Amorphous Magnesium–Calcium Carbonate Phosphate: Crystallization Paced by the Reaction Solution Concentration

The presence of magnesium and phosphorus in calcium carbonate-based biominerals is increasingly found. Both elements play a significant role in the biomineralization process of amorphous calcium carbonate (ACC). While extensive research has focused on the effects of their compositions, less attention is given to the influence of precursor solution concentrations, which is essential for unraveling the crystallization mechanism. Herein, taking amorphous magnesium calcium carbonate phosphate (MgACCP) (molar ratio of Ca2+/Mg2+/CO32–/PO43– fixed at 4:1:4:1) as the example, we report that the amorphous stability highly depends on the precursor solution concentrations. Moderate concentrations (0.04–0.6 M) lead to faster crystallization within a week and the production of bundled nanofibers. In more diluted solutions (0.01 M), the accumulation of Ca2+ and CO32– at the boundaries of colloidal nanobubbles leads to hydration, which stabilizes ACC. Conversely, in more concentrated solutions, a greater amount of Mg2+ in the homogeneous solution binds with water to preserve the amorphous state of MgACCP. The hydration level is determined to be a critical factor in determining the crystallization rate. These findings offer new insights into the crystallization mechanism and morphology control of bioceramics.