Crystallization of some modified fluor-miserite Kx(Ca,Ce)5−xSi8O22F2 glasses

Fluorine-rich miserite glass compositions with variable K2O to CaO ratios, Kx(Ca5.5−x,Ce0.5)Si8O21.5F3 where x = 1, 2 or 3, have been investigated in term of crystallization path and Ce3+ substitution. For sake of comparison a second series without ceria was also prepared: KxCa6.5−xSi8O21.5F3. Xonotlite, tokkoite, fluorite and cristobalite were developed in all crystallized glasses; however, miserite was only identified in Ce-containing samples. Increase of K2O/CaO ratio, in Ce-free sample, enhanced the formation of tokkoite, and reduced the contents of xonotlite and fluorite. On the other hand, in Ce-containing samples, an increase of K2O/CaO ratio promotes the formation of miserite crystals. Quantitative Rietveld analyses assessed the presence of Ce3+ cations in miserite structure. The microstructure of Ce-free glass-ceramics was relatively coarse in the form of interlocked flakes and rods; however, it became unpronounced in the sample of highest K2O/CaO ratio. In Ce-containing glass-ceramics, irregular nanosized grains were spread in cryptocrystalline groundmass, whose fineness increased with increasing K2O/CaO ratio. The CTE values of the parent glasses and their glass-ceramics varied from 10.31 to 11.05 and from 7.52 to 9.84 × 10−6 °C−1 respectively with lower values for Ce-free glass-ceramics. Density and micro hardness values were also measured.