X-Ray Investigation of Tridymite-glass

Abstract

THERE has been a lengthy controversy between G. W. Morey¹ and A. Dietzel² concerning the nature of the crystallites in devitrified glass in region B of the phase diagram of the system: SiO₂ Na₂O CaO (see²). Morey, from microscopic investigations, thought that tridymite was present in this devitrified glass, whereas Dietzel, with the further help of expansion curves, decided in favour of cristobalite. Microscopic work on such a problem is very difficult, especially when dealing with crystallites in crusted in a vitreous mass. On the other hand, expansion curves are quite useless. There are three anomalies in the expansion of pure tridymite. The first occurs about 120° C., the second around 165° C. and the last at 420° C. A solid rod of 36 mm. of glass B does not give any appreciable anomaly of expansion. A rod of the same dimensions made from a powder of this devitrified glass also gives a negative result. X-ray powder analysis on the contrary gives a definite solution of the problem. A devitrified glass of the initial percentage composition: SiO₂ 78, CaO 12, and Na₂O 10 (glass B) gives an unmistakable pattern of tridymite (Co-K-50 m.a.h.).