Early stages of crystallization in canasite-based glass ceramics

Previous work by Miller et al. (2000-2004) has demonstrated that canasite-based glass ceramics have potential for use as biocompatible glass ceramics in hard-tissue augmentation. Several compositional modifications with respect to the stoichiometric formula (K2Na4Ca5Si12O30F4) were studied and biocompatibility in simulated body fluid was reported. However, the mechanism(s) of crystallization were not investigated in detail. The purpose of this study was to examine the early stages of nucleation and growth in four glass compositions using X-ray diffraction and transmission electron microscopy. In stoichiometric compositions (CAN1), laths of predominantly frankamenite homogeneously nucleate throughout the glass at similar to 700 degrees C without the presence of a nucleating phase. However, in Na2O-deficient compositions (CAN2), CaF2 particles (650 degrees C) act as nucleating sites for canasite laths (700 degrees C). In CaO-rich compositions (CAN3), CaF2 particles (650 degrees C) once again act as nucleating sites but for xonotlite (700 degrees C) rather than canasite laths. Instead, frankamenite and canasite crystallize to become the dominant phases at > 700 degrees C. In P2O5-modified compositions (CAN4), CaF2 and fluorapatite, present on cooling, act as nucleating agents for canasite (750 degrees C).