Turquoise is a secondary or supergene mineral, not present in the original copper porphyry. Deposition takes place mostly in the potassic alteration zone, which is characterized by conversion of existing feldspar to potassium feldspar and deposition of quartz and micas at a temperature of 400–600 ☌ (752–1,112 ☏) The copper is redeposited in more concentrated form as a copper porphyry, in which veins of copper sulfide fill joints and fractures in the rock. This takes place when hydrothermal fluids leach copper from a host rock, which is typically an intrusion of calc-alkaline rock with a moderate to high silica content that is relatively oxidized. However, a typical turquoise deposit begins with hydrothermal deposition of copper sulfides. Turquoise deposits probably form in more than one way. "Big Blue", a large turquoise specimen from the copper mine at Cananea, Sonora, Mexico Turquoise forms a complete solid solution series with chalcosiderite, CuFe 6(PO 4) 4(OH) 8♴H 2O, in which ferric iron replaces aluminium. Turquoise is distinguished from chrysocolla, the only common mineral with similar properties, by its greater hardness. Intergrowth with other secondary copper minerals such as chrysocolla is also common. Odontolite is fossil bone or ivory that has historically been thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Typically the form is a vein or fracture filling, nodular, or botryoidal in habit. Crystals, even at the microscopic scale, are rare. Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining. Despite its low hardness relative to other gems, turquoise takes a good polish. Its streak is white to greenish to blue, and its fracture is smooth to conchoidal. Ĭrushed turquoise is soluble in hot hydrochloric acid. The refractive index of turquoise varies from 1.61 to 1.65 on the three crystal axes, with birefringence 0.040, biaxial positive, as measured from rare single crystals. The blue is attributed to idiochromatic copper while the green may be the result of iron impurities (replacing copper.) : 29 Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue and from a blue-green to a yellowish green. The lustre of turquoise is typically waxy to subvitreous, and its transparency is usually opaque, but may be semitranslucent in thin sections. With lower hardness comes greater porosity. X-ray diffraction testing shows its crystal system to be triclinic. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals, and all of its properties are highly variable. The finest of turquoise reaches a maximum Mohs hardness of just under 6, or slightly more than window glass. Pliny the Elder referred to the mineral as callais (from Ancient Greek κάλαϊς) and the Aztecs knew it as chalchihuitl. According to the same source, the gemstone was first brought to Europe from Turkestan or another Turkish territory. However, according to Etymonline, the word dates to the 14th century with the form turkeis, meaning "Turkish", which was replaced with turqueise from French in the 1560s. The word turquoise dates to the 17th century and is derived from the Old French turquois meaning "Turkish" because the mineral was first brought to Europe through the Ottoman Empire. The robin egg blue or sky blue color of the Persian turquoise mined near the modern city of Nishapur, Iran, has been used as a guiding reference for evaluating turquoise quality. Like most other opaque gems, turquoise has been devalued by the introduction of treatments, imitations, and synthetics into the market. It is rare and valuable in finer grades and has been prized as a gemstone for millennia due to its unique hue. Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula Cu Al 6( PO 4) 4( OH) 8♴ H 2O. JSTOR ( June 2022) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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