Tridymite name is derived from the Greek tridymos meaning triplet due to the twinning tendency of the mineral to form trillings.
Tridymite is one of few known polymorphs of silicon dioxide. It was firstly described in 1868 at the type locality Hidalgo, Mexico. Up till now seven forms of tridymite have been described. The most stable forms are α-tridymite (Orthorombic) and β-tridymite (Hexagonal). When the crystallized β-tridymite is cooled down it converts to the α form leaving false pseudohexagonal crystals. At the room temperature the monoclinic symmetry should be observed. Tridymite is metastable and slowly changes into the quartz. Impurities, such as aluminium and sodium, stabilize the structure. For the pure quartz, at elevated temperatures, phase transition to the cristoballite is observed. The crystal forms include well-shaped tiny pseudohexagonal crystals (pseudomorphs of β-tridymite). The platy crystals often twin to form trillings. The plates of the mineral intersect at the angle of ca. 35°. The mineral is formed in volcanic rocks and occurs notably in Mexico, USA (Colorado, Oregon), Italy. The pictures show the specimen from Slovakia, Slanské vrchy mountain range (valued 25$).
Tridymite, Vechec quarry, Slovakia [Photo: Michal Hegedus]
Tridymite, Vechec, Slovakia [Photo: Oscar Fernández Arcís]