Images
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)
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
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 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).
Back to Minerals