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
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
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.
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.
Back to Minerals