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Amorphous calcium carbonate (ACC) is the most important intermediate phase in the nucleation/crystallization process of CaCO3, and thus the proper interpretation of how ACC transforms into final crystals at the molecular level is crucial to understand various biomineralization phenomena. Herein, we successfully monitored the moisture-induced crystallization process of ACC via in situ FTIR spectroscopy, which is very sensitive to the specific changes of the different vibrational modes of carbonates and water molecules. In combination with the tools of perturbation correlation moving window and two-dimensional correlation spectroscopy, it is found that the driving force of ACC crystallization is the fracture of hydrogen bonds formed by H2OCO32-. The bending vibrations of carbonate are more sensitive to moisture permeation than the stretching modes, and the whole crystallization process can be divided into three sequential stages, i.e., the hydrated ACC first loses its structural water and converts to the dehydrated ACC, which then gradually transforms into vaterite, followed by the final growth of vaterite crystals. Anhydrous ACC microdomains are found to be already existing in the as-prepared ACCs.
Cheng et al. (2019) studied this question.