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Formula: SiO2 tectosilicate (framework silicate)
Tridymite is the low pressure, high temperature, triclinic paramorph of
quartz.
Specific gravity: 2.26 to 2.33
Hardness: 6½ to 7
Streak: White
Colour: White, colourless
Solubility: Soluble in HF and boiling Na2CO3
Environments:
Volcanic igneous environments
Meteorites and on Mars
Tridymite is chiefly found in magmatic rocks, especially felsic volcanic rocks. It is also a
constituent of some opal and
chert
(Dana).
Large tridymite deposits have been detected on Mars
(Mindat).
Alteration
At atmospheric pressure, with increasing temperature beta quartz alters to tridymite
at 870oC, and at 1,470oC tridymite alters to cristobalite.
Tridymite,
cristobalite and beta quartz can co-exist in
equilibrium at a point with temperature about 1,400 oC and pressure 30 kbar (The quartz page).
In the San Juan Mountains, Colorado, USA, tridymite is abundant in groundmass and as phenocrysts
in andesite-rhyolite
(Dana).
At Yellowstone Park, Montana, USA, tridymite occurs in cavities in
rhyolite and
obsidian
(Dana).
At Topaz Mountain, Utah, USA, tridymite occurs in cavities in
rhyolite(Dana).
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