Calcium carbonate is a major natural mineral in surface environment and plays an important role in the global carbon cycle. Calcium carbonate has three polymorphs: calcite, aragonite and vaterite. Carbonates with the calcite structure tend to capture impurity ions whose ionic radius is smaller than that of Ca2+ (e.g., Mg2+, Fe2+, Mn2+, etc.). In contrast, carbonates with the aragonite structure tend to capture impurity elements whose ionic radius is larger than Ca2+ (e.g., Sr2+, Pb2+, Ba2+, etc.). Actually, strontium is an incompatible element with calcite. In addition to the three polymorphs of calcium carbonate, amorphous calcium carbonate (ACC) has attracted attentions from many disciplines. ACC is known as a precursor material of biominerals and appears at an early stage of precipitation from a supersaturated solution of CaCO3. Many organisms form skeletons of Mg-rich calcite with Mg contents greater than 10 mol %. The high Mg content in biomineral calcite is explainable by the high Mg content in the precursor ACC. Inspired by biomineralization, other ions might be captured into the calcite lattice through ACC. In this study, we have developed a new method for introducing incompatible Sr into calcite with pressure-induced crystallization of ACC. Strontium-doped ACC was synthesized by mixing 0.1 M Na2CO3 and 0.1 M blended solutions of CaCl2 and SrCl2 with varying Ca/Sr ratios at 0 °C. Precipitates were filtered immediately using a membrane filter, washed with acetone, and dried for 1 day in a vacuum desiccator. Obtained Sr-doped ACC samples were confirmed to contain no crystalline phase from X-ray diffraction (XRD) patterns. The ACC samples were pressurized at 0.8 GPa for 10 min at room temperature. After decompression, the recovered samples were kept in a vacuum desiccator for removal of water emitted after pressure-induced crystallization. After compression, Sr-doped ACC samples were transformed to calcite. The lattice volume of calcite increased to 375.7 Å3 (2.2 % increase from a Sr-free calcite) with increasing Sr content in ACC. The Sr content (Sr/(Sr+Ca)) in the Sr-doped calcite was 16 mol% at maximum. Amorphous materials can accomodate elements that are structually incompatible with the corresponding crystalline phases with high concentration. Results of this study are expected to present new avenues for cereating new materials by doping incompatible ions into a crystal phase through an amorphous state.

Strontium-doping into calcite through pressure-induced crystallization from amorphous calcium carbonate

251st National Meeting (2016)

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