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In SiO2−Al2O3−CaO−P2O5−K2O−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 31P 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.

It is shown that fluorapatite needles formed in SiO 2 -Al 2 O 3 -CaO-P 2 O 5 -K 2 O-Fglasses 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 apatiteneedle 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 longterm 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 effects. But although the transition from the growth habit to the equilibrium shape limits the maximum aspect ratio of fluorapatite needles (15: 1), the extruded glass ceramics exhibit an extraordinary degree of texturing.