Monohydrocalcite in Comparison with Hydrated Amorphous Calcium Carbonate: Precipitation Condition and Thermal Behavior

Monohydrocalcite (MHC) and hydrated amorphous calcium carbonate (ACC) indicate many similarities in chemical composition, precipitation condition, and thermal behavior, from which the comparable thermodynamic state of these hydrated calcium carbonates can be deduced. In a reaction system of a mixed aqueous solution of Ca2+ and Mg2+ ions with CO32- ion, the MHC single phase is precipitated in a very restricted condition of the molar fraction of Mg2+ from 15 to 25% and in the temperature region from 288 to 303 K, which is surrounded by the precipitation conditions of calcite, hydrated ACC, and hydromagnesite. Two MHC samples with different crystallinities and morphologies are obtained within the present precipitation condition of MHC single phase. Although these MHC samples indicate apparently different thermal behaviors, those behaviors of thermal dehydration and subsequent transformations of as-produced anhydrous calcium carbonate can be correlated to those reported for hydrated ACC. The similarities in the precipitation conditions and thermal behaviors of MHC and hydrated ACC indicate the close relevancy of these compounds in the biomineraliztion and biomimetic mineralization processes of calcium carbonate polymorphs.