What is "tridymite"

Tridymite \Trid"y*mite\, n. [Gr. ? threefold. So called in allusion to the supposed threefold nature of the twin crystals.] (Min.) Pure silica, like quartz, but crystallizing in hexagonal tables. It is found in trachyte and similar rocks.

n. (context mineral English) A rare mineral of volcanic rocks that solidified at a high temperature, with the chemical composition silicon dioxide, siliconoxygen₂. Tridymite is chemically identical to quartz, but has a different crystal structure. Sometimes contains sodium aluminum silicate, which seems to be expelled when quartz solidifies at lower temperatures.

n. a mineral form of silica

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. 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 (compound crystals comprising three twinned crystal components).