Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O 2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico.

From the Greek "Tridymos", triplet, alluding to its common twinning as trillings. Tridymite is a low pressure, mostly high-temperature-stable polymorph of silica that can also form or persist metastably at low temperatures.

Tridymite forms thin hexagonal plates that are generally twinned, often in groups of three; its name alludes to this habit. It commonly occurs in igneous rocks, more abundantly than cristobalite, as in the trachytes of Rhineland-Palatinate, Germany; northern Italy; and in the Massif Central, France.

Detailed properties, locality information guide about the mineral tridymite (quartz polymorph).

Comments: Clear, pseudohexagonal crystals of tridymite on matrix. Location: Bellerberg quarry, Laacher See, Rhineland-Palatinate, Germany. Scale: Not Given. Colorless, White, Yellowish white, Gray. Brittle - Conchoidal - Very brittle fracture producing small, conchoidal fragments. Platy - Sheet forms (e.g. micas).

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Tridymite and cristobalite are high-temperature, low-pressure polymorphs of silica, forming stably above 870°C (tridymite) and 1470°C (cristobalite). You might find these chapters and articles relevant to this topic.

Beta-Tridymite (β-tridymite or high-tridymite) is a low pressure, high-temperature-stable polymorph of silica that occurs most notably as vapour-deposited, platey crystals in vesicles in some volcanic rocks, also rarely as phenocrysts in some felsic volcanics, or as a contact metamorphic material in some hornfels.

The meaning of TRIDYMITE is a mineral SiO2 that is a silica, differs from quartz in its usually minute thin tabular orthorhombic forms of crystallization, and is found in cavities in trachyte and similar rocks (hardness 7, specific gravity 2.28—2.33).

Tridymite is a polymorph of quartz, meaning that it is composed of the same chemistry, SiO 2, but has a different structure. Both quartz and tridymite are polymorphs with all the members of the Quartz Group which also include coesite, cristobalite and stishovite.