GANTESTA I :IIIILII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 01409 6493 This is to certify that the thesis entitled THE EFFECTS OF DISSOLVED SULFATE ON HIGH TEMPERATURE NUCLEATION AND KINETIC GROWTH OF DOLOMITE presented by MICHELLE LEA BORKOWSKI has been accepted towards fulfillment of the requirements for MASTER degree in SCIENCE Major professor 97 (4 fléQ/ZVZ/[iZM/ét/ ’7 n a,” Date f/ fax/(’72: / V 97 5 1/ 0.7639 MS U is an Affirmative Action/Equal Opportunity Institution LIBRARY Michigan State University PLACE N RETURN BOX to remove thle checkout from your record. To AVOID FINES retun on or before date due. DATE DUE DATE DUE DATE DUE MAGIC 2 [:1__I -E- I—I—‘I MSU ie An Afflmettve Action/Equal Opportunity inetitrtion W ”3-9.1 THE EFFECTS OF DISSOLVED SULFATE ON HIGH TEMPERATURE NUCLEATION AND KINETIC GROWTH OF DOLOMITE By Michelle Lea Borkowski 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 ABSTRACT THE EFFECTS OF DISSOLVED SULFATE ON HIGH TEMPERATURE N UCLEATION AND KINETIC GROWTH OF DOLOMITE By Michelle Lea Borkowski The effects of dissolved sulfate on the rates of nucleation and crystal growth of the dolomitization of reagent grade calcite were investigated in 0.5 M, 0.66 Mg to Ca ratio solutions without sulfate and with the addition of 0.001 M 804. The reactions were heated at 205°C for a predetermined amount of time and the products were analyzed by x-ray diffraction. Products of the experiments were very high magnesium-calcite (VHMC), nonstoichiometric dolomite, and stoichiometric dolomite. More time was needed to produce these products with sulfate-bearing solutions. To determine which stage, nucleation or slow crystal growth, was suppressed by the sulfate, three reaction models were developed: (1) nucleation slowed only (2) crystal growth slowed only (3) both nucleation and crystal growth slowed. Comparing the models to the data indicates that both nucleation and crystal growth were slowed by the sulfate. Both solutions showed no difference in the mo! % of MgCO3 in the products produced (VHMC and dolomite). This indicates that the free energy drive for the reaction was not afl‘ected by the sulfate, which suggests that sulfate is a kinetic factor. However, sulfate slowed both the nucleation and crystal growth, which suggests that sulfate is behaving like a thermodynamic factor. Because of this behavior it is dificult to distinguish the effects of the sulfate from other thermodynamic factors in ancient dolomites. ACKNOWLEDGMENTS I would like to express my sincerest gratitude to Dr. Duncan Sibley for his guidance and assistance throughout all phases of this project and for the many wonderful conversations during mice and hot chocolate. Special thanks are extended to Drs. Nathaniel Ostrom and David Long for their review and comments on the text of my thesis. I am especially gratefiil to my parents, Tim and Carol, for their support, patience, understanding, advice, love, and encouragement through P-Chem. A special thanks to my brother Tim and his family (Julie and Nicole) for their love and laughter. To Alan, Keith, and Jon, thanks for some of the greatest times in my life. I would have never of made it without you guys. TABLE OF CONTENTS Page No. LIST OF TABLES ......................................................................................... v LIST OF FIGURES ....................................................................................... vi INTRODUCTION ......................................................................................... l PREVIOUS EXPERIMENTAL WORK ........................................................ 2 DOLOMITE AND SULFATE IN NATURAL MARINE SETTINGS ........... 7 EXPERIMENTAL PROCEDURE ................................................................ 10 EXPERIMENTAL RESULTS ...................................................................... 11 DISCUSSION .............................................................................................. 13 CONCLUSION ............................................................................................ 23 REFERENCES CITED ................................................................................ 25 iv LIST OF TABLES Table Page 1. Experimental Conditions ......................................................................... 10 2. Experimental Results of Runs Made In Sulfate Free Solutions ................. 12 3. Experimental Results of Runs Made In Sulfate Present Solutions ............. 12 LIST OF FIGURES Figure Page 1. Generalized Results of Dolomitization Experiments ....................................... 6 2. Results From the Isothermal Experiments Without and With Sulfate ............. 15 3. Comparison of Real Data to Models With Nucleation and/or Crystal Growth Slowed by Dissolved Sulfate ............................................................ 16 4. Comparison of Mol % of MgCO3 to the Amount of Product Produced For Sulfate-free and Sulfate Present Solutions ............................................. 20 vi INTRODUCTION Dolomite is the most thermodynamically stable phase of carbonates in seawater (Drever 1988), however, there is very little dolomite in modern marine sediments. As a result of this, questions exist concerning the kinetic and thermodynamic factors that influence dolomitization (Machel and Mountjoy, 1986). Factors that affect the rate of dolomitization include temperature, the Mg/Ca ratio of the dolomitizing solution, mineralogy of the reactant, the surface area of the reactant, and inhibitors in the solution (Land 1967, 1980; Gaines 1974, 1980; Katz and Matthews 1977; Baker and Kastner 1981; Sibley, Dedoes, and Bartlett 1987; Morrow and Ricketts 1988; Sibley 1990; and Sibley, Nordeng, and Borkowski in press). These researchers showed that an increase in temperature or in Mg/Ca ratio of the solution increases the rate of dolomitization of calcium carbonate. The mineralogy of the initial reactant also affects the rate of the reaction; aragonite is dolomitized faster than calcite (Katz and Matthews 1977; Baker and Kastner 1981; and Sibley and Bartlett 1987). The rate of the reaction is directly related to the surface area of the reactant (Sibley and Bartlett 1987). This observation is consistent with the fact that natural dolomite ofien selectively replaces finer crystalline calcium carbonate (Murray and Lucia 1967; and Choquette, Cox, and Meyers 1992). The presence of dissolved sulfate decreases the rate of dolomitization (Baker and Kastner 1981, Morrow and Ricketts 1988). Sulfate is the second most abundant anion in sea waters and is a kinetic factor that inhibits dolomitization (Morrow and Ricketts 1988). Therefore, the lack of dolomite in modern marine environments may be due to the effects of dissolved sulfate. Experiments 1 2 described here were run to determine processes that are inhibited by the kinetic reactions of dissolved sulfate (nucleation vs. crystal growth) during high temperature (> 200°C) dolomitization. The correlation between experimental data and natural marine dolomitization is as follows: if dissolved sulfate slows crystal growth, dolomite would be plentiful but the crystals would be small. If sulfate slows nucleation, then dolomite crystals would be few, which would account for the lack of dolomite in natural marine settings. Because dissolved sulfate might affect the reaction rate by changing the composition of the initial products formed during dolomitization of calcium carbonate, I also determined the affect of dissolved sulfate on the stoichiometry of the products. PREVIOUS EXPERIMENTAL WORK Many factors can affect the rate of dolomitization such as temperature, the Mg/Ca ratio of the dolomitizing solution, the available surface area of a reactant as well as the mineralogy, and the presence of an inhibitor or catalyst (Land 1967, 1980; Gaines 1974, 1980; Katz and Matthews 1977; Baker and Kastner 1981; Sibley, Dedoes, and Bartlett 1987; Morrow and Ricketts 1988; Sibley 1990; and Sibley, Nordeng, and Borkowski in press). These investigators have studied these factors by analyzing synthetic dolomite produced in laboratory experiments. Experimental procedures for dolomite precipitation are performed at temperatures greater than 100°C because of the inability to obtain unequivocal precipitates of dolomite at temperatures less than 100°C (Land 1967; Lippman 1973; Gaines 1974; Katz and Matthews 1977; Gaines 1980; Baker and Kastner 1981; Gregg 1983; Bullen and Sibley 1984; Sibley and Bartlett 1987; Morrow and Ricketts 1988; Sibley 1990; Zempolich and Baker 1993', and Nordeng and Sibley 1994). The only study that demonstrates an increase in the rate of dolomitization due to an increase in temperature under constant solution and 3 reactant composition was by Katz and Matthews (1977). They found that dolomitization of calcite required 100 hours of heating at 252°C in a hydrothermal bomb before 100% stoichiometric dolomite was formed. At a temperature of 295°C, 100% stoichiometric dolomite was formed in approximately 4 hours. From this change in rate with temperature, they determined the activation energies for very high magnesium calcite (3 5- 40 mole % MgCO3, VHMC) (Katz and Matthews 1977) and dolomite (> 40 mole % MgCO3) using Arrhenius-type plots. The activation energy for dolomite is approximately 49-50 kcal/mol (Katz and Matthews 1977). The large activation energy for dolomite when compared to the 10 kcal/mol for calcite (Morse 1983) explains why high temperatures are needed to form dolomite in the laboratory. Several investigators (Land 1967; Katz and Matthews 1977; Gaines 1980; Sibley et al. 1987; Sibley 1990) used hydrothermal bomb experiments at high temperatures (> 100°C) to determine the affects of various Mg/Ca ratios in solution on the reaction rate. Their results were similar; as the Mg/Ca ratio increased the reaction rate for dolomite increased. However, a study performed by Gaines (1980) found that dolomitization increased when the Mg/Ca ratio of the solution was increased from 3 to 5, but the reaction rate decreased when the ratio was increased from 5 to 7. This decrease in the dolomitization rate when the Mg/Ca ratio in solution is greater than 5 may be due to the precipitation of magnesite on the reactant surface (Sibley, Nordeng, and Borkowski in press). Dolomitization rates increase with the reactant surface area (Katz and Matthews 1977; Gaines 1980; Sibley et al. 1987 ). The surface area supplies potential nucleation sites for dolomite formation (the number of nuclei is assumed to be proportional to surface area). Also, a larger surface area increases the dissolution rate of calcium carbonate which supplies the carbonate needed for dolomite formation. Therefore, the greater the surface area, the greater the potential is for dolomitization. 4 The mineralogy of the carbonate reactant affects the rate at which dolomite forms. Experimental studies show that aragonite is dolomitized before calcite (Katz and Matthews 1977; Baker and Kastner 1981; Sibley and Bartlett 1987; Sibley et al. 1987; Sibley, Nordeng, and Borkowski in press). This occurs because of the chemical and physical properties of aragonite (chemical make-up, crystal structure, grain size, and bond strength). The aragonite used by Sibley and Bartlett (1987) was composed of aggregates of smaller sized crystals than the calcium carbonate. These smaller aragonite grains aided in the rapid rate of dolomitization by providing a greater number of nucleation sites than the calcite due to the increase in surface area to volume ratio. Also, the aragonite to dolomite transformation has a greater chemical potential than the calcite to dolomite transformation because aragonite is more soluble than calcite. As a result, dolomite may not precipitate as readily in an environment with calcite as it would with aragonite. This observation is consistent with natural forming dolomite because in modern sabkhas dolomite selectively replaces aragonite (McKenzie 1981; Patterson and Kinsman 1982). There are several substances which, when added to the dolomitizing solution, increase the reaction rate. Gaines (1980) found three substances that increased the reaction, lithium, NaCl, and an oxidant. The mechanism(s) responsible for the increase in reaction rate of dolomitization due to the addition of 0.05 M Li is unknown. However, the increased rate due to 2 M NaCl may be from the increase in the activity coefficient for Mg“ and the increased rate from the addition of the oxidant (‘OC12) is thought to be from the oxidation of the organic matter (Sibley, Nordeng, and Borkowski in press). Dioxane and high concentrations of bicarbonate has been shown to increase the reaction rates (Oomori and Kitano 1987; Morrow and Ricketts 1988; Sibley, Nordeng, and Borkowski in press). Gaines (1980) showed that 0.01 M aspartic acid and soluble protein (gelatin) decreased the rate of dolomitization. Dissolved sulfate also acts as an inhibitor of 5 dolomitization (Baker and Kastner 1981; Morrow and Ricketts 1988; Sibley, Nordeng, and Borkowski, in press). Baker and Kastner (1981) synthesized dolomite at 200°C using hydrothermal bombs filled with 10 mL of 0.080 M MgC12 + 0.060 M CaC12 + NaCl + Na2804 (Mg/Ca molar ratio = 1.33) and 20 mg of a calcite reactant. NaCl and NaZSO4 were added to the solution in various amounts in order to maintain an ionic strength of 0.7. The sulfate concentration varied from zero to 0.004 M. Even minor amounts of sulfate (0.001 M) strongly inhibited the rate of dolomitization of calcite (Baker and Kastner 1981). Normal sea water has an average concentration of 0.028 M of sulfate, therefore dolomite would be expected to precipitate more rapidly if the sulfate concentrations in the ocean were diminished. Morrow and Ricketts (1988) continued that dissolved sulfate inhibits dolomitization. They suggested two reasons why sulfate suppresses dolomite precipitation: (l) sulfate slows the rate of calcite dissolution and (2) sulfate may react with the calcium from calcite dissolution and form CaSO4, which then creates a protective layer on the surface of the calcite crystal therefore reducing potential nucleation sites. The formation of synthetic dolomite has a long induction period (Figure l) ( Katz and Matthews 1977; Sibley and Bartlett 1987). The induction period is the time from the onset of the experiment to the first appearance of the reaction product (Walton 1969). During the induction period, significant nucleation occurs early in the reaction at the expense of the reactant followed by slow initial growth of the metastable phases, very high magnesium-calcite (3 5-40 mole % MgCO3) and nonstoichiometric dolomite (45-48 mole % MgCO3) ( Sibley et al. 1987; Nordeng and Sibley 1994). Following the induction period, there is a rapid growth stage of the metastable phases and stoichiometric dolomite (Katz and Matthews 1977; Baker and Kastner 1981; Sibley et al. 1987; Sibley 1990). 9&2 33% v5 mac—v.55 .3855on serge—2% mo 2.88 ecu—8080 ._ 8ng TI 85m 58:3 Ir— mEE. o Am m . ............. a. $ma 0 CD 2 05 ®~OE Cm Iv Uzmxr \SQNh m m mt m m. mm mm .m m. m m. P. u m. m. ....da m m mm ... l a 0:828 .5638: + USE > .v/ m. s m. .... a... m. w m ozfimmem oEeEoEomBm .............................................. o3 7 DOLOMITE AND SULFATE IN NATURAL MARINE SETTINGS Previous experimental work does show that sulfate slows the reaction rate of dolomite formation in the laboratory (Land 1967, 1980; Gaines 1974, 1980; Katz and Matthews 1977; Baker and Kastner 1981; Sibley, Dedoes, and Bartlett 1987; Morrow and Ricketts 1988; Sibley 1990; and Sibley, Nordeng, and Borkowski in press). However, the question remains, is sulfate one of the inhibiting factors of dolomitization in nature? The relationship between this study and the question proposed is; if sulfate slows only crystal growth, than dolomite would form but the crystals would be small, if sulfate slows only the nucleation then dolomite would be scare in natural marine settings. Dolomites that do form in sulfate-bearing water appear to have physical, thermodynamic, or kinetic factors that enable the dolomite process to overcome the sulfate barrier. Recent dolomites are known to form in a variety of natural marine environments that contain sulfate. One of the most common environments for the formation of dolomite in nature is the presence of organics that reduce sulfate. Greater than 1 wt. °/o diagenetic dolomite appears to be restricted to organic rich deep marine sediments (sulfate-reducing) (Baker and Burns 1985) and tide flats (McKenzie 1981, Patterson and Kinsman 1982, and Gunatilaka 1991). Other common environments in which dolomite formation occurs in sulfate-bearing waters is when, the sedimentation rate creates a reducing environment, the water temperature is elevated, or the Mnga ratio is abnormally high. Sulfate reduction is a reaction in which bacteria use oxygen from 804'2 to oxidize organic matter to C02 (Drever 1988). 804-2 + 2C + 2H20 #st + 2HCO3' organic Sulfate reduction increases the carbonate alkalinity of the solution which results in interstitial porewater highly saturated with respect to dolomite (Baker and Burns 1985; Burns and Baker 1987; Compton 1988; and Slaughter and Hill 1991). The carbon (CO32') 8 that is produced during sulfate reduction may be incorporated in dolomite. This carbon has a characteristic isotopic signature of -15 °/oo to -20°/oo PDB (Burns and Baker 1987). Dolomites that have 13C depleted values are assumed to have formed in porewaters rich in carbon that was derived from the oxidation of organic matter and the reduction of sulfate. Dolomite has been found in many modern marine, organic-rich sediments; Gulf of California, California borderlands, Peru Shelf, Japan Trench, the Cariaco Basin in offshore Venezuela (Garrison, Kastner, and Zenger 1984), Monterey Formation California (Shell Beach and Mussel Rock) (Pisciotto and Mahoney 1981; Baker and Burns 1985; Burns and Baker 1987), Green River Formation of Colorado (Cole and Dyni 1985), Tripoli Formation Sicily (Bellanca, Calderone, and Neri 1986), Florida's west coast (Randazzo and Cook 1987), Kau Bay, Indonesia (Middelburg, DeLange,and Kruelen 1990), and Al- Khiran, Kuwait Arabian Gulf (Gunatilaka 1991). Sedimentation rate is a major control on whether dolomite forms in the zone of sulfate reduction (Pisciotto and Mahoney 1981; Baker and Burns 1985; Burns and Baker 1987; and Compton 1988). In areas of low sedimentation rates, the sediments remain near the water/sediment interface for extended periods of time. This allows the sulfate reducing organisms to create an environment within the first 10-20 meters of sediment that is suitable for dolomitization (Burns and Baker 1987) and allows Mg2+ from the overlying seawater to diffuse into the zone of sulfate reduction. Rapid sedimentation rate also favors sulfate reduction but does not allow magnesium to diffuse through the sediment from the seawater to form appreciable dolomite. According to Baker and Burns (1985) and Slaughter and Hill (1991), sulfate inhibits dolomitization in a natural marine environment through one or more mechanisms: (1) the sulfate adsorbs at active dissolution sites and does not allow the reactant to go into solution (2) sulfate adsorbs on to the active grth sites and restricts nucleation of 9 intermediate stages (VI-{MC and nonstoichiometric dolomite) and/or dolomite, and (3) decreases carbonate alkalinity due oxidation of organic matter. Some dolomites form in saline water (3 to greater than 300 ppt) with sulfate concentrations (0.001-0.004 M) greater than the molarity used in some laboratory studies (Baker and Kastner 1981); Lake Cundare, Lake Eurack, Lake Weering, Lake Beeac, and Pink Lake (DeDeckker and Last 1989) and Basque Lakes (Burton, Hahn, and Machel 1992). The chemistry of the water and the warm climate are common for all of these lakes. These lakes all have high salinity and extremely high Mg/Ca ratios (Mg/Ca ratio for the lakes studied by DeDeckker and Last (1989) is greater than 70 times the normal and for the Basques Lakes between 200-300 times the normal). Carbonate alkalinity (pl-I) is 3-10 times greater than that of sea water. The sulfate concentrations for Lake Cundare, Lake Eurack, Lake Weering, Lake Beeac, and Pink Lake (DeDeckker and Last 1989) ranged from 15 to 80 mmol/l, which is still more than an order of magnitude above the concentrations considered in laboratory studies (Baker and Kastner 1981). Based on the results of Baker and Kastner (1981) and Morrow and Ricketts (1988) the levels of dissolved sulfate in these systems should severely inhibit dolomitization. However, dolomite does precipitate. Extremely elevated Mg/Ca concentration ratios and elevated temperatures have more than compensated for the possible inhibiting effect of the sulfate in alkaline lakes (DeDeckker and Last 1989). Therefore, there is not only one thermodynamic or kinetic factor that controls the dolomitization of calcium carbonate. A combination of these factors ultimately determines if dolomite will precipitate. In environments where sulfate concentrations are zero or very low, the Mg/Ca ratio, temperature, and alkalinity do not need to be elevated beyond normal marine conditions for dolomitization to occur. But in areas of high sulfate, either organics or physical factors must reduce sulfate or thermodynamic or kinetic factors must be capable of overcoming the sulfate barrier. 10 EXPERIMENTAL PROCEDURE Experiments reported here used 0.100 gram reagent grade calcite (Mallinckrodt) and 15 mL of a 0.5 M, 0.66 Mg/Ca solution, with and without sulfate, in 23 ml Teflon- lined, stainless steel, Parr-type bombs (Table 1). Based on the temperature (205°C) of the study, the composition of the solutions fell well within the dolomite stability field relative to the calcite-dolomite phase boundary (Baker and Kastner; 1981 and Morrow, Gorharn, and Wong 1994). Solutions were prepared by mixing stock solutions of 0.5M CaClz-ZHZO and 0.5M MgC12-6H20. Sulfate bearing solutions were prepared by addition of 0.0827 g of Na2S04 to the 0.66 Mg/Ca solution. The amount of sulfate that is added to the solution is too small to affect the ionic strength or Mg2+ activity. Teflon-liners were weighed and 0.100 g of calcite was added to the container. Fifteen mL of either sulfate bearing or sulfate-free solutions were added into the containers. Again the containers were weighed with the reactant and the solutions. Liners were placed into the stainless-steel bombs and heated at 205°C for a predetermined amount of time. At the end of each run, liners were removed from the bombs and reweighed to determine if any fluid was lost. To remain consistent with previous research, (Nordeng and Sibley 1994), experiments in which fluid lost exceeded 0.5 g were discarded. Products from the reaction were filtered, flushed with twice distilled water, and dried in a low temperature oven (45°C). Table 1. Experimental Conditions Temperature 205°C Mg/Ca ratio 0.66 Solution volume 15.0 mL Calcite reactant mass 0.10 g Reagents 0.5 M CaClz-ZHZO 0.5 M MgC12-6H20 11 Products of the reactions were analyzed by x-ray diffraction. Once the products from the bombs dried, a small amount of the sample plus a fluorite standard was homogenized by light grinding with a mortar and pestle. The percentage and composition of the products were examined by x-ray powder diffraction. Smears were scanned with copper radiation from 28 to 32° 20, at a rate of l/2°/rnin. The percent of dolomite was determined by peak height ratios of the d(104) peaks of calcite and dolomite (Royce, Wadell, and Petersen 1971). The dolomite and very high magnesium calcite compositions of the reaction were determined by peak positions of the d(104) reflection relative to the fluorite standard ( Goldsmith, Graf, and Heard 1961; Lumsden 1979). Using the equation from Lumsden and Chimahusky (1980) , M CO3 = 333.33d(104) - 911.99, and the d(104) reflection, the mole % of MgCO3 were detemiined. There may be inaccuracies of 2 to 3 mol % MgCO3 (Reeder and Shepard 1984). EXPERIIVIENTAL RESULTS The data from x-ray diffraction analyses are presented in Tables 2 and 3. These results show three distinct products (Table 2 and 3), very high magnesium calcite (VHMC), nonstoichiometric dolomite (high calcium dolomite), and stoichiometric dolomite. These products can be distinguished by the mol % of MgCO3. The first product to be detected, VHMC, has a 35.5-39.5 mol % MgCO3. The second product, nonstoichiometric dolomite, was often detected along with the VHMC. The mol °/o of MgCO3 for this product is approximately 45.0-47.7 %. The last product, stoichiometric dolomite, was identified by its high MgCO3 content of > 47.7 mol %. The stoichiometric dolomite phase occurred by itself, after the intermediate phases had become dolomitized. The sulfate-free experiments produced VHMC (39 mol % MgCO3) in approximately 72 hours. Also, the first appearance of nonstoichiometric dolomite (45.1 mol % MgCO3) occurred with the VHMC and calcite after 72 hours of heating. 12 Table 2. Experimental Results of Runs Made In Sulfate Free Solutions Sample Heating 2 0 Mole % 2 0 Mole % % % °/o Time Cu ka Mg Cu ka Mg Dolomite VHMC Prod 5-1-01 48 0 0 0 5-1-02 48 O O 0 4-22-01 72 30.83 45.1 52 0 52 4-22-02 72 30.60 39.0 30.85 45.7 39 37 76 4-18-01 96 30.64 39.3 30.89 47.6 34 21 55 4-18-02 96 30.55 36.3 30.86 46.2 33 29 62 4-28-01 120 30.55 36.3 30.89 47.6 42 25 67 4-28-02 120 30.91 47.7 94 0 94 2-23-01 120 30.99 51.2 100 0 100 8-16-03 132 30.92 47.9 98 0 98 2-24-01 144 30.94 48.5 98 0 98 2-25-01 168 30.93 48.2 100 0 100 Table 3. Experimental Results of Runs Made In Sulfate Present Solutions Sample Heating 2 0 Mole % 2 0 Mole % % % % Time Cu ka Mg Cu ka Mg Dolomite VHMC Prod 5-25 24 0 0 0 5-26 48 O O 0 8-30 24 0 0 0 8-31 48 0 0 0 9-15 96 30.52 35.5 0 2 2 2-23-02 120 30.60 39.0 0 13 13 8-16-01 144 30.62 38.9 0 20 20 2-24-02 120 30.70 42.6 53 0 53 2-25-02 168 30.62 38.9 0 22 22 8-9-01 192 30.72 43.3 69 0 69 9-19 204 30.67 41.1 38 0 38 5-17-02 216 30.87 46.3 50 0 50 5-17-01 216 30.63 39.1 0 70 70 9-20 228 30.61 38.3 0 38 38 5-18-02 240 30.97 49.4 100 0 100 3-14 264 30.64 39.3 0 82 82 9-22 264 31.10 50.8 100 0 100 6-7-01 288 30.91 47.7 100 0 100 6-24-02 336 30.96 49.1 100 0 100 6-24-01 360 30.95 48.8 100 0 100 l3 Stoichiometric dolomite was first detected after approximately 120 hours and made up 100 % of the sample, there was no detectable reactant or intermediate phase left (Table 2). The sulfate solution results in a similar reaction sequence but the reaction was much slower. This experiment required 96 hours before VHMC (35.5 mol % MgCO3) was detected. Nonstoichiometric dolomite was found afier 120 hours (42.6 mol % MgCO3). Stoichiometric dolomite (49.4 mol % MgCO3) was found at 240 hours and like the sulfate-free experiments, composed 100 % of the sample. DISCUSSION For this study, it was important to be able to distinguish between nucleation and crystal grth to determine how dissolved sulfate effected each of these stages. The data from the experiments that were conducted fit the model that was developed by Nordeng and Sibley (1994) (Figure 1). This model helped to characterize and define the nucleation and growth stages. The model suggests that three distinct stages occur during the dolomitization of calcium carbonate; nucleation and slow crystal growth, which make up the induction period, and rapid crystal growth. These stages are described in detail by Sibley, Nordeng, and Borkowski (in press) as follows: Stage ( l). Nucleation: Nucleation of VHMC and/or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on CaCO3. Stage (2). Induction Period: Both nucleation and slow crystal growth occur in this time period. Nucleation occurs early in the induction period leaving the majority of time for post-nucleation growth (slow growth) of HMC and/or nonstoichiometric and stoichiometric dolomite. 14 Stage (3). Replacement Period (rapid growth stage): 3) Calcium carbonate is replaced by VHMC or nonstoichiometric dolomite. b) Calcium carbonate, VHMC, and nonstoichiometric dolomite are replaced by stoichiometric dolomite. The results from this study show that dissolved sulfate inhibits the dolomitization of calcium carbonate (Figure 2). The data do show that the crystal growth was slowed because the slope of the rapid growth stage decreased, assuming that the rapid growth stage only involves crystal growth. The non-sulfate solution reaches the rapid growth stage at approximately 50 hours of heating, where as the sulfate solution required almost 100 hours to begin the rapid growth (Figure 2). The non-sulfate experiment first produced 100% stoichiometric dolomite in approximately 120 hours. If growth were to proceed at the same rate as the non-sulfate experiment then the sulfate solution should produce dolomite in about 170 hours. However, the sulfate solution required approximately 240 hours of heating to produced 100% stoichiometric dolomite. This significant increase in time clearly shows that crystal growth has been suppressed by 504-2. The induction period was also increased by addition of sulfate. This could be the result of: (1) slowed nucleation only (2) slowed growth only or (3) slowed both nucleation and growth. These three possibilities result in different transformation curves (Figure 3). The curves are based on fitting the data to the sulfate free reaction. Nucleation in the sulfate fi'ee solution is assumed to have occurred within the first 35 hours of heating and the relative rates of slow growth and rapid growth are assumed to remain constant. These assumptions are based on the results from the high temperature hydrothermal bomb experiments of Nordeng and Sibley (1994). This work supports these assumptions because dolomite nucleation for their study required more than 12 but less than 48 hours of heating. Also, a comparison between isothermal experiments and cycled experiments (between 193°C and room temperature) indicate that nucleation is suppressed by cycling. Even though nucleation is slowed for the cycled experiments, the growth IS .0323 5? EB 3223 358.598 383502 2: ES... £28m .m Emmi 4 wh<5qmum I._._>> m._.506 oz