Tridymit

Tridymit is a rare high-temperature crystalline polymorph of silica (SiO2). It is one of the crystalline forms of silica, alongside quartz, cristobalite, coesite, and stishovite, and is typically associated with silica-rich magmatic systems. The name is sometimes encountered as tridymite in English, with limited alternate spellings in other languages.

Occurrence and formation

Tridymit forms in silicic volcanic environments where high temperatures promote its crystallization from silica-rich melts or from quartz. It is most commonly found as small crystals or microscopic needle-like forms in volcanic glass, pumice, and vesicles in rhyolitic rocks and obsidian. Outside Earth, tridymite has been identified in some lunar samples and meteorites, where rapid cooling of silica-rich vapors or melts can stabilize the mineral.

Structure and stability

In terms of structure, tridymit belongs to the silica family with a framework that differs from quartz. It exists in several polymorphs depending on temperature, and the high-temperature form tends to exhibit a hexagonal-like arrangement. At ambient conditions tridymite is metastable in many natural settings and can slowly transform to quartz or cristobalite over time, especially with changes in temperature or disturbance.

Physical properties

Tridymite is colorless to white and may be transparent to translucent. It forms fine crystals that are typically small and slender. The mineral has a Mohs hardness around 6, with a relatively low to moderate specific gravity, and it is generally brittle.

See also

Silica polymorphs; quartz; cristobalite; coesite; stishovite.