Crystallization of Canasite/Frankamenite-Based Glass-Ceramics

Glass-ceramics based on the naturally occurring quadruple chain silicate, canasite (Na4K2Ca5Si12O30F4), have been produced. The phase evolution of these materials has been investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM). Two sequences of nucleation and growth have been identified. When CaF2 crystals are present prior to the formation of the chain silicate phases, canasite and frankamenite (a fluorine-rich form of canasite) are the major crystalline phases at (>750 °C) along with minor phases of xonotlite and cristobalite depending on temperature. If CaF2 forms, frankamenite is the major crystalline phase between 750 and 850 °C but is replaced by xonotlite at >850 °C. It is postulated therefore that the presence of CaF2 crystals is a prerequisite for the formation of true canasite glass-ceramics.