Tridymite

tectosilicate

cristobalite

Formula: SiO2 tectosilicate (framework silicate) Silica minerals stability diagram
Tridymite is the low pressure, high temperature, triclinic polymorph 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

Tridymite is chiefly found in magmatic rocks, especially felsic volcanic rocks. It is also a constituent of some opal and chert (Dana).

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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