Crystallization of a Tetrasilicic Fluormica Glass

The crystallization of a tetrasilicic synthetic mica glass of nominal composition K₂Mg₅Si₈O₂₀F₄ was investigated at selected temperatures from 560° to 1150°C using DTA, density measurements, electron microscopy, and X-ray diffraction techniques. Crystallization is shown to occur in 2 stages. (1) Initial structural ordering occurs at ∼600°C, as indicated by a strongly exothermic reaction, an increase in density, and the appearance of X-ray diffraction lines. (This transformation temperature is related to fluorine content.) (2) The crystal morphology of the glass-ceramic changes at 900° to 1150°C. A mechanism is proposed for the transformation from amorphous to crystalline structure in this synthetic mica system.