High-Magnesium Calcite Mesocrystals: Formation in Aqueous Solution under Ambient Conditions.

Mesocrystals of high-magnesian calcites are commonly found in biogenic calcites. Under ambient conditions, it remains challenging to prepare mesocrystals of high-magnesian calcite in aqueous solution. We report that mesocrystals of calcite with magnesium content of about 20 mol % can be obtained from the phase transformation of magnesian amorphous calcium carbonate (Mg-ACC) in lipid solution. The limited water content on the Mg-ACC surface would reduce the extent of the dissolution-reprecipitation process and bias the phase transformation pathway toward solid-state reaction. We infer from the selected area electron diffraction patterns and the dark-field transmission electron microscopic images that the formation of Mg-calcite mesocrystals occurs through solid-state secondary nucleation, for which the phase transformation is initiated near the mineral surface and the crystalline phase propagates gradually toward the interior part of the microspheres of Mg-ACC.