Tridymite

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 is Si 2. Tridymite was first described in 1868 and the type location. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings (compound crystals comprising three twinned crystal components).