Effect of sodium content on the crystallization behavior of fluoramphibole glass-ceramics

The purpose of this study was to evaluate the effect of sodium content on the crystallization behavior of glass-ceramics in the system SiO(2)-MgO-CaO-Na(2)O-K(2)O-F. Five glass compositions were prepared with increasing sodium content. The compositions were melted at 1400 degrees C for 2 h and cast into 60 x 12-mm ingots. Differential thermal analyses were performed on the glasses up to 1400 degrees C. Discs (1.5 x 12 mm) were cut from the ingots, nucleated in the temperature range 600-650 degrees C for 1-2 h, and crystallized in the temperature range 900-1000 degrees C for 0.5-4 h. The crystalline phases were analyzed by x-ray diffraction. The microstructure was investigated by scanning electron microscopy. Differential thermal analyses showed that the crystallization exotherm occurred in the temperature range 600-750 degrees C. There was a linear relationship between the amount of sodium and the transformation temperatures. X-ray diffraction revealed the presence of mica and diopside as major crystalline phases in the sodium-free composition. Mica, diopside, and fluorrichterite were present in all other glass-ceramics. The microstructure of the sodium-free glass-ceramic was characterized by the presence of hexagonal mica crystals and prismatic diopside crystals. All other compositions exhibited needle-shaped fluorrichterite crystals (2-5 microm in length) in addition to mica and diopside crystals.