A mineral form of silica

Wikipedia Word definitions in Wikipedia
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 ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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.

WordNet Word definitions in WordNet
n. a mineral form of silica

Wiktionary Word definitions in Wiktionary
n. (context mineral English) A rare mineral of volcanic rocks that solidified at a high temperature, with the chemical composition silicon dioxide, siliconoxygen 2 . Tridymite is chemically identical to quartz, but has a different crystal structure. Sometimes ...