Turquoise

turquoise

chalcosiderite

fluellite

Images

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).

At Nacozari, Mexico, turquoise pseudomorphs after apatite have been found (KL p211).

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).

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