cristobalite

Cristobalite is a mineral form of silicon dioxide (SiO2) and a high-temperature polymorph of silica. It is one of the crystalline forms of silica, alongside quartz and tridymite. Cristobalite occurs in two main polymorphs: alpha-cristobalite, stable at lower temperatures, and beta-cristobalite, stable at higher temperatures. The two forms differ in crystal structure and occur in different temperature ranges. In nature, beta-cristobalite is typically found in volcanic glass and high-temperature silica-rich rocks, while alpha-cristobalite can occur in some natural deposits and in synthetic products.

Natural occurrence and formation: Cristobalite forms when silica-rich melts or glassy materials crystallize at high temperatures, such as in volcanic environments or during the firing of silica-containing ceramics. It can also be produced synthetically for industrial use, especially in glass-ceramics and refractory materials.

Properties: Cristobalite has the chemical formula SiO2. It is white to colorless and transparent to translucent. It has a hardness around 6.5 on the Mohs scale and a refractive index close to 1.46. Its density is about 2.2–2.3 g/cm3. Compared with some other silica forms, cristobalite can exhibit a relatively high thermal expansion, which influences its behavior in heat-treated ceramics and glasses.

Stability and transformation: Cristobalite is metastable at ambient conditions and can slowly transform to quartz over time or with aging and heating. This transformation can alter the structure and properties of silica-containing materials and may contribute to devitrification or cracking in certain ceramics.

Applications and safety: In industry, cristobalite is encountered in glass-ceramics, refractories, and other silica-based materials. It is also a crystalline silica form that poses health risks when inhaled as dust; appropriate safety measures are important in mining, processing, and occupational settings.