Tridymite

(mineralogy) SiO₂ A white or colorless crystal occurring in minute, thin, tabular crystals or scales; a high-temperature polymorph of quartz.

A physical form of silica used in combination with cristobalite to limit thermal expansion.

Environment

Rare mineral of volcanic rocks.

Usually seen in thin rock slices when they are examined under the microscope, but may sometimes be observed in small cavities in volcanic rocks. They appear as thin tabular crystals, often developed so that they look hexagonal, but commonly grouped into sheaves of three crystal plates (reminiscent of a half-opened book on its side), or in intergrowths of three snow-crystal-like shapes in the commonly pseudohexagonal twinning.

White or colorless. Luster glassy; hardness 7; specific gravity 2.3; fracture conchoidal; cleavage prismatic. Fragile plates; transparent to translucent.

Silicon dioxide; like quartz, but commonly contains a significant percentage of sodium aluminum silicate.

Test behavior same as for quartz (p. 269), but the amateur collector will have to recognize it by its crystal form and manner of occurrence. Might resemble a zeolite in poor developments, but the tip of a flame would make the latter froth and boil, while tridymite just sits there.

The tabular crystals, especially when grouped into sheaves, and the rock associations are typical.

Significant as a high-temperature silicate mineral, forming only in rocks solidifying at high temperatures. Quartz, tridymite, and cristobalite (next) all have the same composition but form under different conditions. Tridymite undoubtedly sometimes forms at temperatures below its theoretically stable limit, 870°C, as does cristobalite. Then it often changes to quartz, and many specimens are really quartz pseudomorphs after tridymite.

Crystals of tridymite are usually microscopic; the largest are under Ɖ in. (1 cm) long and very thin. Good crystals have been found in gas cavities in the lavas of the San Juan Mountains of Colorado and on the flanks of Mt. Lassen in California. Reported in the crystallizing nuclei (lithophysae) of the Yellowstone National Park obsidian, with quartz, feldspar, and fayalite.

The name refers to its usual habit of crystallizing in booklike trillings, or groups of three individuals.

Tridymite in a volcanic rock sample

Crystal structure of tridymite

Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO₂. Tridymite is stable between 870 and 1470 degrees Celsius. It has a Mohs hardness of 6.5 to 7, a specific gravity of 2.28 to 2.33 and refractive indices of nα=1.471 - 1.482 nβ=1.472 - 1.483 nγ=1.474 - 1.488. Optically, tridymite has a birefringence of < .004 and has a 2V between 40 and 90 degrees, it may have wedge, complex or polysynthetic twinning.

Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek Tridymos for triplet as tridymite commonly occurs as twinned crystal trillings.

See also

Wikisource has original text related to this article: EB1911:Tridymite

• cristobalite
• coesite
• stishovite

References

• Mindat
• Webmineral
• Mineral galleries

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