Crystallization of triplet alkalies (Li, K, Na)-containing fluorrichterite glasses

Crystallization characteristics of glasses based on fluoramphibole containing three alkalies were investigated. The thermal behaviour of the glass samples was studied using DTA and their heat-treated samples were characterized by XRD and SEM. Fluorophlogopite, tainiolite, fluorrichterite, roedderite, enstatite and cristobalite phases were developed in the heat-treated glasses. The high Li-containing sample shows modification of the crystalline phases and the phase transformation at each DTA exotherm. Dominance of fluorophlogopite or fluorrichterite was detected in high K- and Na-containing samples respectively with enstatite as well. Crystallization at high temperature lowered the cristobalite phase in the high alkali-content glasses, whereas all samples showed formation of roedderite and enstatite on long duration heat-treatments. The microstructure of the glass-ceramic samples tends to be fine in the case of equal triplet alkalies (Li, K, Na)- and Na-containing samples whereas it became coarse in the high K-content samples. However, it was nonhomogeneous because of dendritic growths in the high Li-content ones. The coefficient of thermal expansion of glasses showed moderate values of 76-89 × 10−7 °C−1 (20-300 °C). However, in the corresponding glass-ceramics, and with exception of the highest Li-content glass-ceramic (72-78 × 10−7 °C−1), these were relatively high especially for samples of highest K- and Na-contents (83-119 × 10−7 °C−1). The density of the glass-ceramic samples (2.821-2.641 gm/cm3) decreased in the order GNKL > GN > GK > GL > Grich.