EVALUATING THE EFFECTS OF FLUID NACL AND KCL CONCENTRATIONS ON REACTION RATE, MAJOR CATION COMPOSITION, AND CATION ORDERING DURING HIGH-TEMPERATURE DOLOMITIZATION EXPERIMENTS

Dolomitization is commonly described as a dissolution-precipitation reaction whereby a CaCO₃ precursor is replaced by MgCa(CO₃)₂. Many factors affect the reaction rate, major cation composition (i.e. stoichiometry), and degree of cation ordering in dolomite, including the Mg/Ca ratio and temperature of the fluid, as well as the mineralogy and surface area of the precursor carbonate. Previous studies have proposed a number of hypotheses about the effects of salts on dolomitization, but none provides a clear test of the variable in question. In this study, the kinetic effects of common salts (KCl and NaCl) in the dolomitizing fluid were evaluated experimentally. Dolomitization of aragonite and calcite reactants took place in a 0.86 M MgCl₂*6H₂O - CaCl₂*2H₂O stock solution (Mg²⁺/Ca²⁺ = 1). Either NaCl or KCl was added to the stock solutions to attain experimental solutions with salinity values typical in common diagenetic environments (e.g., marine and sabkha). Teflon lined stainless steel reaction vessels were charged with 15 mL solution and 100 mg solid reactants, and heated to 215° C in a high precision gravity-driven convection oven. Reaction vessels were removed after predetermined times (2-800 hours) and force cooled with compressed air. Solid products were filtered, rinsed with deionized water, and dried in a vacuum desiccator. Powdered solids were analyzed using standard X-ray diffraction techniques to determine mineralogy, dolomite stoichiometry, and degree of cation ordering. Experimental data clearly indicate that as salinity of the dolomitizing fluid increases, the reaction rate decreases. Higher salinity values and slower reaction rates are also coupled with increases in dolomite stoichiometry (mol% MgCO₃) and cation ordering, which is consistent with the hypothesis that slower dolomitization rates promote the formation of more ideal dolomite. These observations presented here add to our understanding of the fundamental controls on dolomite stoichiometry and cation ordering, which may be useful proxy resources in our effort to elucidate the origin of ancient dolomites.