Opal

minerals

uranophane

boltwoodite

hyalite

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Formula: SiO2.nH2O
Tectosilicate (framework silicate)

Varieties

Hyalite is a colourless variety of opal, opal-AN, which is an amorphous silica-glass containing about 3-8% water (Mindat)

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
Environments:

Volcanic igneous environments
Pegmatites
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.

Localities

At White Cliffs, New South Wales, Australia, opal pseudomorphs after ikaite have been found (KL p259).

At the Mount Deverell variscite deposit, Milgun Station, Western Australia, opal forms surface coatings and thin veins along the margins of variscite veins. The variscite deposits are hosted by marine sedimentary rocks (AJM 20.227.).

At Tongbei, Fujian province, China, pseudomorphs of opal variety hyalite after quartz have been found with spessartine (KL p259).

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, metauranospinite and boltwoodite.

At the Potter-Cramer mine, Vulture Mining District, Maricopa Co., Arizona, USA, opal-AN occurs as colourless to milky white botryoidal masses that fluoresce cornflower-blue, and are usually found in association with red-fluorescing wickenburgite (R&M 91.1.33).

Amity, Town of Warwick, Orange county, New York, USA, is an area of granite intrusions into marble and associated gneiss. The marble is mostly composed of white crystalline calcite that often has small flakes or spheres of graphite and phlogopite. Opal variety hyalite uncommonly occurs as white masses that strongly fluoresce green (R&M 96.5.438).

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