Alpha-tridymite

CAS/DOT IDENTIFICATION 15468-32-3/none SYNONYMS alpha-tridymite, christensenite, tridymite, tridymite 118. 

At a temperature of 870°C, a-quartz transforms irreversibly into alpha-tridymite (a-tri-dymite, orthorhombic) [15468-32-3] with an important volume change of 14.4 vol.% as follows ... 

However, if the temperature is raised to 1470°C, a-tridymite transforms also irreversibly into alpha-cristoballite (a-cristoballite) without any change in volume as follows ... 

While tridymite and cristobalite may exist for indefinite periods of time at room temperature, tire low-temperature alpha-quartz is believed to be the form of silica truly stable at these temperatures. A considerable expansion accompanies the conversion of a-cristobalite to /J-cristobalite at 220-275°C, and a sudden expansion of -2.2% occurs during the inversion of a-quartz at 573°C. The conversion of quartz to tridymite at 870°C is accompanied by an expansion of -15%. 

---