EFFECT OF SOLUTION CHEMISTRY AND MINERALIZATION PATHWAY ON CALCIUM CARBONATE POLYMORPH SELECTION AND COMPOSITION: IMPLICATIONS FOR CARBONATE SEDIMENTS

In addition to providing an amorphous precursor for the biological mineralization of calcified skeletal structures, a number of studies are reporting that amorphous calcium carbonate (ACC) can form as a reactive intermediate to the carbonates produced in unique geological settings. Previous experimental work shows ACC can transform to one or more of the crystalline polymorphs of CaCO₃ (calcite, aragonite, vaterite). This nonclassical pathway to mineralization may provide a missing link to explain long-standing enigmas regarding carbonate formation and chemistry in natural environments that are not readily interpreted by the classical growth process. Specifically, the high Mg carbonates seen in some lake sediments and cements are an intriguing focus of study. The control of mineralization pathway on Mg content is particularly interesting because recent studies show the nonclassical process can produce carbonates with compositions and textures that are significantly different from those grown by the classical step-growth mechanisms.

This study uses a mixed flow reactor method to prepare ACC in an inorganic environment under controlled chemical conditions and establish relationships between polymorph composition, solution chemistry, and environment of transformation. The experimental design controlled the input solution Mg/Ca ratio, total carbonate concentration, and pH to produce ACC with systematic chemical compositions. The resulting ACC was allowed to transform within the output solution under both stirred and unstirred conditions as a proxy for the physical energy of transformation environments. We find that the Mg/Ca ratio of the steady state solution has a primary control on composition, and the time to transformation, and that stirring significantly reduces the time to transformation. The first crystalline phase that forms from the ACC precursor is controlled by the activity of Mg²⁺ and CO₃^2- in the steady state solutions. Polymorph selection is also affected by stirring, with monohydrocalcite and high Mg calcite forming in the stirred and unstirred environments, respectively. The findings provide a physical basis for interpreting the environmental conditions in which calcium carbonate polymorphs form in alkaline settings.