Proto-calcite and proto-vaterite in amorphous calcium carbonates.

Crystallization of calcium carbonate, typically, progresses sequentially via metastabIc phases. Amorphous CaC03 (ACC) generally forms initially, both in vitro and in vivo, and is the precursor of the predominant anhydrous polymorphs (calcite, aragonite, and vaterite). [I-I3] A new picture of the crystallization of calcium carbonate is emerging, which involves transformations of clusters to ACC and eventually to crystalline polymorphs,P4.15] This stepwise manner has implications for the understanding of biomineralization[I6] and of crystallization. ACCs that contain additives display order over atomic length scales that are related to crystalline polymorphs;[I-3] ACC synthesized at high supersaturation