Recycling. Synthesis of Aragonite-Type of Calcium Carbonate from Calcined Scallop Shell. With amorphous calcium carbonate as an intermediate.

Aragonite type of calcium carbonate was synthesized via amorphous calcium carbonate from calcined scallop shell. The effect of aging and Mg2+ ion addition on polymorphism of the product was determined, and the mechanism of formation of aragonite is discussed. Aging of amorphous calcium carbonate first led to the formation of calcite and unstable vaterite, and the vaterite was then converted to aragonite with the longer aging. The addition of Mg2+ ions further enhanced the formation of aragonite. The elemental analysis by atomic absorption spectrometry and the characterization of the products by EPMA and XRD showed that Mg2+ ions were preferentially adsorbed on the calcite nucleus, then involved in the growing calcite (not needlelike crystal like aragonite). This adsorption inhibited the growth of calcite, leading to aragonite formation.