. strongly etched: new material is common but not X-ray detected. 8.1411 4 Reaction time 88 hrs. Products and description Approx. 801 aragontie and 201 dolomite: dolomite has 63 mole 1 CaCO3 . disordered. euhedral rhombs. < 53m. see Figure 16. 61 .Ru_n 5-7 Reection time 92. 98 and 100 hrs. Producte and description Approx. 1001 aragontie: abundant clusters (< 2 m) of spherulitic material on the aragonite surface. not X-ray detected: this material strongly resembles amorphous products from room temperature experiments with Mg and Ca chloride solutions and high Mg/Ca ratios. Run 8 Reaction time 104.5 hrs. Proggcts and description 501 aragonite and 501 dolomite with 58.9 mole 1 CaCO3. poorly-ordered. dense mosaic of interlocking euhedral rhombs about Sum in size. Run 9 Reaction time 112 hrs. Products epd description 1001 dolomite. 52.1 mole 1 CaCO3. fair to poor ordering. texture identical to Run 8. Reactant Ward's aragonite. Sample A12 Run 1 Reaction time 55 hrs. Proggcts and description Approx. 1001 aragonite reactant: abundant crystals on the aragonite surfaces which are not X-ray detected. crystal size is about 5-10um. Bug. 2 Reaction time 60 hrs. Products end description 801 aragonite and 201 dolomite: diffuse and weak dolomite X-ray peak. limpid euhedral rhombs. 5pm in size. randomly oriented. Ru_n 3 Reaction time 68 hrs. Products and description 1001 dolomite. well-ordered. 51.1 mole 1 CaCO3. dense mosaics of interlocking euhedral limpid rhombs. 5 m diameters. Run 4 Reaction time 69 hrs. Products and description 1001 dolomite. well—ordered. 51 mole 1 CaCO3. texturally identical to Run 3. 62 Reactant Ward's aragonite. Sample AI Run 1 and 2 Reaction time 19 hrs. Products end description 1001 aragonite. unaltered. Run 3 Reaction time 25 hrs. Products epd description Approx. 1001 aragonite with abundant amorphous intermediate product. Run 4 Reaction time 36 hrs. Prodgcts and description 1001 dolomite. 52 mole 1 CaCO3. fair ordering. euhedral rhombs. Run 5 Reaction time 34 hrs. Producte:and deeeription Identical to Run 3. M 6 Reaction time 46 hrs. Products and description 1001 dolomite. 62 mole 1 CaCO3. disordered. euhedral rhombs with random orientation. R_U.r1 7 Reaction time 55 hrs. Products and description 1001 dolomite with 51.4 mole 1 CaCO3. fair ordering. Run 8 Reaction time 63.5 hrs. Products and description 1001 dolomite with 50.9 mole 1 CaCO3. well-ordered. texturally identical to Run 6. Reactant 'Ward's Aragonite. Sample AF Run 1 Reaction time 15 hrs. Products and description Approximately 80—901 aragonite and 20-101 disordered dolomite with 60 mole 1 CaCO3. Run 2 Reaction time 20 hrs. Products and description 100% aragonite with minor amorphous material. Bee 3 Reaction time 24.5 hrs. Products and description 1001 dolomite with 52.5 mole 1 CaCO3. poorly-ordered. euhedral rhombs. no precursor texture preserved . Run 4 Reection time 27 hrs. Products and description 1001 dolomite with 51.7 mole 1 CaCO3. fair ordering. Reactant Synthetic Aragonite Needles. Sample AF2 Run 1 and 2 Reaction time 11 and 13.5 hrs. EEQQHE£§__QQQL,descrletlon Approx. 1001 aragonite; rare prismatic crystals of a phase which escapes X-ray un 3 Reaction time 16 hrs. Products end description Dolomite with 64.3 mole 1 CaCO3. disordered. euhedral rhombs about 5 m in size: HMC with 14 mole 1 MgCO3 occurs sparsely as 15 m long euhedral. sharply terminated prisms: dolomite content >> HMC. Bee 4 and 5 Beection time 17 and 24 hrs. Products and description Identical to Run 2. 3.92 6 Reaction time 37 hrs. Products BDd description Dolomite with 62 mole 1 CaCO3. poorly—ordered. euhedral 10 m rhombs: HMC with 16.2 mole 1 MgCO3. rhombic shaped: dolomite content >> HMC. Ban 7 Reaction time 50 hrs. Products and description 1001 dolomite with 52.6 mole 1 CaCO3. fairly well—ordered. individual euhedral rhombs. crystal size (5pm) is finer than HMC intermediates (15pm) detected in earlier runs. Results of External Carbonate and Seeding Reactant Iceland Spar Calcite. Sample CF’ 3290s Beection time 24 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result LMC and magnesite. excluding seeds. 393cc Reectien time 24 hrs. Additive 0.035 gram NaZCO3 Result LMC plus minor magnesite. EuoCc Reection time 35 hrs. Additive 0.035 gram NaZCO3 Result 151 disordered dolomite. 59 mole 1 CaCO3: 851 LMC. EunCd Reaction time 35 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 1001 LMC. excluding seeds. Bu_nCe Reection time 47.5 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 1001 LMC. excluding seeds. un Cf Reaction time 47.5 hrs. Additive 0.035 gram Na2C03 Result 801 dolomite. poorly-ordered. 52.2 mole 1 CaCO3: 201 LMC. MCQ Reaction time 60 hrs. Additive 0.035 gram NaZCO3 Result 851 dolomite with 55 mole 1 CaCO3: 151 LMC. Run Ch Reaction time 60 hrs. Additive 0.035 gram dolomite seeds. ordered. near-stoichiometric Result 1001 LMC. excluding seeds. Baum Reaction time 60 hrs. Additive Control Result 1001 LMC 65 Ego Ci Reaction time 75 hrs Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 1001 dolomite. poorly-ordered. 52.2 mole 1 CaCO3. BeoCk Beectign time 75 hrs. Additive Control Result 501 disordered dolomite. 56.8 mole 1 CaCO3; 501 LMC. Reactant Ward’s Aragonite. Sample AI Bun Ae Reaction time 10.5 hrs Additive 0.035 gram Na2CO3 Result 201 disordered dolomite with 61.5 mole 1 CaCO3: 801 aragonite. Ben AF Reaction time 10.5 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 1001 aragonite Bun Ab Reaction time 15 hrs. Additive 0.035 gram Na2C03 Result 501 disordered dolomite with 60.8 mole 1 CaCO3; 501 aragonite. Run Ag Reaction time 17 hrs. Additive 0.0175 gram dolomite seeds. disordered. 62 mole 1 CaCO3 Result 1001 aragonite. Bun Ah Reaction time 17 hrs. Additive 0.035 gram NaZCO3 Result 901 dolomite. 64.6 mole 1 CaCO3. disordered: 101 HMC with 9.5 mole 1 MgCO3. Ego Ad Reaction time 20 hrs. Additive 0.035 gram NaZCO3 Result 751 dolomite with 59.9 mole 1 CaCO3: 251 aragonite. Run Ac Reaction time 20 hrs. Additive Control 66 Result 121 disordered dolomite with 61.5 mole 1 CaCO3: 881 aragonite. Run Aa Reaction time 20 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 301 dolomite. 54 mole 1 CaCO3: 701 aragonite. MM Reaction time 32 hrs. Additive Control Result 1001 dolomite with 63 mole 1 CaCO3. disordered. MAJ Reection time 32 hrs. Additive 0.0175 gram seeds. ordered. near-stoichiometric dolomite Result 1001 dolomite with 63 mole 1 CaCO3. disordered. BIBLIOGRAPHY BIBLIOGRAPHY Adamson. A.W.. 1982. Physical Chemistry of Surfaces: New York. John Wiley and Sons. Inc.. 664p. Armstrong. A.K.. 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