Fire-resistant mineral and stable form of crystalline silica SiO 2 with a Mohs hardness of 7 (Fig. 1). In addition, quartz is the main component of molding and core sands in the foundry industry (Fig. 2).
Depending on temperature and pressure, different SiO2 modifications are stable:
Low-temperature quartz ↔ High-temperature quartz
Low-temperature quartz represents the most important rock-building material. When heated up to a temperature of more than 573°C, a spontaneous reversible transformation to high-temperature quartz takes place.
High-temperature quartz ↔ High-temperature tridymite
The development of this modification at about 870°C is possible only in presence of forein ions and certain temperatures.
High-temperature quartz ↔ High-temperature cristobalite
When heating quartz up to temperatures of more than 1400°C and holding this temperature (casting conditions), irreversible high-temperature cristobalite develops. The percentage of the transformed phase depends on the impurities content. During the cooling process, low-temperature cristobolate is develop in the temperature area of 270 to 180°C. In case of temperatures above 1700°C there is a melting period from which amorphous fused silica results (see Fused-silica sand).
The volume change during the transformation of single SiO2 modifications affect the fused-silica sand extension behavior (see Special sand, Fig. 1) and in this context also molding material-related casting defects (e.g. buckles).