Formula: SiO2.nH2O
Tectosilicate (framework silicate)
Hyalite is a colourless variety of opal, opal-AN, which is an amorphous silica-glass containing about 3-8% water.
Specific gravity: 1.9 -2.2
Hardness: 5 - 6½
Streak: White
Colour: Colourless, transparent (variety hyalite), whitish, bluish with a play of rainbow colours (precious opal), red to orange, translucent (fire opal), green, red, brown, yellow, opaque (common opal)
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid

Volcanic igneous environments
Sedimentary environments
Basaltic cavities
Hot spring deposits

Opal is a low temperature secondary mineral that develops in a wide variety of rocks as cavity and fracture fillings; it may be deposited by hot springs at shallow depths, and it may replace the cells in wood and the shells of clams. The largest accumulations of opal are formed from silica-secreting organisms.

At Zacatecas, Mexico, opal variety hyalite occurs in rhyolite. It exhibits green daylight fluorescence due to dispersed uranyl ions, and it is associated with meta-autunite, haiweeite, uranophane, meta-uranospinite and boltwoodite.

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