CRYSTALLIZATION ENERGETICS OF AMORPHOUS CALCIUM CARBONATE

Calcium carbonate is an inorganic mineral that plays an important role in geological mineralization of CO₂ as well as biomineralization of the hard skeletal tissues. In addition to five crystalline forms, calcium carbonate also exists in one or more amorphous forms. The amorphous calcium carbonate (ACC) is a metastable phase and readily transforms into one of the crystalline forms. Many organisms such as larvae of sea urchin and molluscan bivalves use this amorphous phase as a transient phase for the construction of the higher order complex structures of crystalline calcium carbonate. We measured the crystallization enthalpies of ACC obtained from synthetic and biogenic spicules at ambient temperature to understand the biomineralization and crystallization process of ACC. Using acid solution calorimetry, we measured the enthalpies of crystallization of synthetic hydrated ACC to calcite at ambient temperature as -17.6 ± 0.7 to -25 ± 2 kJ/mol. The crystallization enthalpy of biogenic ACC from spicule (-12.2 ± 0.3 kJ/mol) is comparable to that obtained from the DSC measurement for synthetic dehydrated ACC material (-13.3 ± 0.7 to -16.6 ± 0.4 kJ/mol). The initial hydrated synthetic ACC is significantly more metastable than the dehydrated or spicule samples. This suggests that the hydrous and anhydrous materials are quite different in structure. ACC is much more metastable relative to calcite than any of the crystalline polymorphs. Therefore, it is energetically downhill to make any of the crystalline polymorphs from ACC.