Crystallization in the Na2O–CaO–Al2O3–SiO2–(LiF) glass compositions

Spodumene-nepheline glass-ceramics was prepared from Na2O–CaO–Al2O3–SiO2 glass compositions. Addition of LiF changes the course of the reaction toward the formation of β-spodumene together with nepheline. The crystallization of β-spodumene strongly reduced the thermal expansion coefficient from 70, in the base sample, to 17, in that containing 6 wt.% LiF × 10−7 °C−1 (20–700 °C). Addition of 3 wt% LiF is considered the best as it reduces the expansion coefficient and enhances the microstructure uniformity. Higher content of LiF results in impairing the machinability as the content of β-spodumene tends to exceed 50%. Li+ substitute Na+ cations resulting in the formation of β-Spodumene ss. However, shift in the nepheline d-spacing lines with the appearance of some unidentified ones may reflect a state of solid solution formation. The presence of fluorine in such glass structure is found to promote the nepheline growth. The transformation temperature (Tg) and the softening temperature (Ts) were reduced as a result to the addition of LiF.