Topotaxial Conversion of Chlorapatite and Hydroxyapatite to Fluorapatite by Hydrothermal Ion Exchange

The ion exchange of Cl - and OH - with F - in synthetic calcium chlorapatite and calcium hydroxyapatite single crystals was investigated under hydrothermal conditions. The hydrothermal treatments were conducted using 5 and 10 M KF solutions, at temperatures of 500 and 600 °C under a constant internal pressure of 100 MPa, for intervals between 6 and 336 h. The ion exchange of Cl - in the chlorapatite crystals proceeded faster than that of OH - in the hydroxyapatite crystals during hydrothermal treatments at 500 °C in 5 M KF solution. Under these conditions, the chlorapatite crystals were completely converted to fluorapatite crystals. In contrast, the hydroxyapatite crystals were only partially converted to fluorapatite. In both cases, the topotaxial ion-exchange reaction was achieved by a mechanism of dissolution−precipitation of clusters. The partially and completely converted fluorapatite crystals had a characteristic texture, exhibiting tiny tunnels along the c axis of the crystals, and a thin layer of CaF 2 was also produced on the surface of the converted crystals. The influence of the reactivity of the individual apatites on the control of the ion exchange rate is discussed.