Conversion of Calcium Fluorapatiteinto Calcium Hydroxyapatite under Alkaline Hydrothermal Conditions

The exchange of F- ions in synthetic calcium fluorapatite single crystals with OH- was investigated under alkaline hydrothermal conditions. Experiments were performed at various temperatures from 600 to 800°C at 100 MPa, for intervals between 6 and 192 h, in 5 and 10 M KOH solutions. The fluorapatite single crystals were sluggishly dissolved at temperatures over 700°C, which resulted in the ion exchange of F- with OH-. The rate of conversion was accelerated by increasing the reaction temperature or concentration of the alkaline solution. The ion-exchange reaction started from the prismatic ends of the fluorapatite single crystals to form acicular small crystals and holes in the original crystal along the c-axis. The acicular crystals had a lower content of F- resulting from the ion exchange, and they broke away from the original crystal as the ion exchange proceeded. This phenomenon may be due to the accumulation of residual stress, which was generated by the lattice expansion along the a-axis caused by the ion exchange. The ion-exchange reaction proceeds by a dissolution and precipitation process. The influence of the apatite structure on the conversion mechanism developed under alkaline hydrothermal conditions is proposed.