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Formula: CuAl6(PO4)4(OH)8.4H2O
Hydrated phosphate containing hydroxyl, turquoise group
There is nearly always some replacement of Al3+ by Fe3+, forming a partial
solid solution with chalcosiderite.
Crystal System: Triclinic
Specific gravity: 2.6 to 2.9 measured, 2.91 calculated
Hardness: 5 to 6
Streak: White
Colour: Azure-blue to apple-green
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Fe,Ca
Environments:
Sedimentary environments
Hydrothermal environments
Turquoise is a secondary mineral deposited by copper-bearing
hydrothermal waters or meteoric waters containing dissolved copper (R&M 87.4.333). It is usually found in
small veins
in decomposed volcanic rocks in arid regions.
It accumulates in crevices, particularly in trachyte,
slate and sandstone.
Localities
At the Mount Lofty ranges, South Australia, turquoise is found at the Moculta quarry associated with
apatite and jarosite. At the Fairview workings
it is associated with wavellite and fluellite
(AJM 17.1.26).
At the Mount Deverell variscite deposit, Milgun Station, Western Australia,
turquoise
occurs in altered variscite associated with
wardite
and millisite. The variscite deposits
are hosted
by marine sedimentary rocks
(AJM 20.2.31).
In Hubei, China, nodules of turquoise have been found, some distinctly green coloured, possibly due to iron impurities
replacing aluminium
(AESS).
Turquoise from Hubei
In Tibet, China, nodules of turquoise are widely used for jewellery
(AESS).
Turquoise from Tibet - Image
At Nacozari, Mexico, turquoise pseudomorphs after
apatite have been found
(KL p211).
Turquoise from Nacozari - Image
At the Butte Mining District, Silver Bow County, Montana, USA, field evidence and mineral textures indicate that blue,
endmember turquoise formed with chalcocite in the enrichment blanket,
and that green, iron-bearing turquoise formed at a higher elevation, above
the pre-mining water table. Indications are that iron-bearing turquoise is
an alteration product of turquoise that formed below the water table and was later stranded in the oxidised zone
due to relative uplift associated with extensional faulting. The fact that turquoise and
tinticite were never observed in exposures of unmodified
hypogene mineralisation deeper in the mine implies that these
minerals are indeed of supergene origin and did not form as
primary hydrothermal minerals
(CM 60.1013-1026).
Turquoise from the Butte Mining District - Image
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