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 (Mindat)
Opaline occurs as pseudomorphs of opal after serpentine

Crystal System: Amorphous
Specific gravity: 1.9 to 2.3 measured
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 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 county, 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).

At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the pegmatites are beryl-type rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most complex zonation and diverse mineralogy of any of the Keyes pegmatites. Six zones are distinguished, as follows, proceeding inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to 1.2 meters in size that were altered to vuggy albite-muscovite with fluorapatite crystals. This unit presumably was the source of the albite, muscovite, fluorapatite, quartz and other crystallised minerals found in pieces of vuggy albite rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including tourmaline, graftonite, triphylite, vivianite, pyrite, pyrrhotite, and beryl crystals to 30.5 cm long and 12.7 cm across.
Opal, variety hyalite, was found on the base of a 15-cm smoky quartz crystal, forming a thin, colourless, bubbly crust that fluoresces bright green under shortwave UV (R&M 97.4.322).

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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