tridymiit

Tridymite is a high-temperature polymorph of silica (silicon dioxide, SiO2). It is one of the three common polymorphs of silica, the others being quartz and cristobalite. Tridymite is stable at temperatures between approximately 870 °C and 1470 °C at atmospheric pressure. Below 870 °C, it transforms into various low-temperature forms of quartz, and above 1470 °C, it transforms into cristobalite. These transformations are reconstructive, meaning they involve significant breaking and reforming of Si-O bonds, and are therefore generally slow and can lead to metastable occurrences.

Tridymite typically crystallizes in the monoclinic crystal system, though at higher temperatures it exists in hexagonal forms. It is often found as white or colorless crystals. Microscopically, it can appear as twinned aggregates or intergrowths with other silica polymorphs. Its density is lower than that of quartz.

It is commonly found in volcanic rocks, particularly those with intermediate to felsic compositions, such as rhyolites and andesites, where it forms during rapid cooling from high-temperature magmas. It can also be found in meteorites and in industrial products like refractory bricks where silica is used at high temperatures. Due to its reconstructive phase transitions, pure tridymite is relatively rare and often found as metastable relics within samples of other silica polymorphs.