In vitro synthesis of high Mg calcite under ambient conditions and its implication for biomineralization process

Instead of calcite or aragonite, thermodynamically unstable magnesium-bearing calcite occurs in many marine organisms such as sea urchin, sea star, corallina algae, and so on. However, the formation process of biogenic high-magnesium calcite has been an enigma. In this work, high-magnesium calcite in pure phase was synthesized in large yield via polymer-stabilized amorphous calcium magnesium carbonate (ACMC) precursors under mild conditions for the first time. The magnesium contents in Mg-bearing calcite can be exquisitely tuned from 15 to 40 mol % by changing the Mg/Ca ratio in mother solutions. We found appropriate polymer concentration windows for the synthesis of pure phase high Mg-calcite under defined crystallization conditions. It is proposed that the role of dextran sulfate sodium salt was to restrain the formation of aragonite, and the main role of poly(acrylic acid) was to stabilize ACMC. In this work, we designed a strategy to prepare thermodynamically unstable high Mg-calcite; that is, the preparation of polymer-stabilized precursor phase and the crystallization process were separated into two steps. This strategy might be extended to the synthesis of other thermodynamically unstable phases. The in vitro fabrication of pure high-magnesium calcite may give some indirect clues to the formation mechanism of biogenic Mg-calcite.