Microstructure of SiO2−Al2O3−CaO−P2O5−K2O−F- Glass Ceramics. 2. Time Dependence of Apatite Crystal Growth

It is shown that fluorapatite needles formed in SiO2−Al2O3−CaO−P2O5−K2O−F- glasses upon heat treatment at 1200 °C undergo a time-dependent coarsening that fulfills the criteria of Ostwald ripening. Thermodynamic considerations imply that different growth mechanisms (normal and spiral growth) along different crystallographic directions are responsible for the strongly direction-dependent growth rates observed at 1200 °C, leading to an apatite-needle aspect ratio as large as 15:1 after 15 h at 1200 °C. The microstructure is textured by postcrystallization extrusion, facilitating a thorough investigation of the apatite needle morphology and the determination of anisotropic crystal-growth rates. During very long-term heat treatments at 1200 °C, however, the formation of isometric, blocklike apatite crystals is encountered. The latter crystals are formed at the expense of apatite needles and their occurrence proves that the needle morphology is not a result of the equilibrium shape but caused by kinetic growth...