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Calcite and Aragonite Precipitation Under Controlled Instantaneous Supersaturation: Elucidating the Role of CaCO₃ Saturation State and Mg/Ca Ratio on Calcium Carbonate Polymorphism

¹ Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, U.S.A.
² Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, U.S.A.; lgonzlez{at}ku.edu

For many decades researchers have attempted to clarify the factors controlling the precipitation of calcium carbonate polymorphs and the incorporation of Mg into calcite precipitating in natural systems. In this study a novel experimental approach delineates the role of instantaneous supersaturation and solution Mg/Ca ratio in calcium carbonate mineralogy and composition. The results of this study show that in solutions with different Mg/Ca ratios, the transition between the aragonite and the calcite + aragonite precipitation fields is controlled by a combination of the saturation state of the solution with respect to CaCO₃ and the Mg/Ca ratio in solution. The data show that for increasing solution Mg/Ca ratios, progressively higher supersaturation level is required for calcite precipitation.

The predominance of aragonite at high solution Mg/Ca and low supersaturations is attributed to a relative decrease in calcite growth rates as a result of increasing incorporation of Mg²⁺ in the calcite lattice at higher solution Mg/Ca ratios. As suggested by abundant experimental work, increasing solution Mg/Ca decreases the growth rate of calcite while aragonite growth rates stay unaffected. Our experiments do demonstrate that despite decreased growth rate the percentage of MgCO₃ in calcite increases with increasing solution Mg/Ca ratio. As calcite growth rates decrease, aragonite growth rates stay constant and it becomes the dominant mineral phase in solutions with high Mg/Ca ratio and low supersaturations.

These findings are relevant to present-day mineralogical distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogic changes through geologic time. The observed dependence of calcium carbonate mineralogy on solution composition and supersaturation suggests that carbonate mineralogy is controlled by kinetics of growth.