Crystallisation and substitutions of fluor-magnesio-richterite (Li,Na,K)2Mg6Si8O22F2 glasses

The effect of Li+ and K+ substitutions for Na + on the crystallisation of the fluor-magnesio-richterite, Na 2Mg6Si8O22F2, glasses was investigated. Transparent colourless glasses were obtained in all cases except when sodium was fully substituted by Li when cristobalite crystals where found in the melt. XRD measurements showed the presence of fluor-magnesio-richterite, proto-amphibole,fluorophlogopite, taeinolite, enstatite, forsterite, lithium disilicate and cristobalite in the samples crystallised in the 600-1000°C temperature range. The coefficient of thermal expansion values of the glass-ceramic samples were in the range 98-116×10-7°C -1 (20-500°C) depending on the composition: fluoramphibole giving relatively higher values while enstatite and fluormica gave relatively lower values. The glass-ceramics had medium densities of 2.794-2.925 g/cm3. SEM images showed homogeneous microstructures of interlocked fibrous and flake-like crystals in a cryptocrystalline groundmass.