Hydrothermal conversion of calcite crystals to hydroxyapatite

Hydroxyapatite (HAp) has been successfully synthesized by hydrothermal treatment of calcite crystals in H₃PO₄ suspensions at 120 or 180 °C. Synthesis of HAp crystals under different H₃PO₄ concentrations and time conditions were studied. The results showed that HAp can be produced by the hydrothermal conversion of calcite crystals in the presence of a neutral suspension consisted of Ca 50 mM and PO₄ 60 mM, at 120 and 180 °C for 6 h. Unreacted CaCO₃ remained when PO₄ concentration was less than this at lower temperature. The major formation mechanism of HAp seems to be dissolution/precipitation.