Tridymite - San Pietro Montana (Padova) Italia (XX 6mm) — Photo: Didier Descouens · CC BY-SA 3.0
Tridymite
The high-temperature silica polymorph — tiny pseudo-hexagonal plates in volcanic cavities.
Intro adapted from Wikipedia (CC BY-SA).
Tridymite is an IMA-recognized mineral species in the silicate class, with the chemical formula SiO₂.
It crystallizes in the monoclinic crystal system (space group Cc). It is a member of the Tridymite structural group.
Key properties: Mohs hardness 7; specific gravity 2.25–2.28; typically white or colorless or gray; white streak; Vitreous, may be pearly on {0001} luster; refractive index 1.477; optically biaxial.
IMA symbol Trd, grandfathered status — known before the IMA’s founding, named in 1868.
Deposited from hot gases in vesicles and lithophysae; as phenocrysts in felsic volcanic rocks, and less commonly, in basalts; in contact metamorphosed sandstone.
It is found associated with Cristobalite, sanidine, quartz, augite, fayalite, \hornblende, " hematite, ferroan enstatite, troilite.
Its name comes from the Greek meaning three-twin, in allusion to the common occurrence of trillings.
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