Effect of Strontium Ions on Crystallization of Amorphous Calcium Carbonate

In biomineralization, amorphous precursors enable formation of complicated crystal shapes and incorporation of organic and inorganic impurities. In some cases the concentrations of the inorganic impurity atoms greatly exceed the thermodynamical (equilibrium) solubility limits. To shed additional light on this, crystallization experiments with Sr-rich amorphous calcium carbonate (ACC) are carried out. The Sr-concentration in ACC, Sr/(Ca+Sr) approximate to 0.3, is more than 60 times larger than its solubility limit in calcite. Crystallization of ACC is accomplished using two different pathways: i) in a high humidity environment at room temperature, and ii) upon heating at 300 degrees C. In both cases, X-ray diffraction measurements revealed the formation of particular crystal phases with an ability to take up substantial amounts of Sr. Crystals formed via the humidity route comprised spatially separated regions of almost pure rhombohedral calcite and orthorhombic aragonite/strontianite solid solution with a Sr/(Ca+Sr) ratio equal nearly 0.5. Crystallization of ACC upon heating resulted in the appearance of a new calcite-like phase with reduced space symmetry, R3 over bar m, instead of R3 over bar c for regular calcite. Symmetry reduction in this Sr-rich R3 over bar m-phase is due to the rotational disorder of flat carbonate groups about the c-axis, which is responsible for the enhanced lattice capacity of Sr incorporation.