The Mechanism of Precipitation of Biological Minerals. The Phosphates, Oxalates and Carbonates of Calcium*

The precipitation of the phosphates, oxalates, and carbonates of calcium is complicated by the possible formation of different precursor phases involving polymorphs, hydrates, and acid salts. In order to elucidate the mechanisms of the reactions, it is necessary to study the kinetics under conditions of controlled supersaturation calculated from the activities of free ionic species. In general, the rates of formation of the salts are proportional to the (supersaturation)", where n = 1.25-2.0 suggesting a surface controlled process. However, in the case of the calcium phosphate phases, the precipitation of the thermodynamically most stable hydroxyapatite is often complicated by the formation of precursor phases which form and subsequently dissolve during the overall reactions. The sensitivity of the various solid phases to the presence of crystal growth inhibitors is markedly different. Thus in the case of calcium carbonate, it is possible to selectively inhibit calcite and aragonite by adding traces of phosphonate inhibitor, thereby encouraging the formation of vaterite, the most thermodynamically unstable phase. Such selective inhibition may explain the existence of thermodynamically unstable phases in biological systems.