Crystallization and oriented attachment of monohydrocalcite and its crystalline phase transformation

Crystallization and oriented attachment of monohydrocalcite (MHC) for the self-organization of dumbbell-like superstructures in the absence of any additives, as well as the crystalline phase transformation of MHC to anhydrous calcium carbonate (CaCO3), have been systematically described in this paper. In Mg2+-containing artificial seawater at 4 °C, the dissolution of initially formed amorphous calcium carbonate (abbreviated as ACC) cannot be detected prior to the crystallization of MHC, and the detected inter-particle hydrogen bonds probably induce the oriented attachment of nanocrystalline MHC building blocks with a protective Mg-rich layer. As for the crystalline phase transformation of MHC to an anhydrous CaCO3 (i.e., vaterite, aragonite or calcite), two novel aspects have been highlighted: (i) the unexpected detection of unstable vaterite for the calcination of solid-state MHC, (ii) the unique product of cube-shaped or dumbbell-like superstructures of tiny calcite rhombohedrons from Mg2+-free artificial seawater at 30 °C. In particular, the effect of Mg2+-chelators (organic acetylacetone or inorganic sodium hydroxide) on the abnormal solution-mediated transformation of MHC to calcite may be potentially applied for biomimetic synthesis purposes.