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Phase transformation of amorphous calcium carbonate (ACC) into vaterite and its subsequent stability was investigated at a constant pH (~8.2), ionic strength, and temperature that simulated the biological environment. Solutions containing the same concentrations of CaCl 2 , Na 2 CO 3 , and tris(hydroxymethyl)aminomethane buffer and various concentrations of PO 4  (0-62.5 μM) were prepared, and precipitates in the solutions were sampled at a constant interval to observe the morphology and type of calcium carbonate polymorphs that appeared. The change in the Ca-ion concentration over time, which served as a guide for phase transformation of ACC into crystalline phases, was measured in relation to the PO 4  concentration. The starting time of phase transformation was at the minimum point when the concentration was ~2-3 μM. Vaterite spherulites consisting of needle-like crystals (0.5-2 μm in length) formed only in this PO 4  range and survived the experimental procedure (~2.5 h). In contrast, the starting time of phase transformation increased exponentially with the PO 4  concentration when it was higher than 5 μM. The vaterite spherulites and calcite crystals co-precipitated, and both polymorphs grew over time. The PO 4  was shown to be an accelerator for phase transformation from ACC into vaterite at low concentrations (Ca/PO 4  molar ratio &lt;3000) and an inhibitor for transformation at high concentrations. We investigated the kinetics of vaterite formation in the presence of PO 4  and derived an advanced concept for cluster-based phase transformation. This investigation showed that the appearance and stability of calcium carbonate polymorphs is easily controlled by adjusting the PO 4  concentration.

american mineralogist

Amorphous materials: Properties, structure, and durability: Acceleration and inhibition effects of phosphate on phase transformation of amorphous calcium carbonate into vaterite