Crystallization and oriented attachment of monohydrocalcite and its crystalline phase transformation

Abstract

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.