EFFECT OF SOLUTION CHEMISTRY AND MINERALIZATION PATHWAY ON CALCIUM CARBONATE POLYMORPH SELECTION AND COMPOSITION: IMPLICATIONS FOR CARBONATE SEDIMENTS
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 CO32- 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.