• journal_title：American Mineralogist
• Contributor：Yuki Sugiura ; Kazuo Onuma ; Yuki Kimura ; Katsuo Tsukamoto ; Atsushi Yamazaki
• Publisher：Mineralogical Society of America
• Date：2013-01-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4212
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：1
• firstpage：262
• section：Amorphous Materials: Properties, Structure, and Durability

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 CaCl₂, Na₂CO₃, and tris(hydroxymethyl)aminomethane buffer and various concentrations of PO₄ (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 PO₄ 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 PO₄ range and survived the experimental procedure (~2.5 h). In contrast, the starting time of phase transformation increased exponentially with the PO₄ concentration when it was higher than 5 μM. The vaterite spherulites and calcite crystals co-precipitated, and both polymorphs grew over time. The PO₄ was shown to be an accelerator for phase transformation from ACC into vaterite at low concentrations (Ca/PO₄ molar ratio <3000) and an inhibitor for transformation at high concentrations. We investigated the kinetics of vaterite formation in the presence of PO₄ 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 PO₄ concentration.