Microstructure of SiO₂−Al₂O₃−CaO−P₂O₅−K₂O−F⁻ Glass Ceramics. 1. Needlelike versus Isometric Morphology of Apatite Crystals

In SiO 2 −Al 2 O 3 −CaO−P 2 O 5 −K 2 O−F - glasses, either spherical or needlelike fluorapatite crystals can be formed depending on the heat-treatment schedule. If the annealing is performed at 800 °C or low heating rates are applied, isometric apatite crystallizes whereas the heat treatment at 1200 °C in conjunction with high heating rates results in apatite needles. On the basis of the combination of 31 P MAS NMR spectrometry, transmission-electron microscopic imaging, and energy-dispersive X-ray spectrometry, a crystallization model capable of explaining the morphology change is derived. Phase-separation processes are shown to have a major impact on the composition of the residual glass hosting apatite crystals and the structure and composition of the glass matrix in turn determine the fluorapatite morphology observed. At 800 °C, spherical crystals are formed since the two different growth modes parallel and perpendicular to the crystallographic c axis of apatite (provoking anisometric growth at high temperature) are outweighed by hampered diffusion.