The Mechanism of Thermal Reactions in Apatites–Silica–Water Vapor Systems

Abstract

Abstract The reactions of fluor-, hydroxy-, and chlorapatites with silica in the presence of water vapor have been studied under various conditions−i.e., various amounts of added silica, various temperatures (up to 1300°C), and various heating times and water-vapor pressures (0.6–760 mmHg). As the pressure increased, the decomposition rate of hydroxyapatite with silica decreased considerably. Fluorapatite and chlorapatite were found to be decomposed with silica after being partially defluorinated or dechlorinated. In the case of fluorapatite, no effect of silica addition was recognized under a low water-vapor pressure (0.6 mmHg). This effect was observed under a high water-vapor pressure (760 mmHg), accompanied by rapid increases in the fluorine volatility and the destruction rate of the apatite in their time-dependence plots. In the reaction using chlorapatite, a similar effect was found under both low and high pressures. The results for the defluorination and dechlorination could mostly be interpreted by assuming the following changes: (1) halogen-oxyapatite formation, (2) the substitution of hydroxyl ions for halogen ions, and (3) the decomposition of apatite with silica.