Turquoise: How Is Turquoise Formed?
Turquoise is a valuable mineral and is possibly the most valuable, non-transparent, non-metal mineral in the jewelry trade.
Turquoise is probably one of the oldest gemstones known. Yet, only its prized blue color, a color so distinctive that its name is used to describe any color that resembles it, results in its being used as a gemstone.
Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula Cu(Al,Fe3+)6(PO4)4(OH)8 · 4H2O.
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| Natural Turquoise specimen. |
The word turquoise dates to the 17th century and is derived from the French turquois for "Turkish" because the mineral was first brought to Europe through Turkey, from mines in the historical Khorasan Province of Persia.
The finest of turquoise reaches a maximum Mohs hardness of just under 6, or slightly more than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals, and all of its properties are highly variable. X-ray diffraction testing shows its crystal system to be triclinic.
Chemically, a hydrated phosphate of copper and aluminum, turquoise is formed by the percolation of meteoric or groundwater through aluminous rock in the presence of copper. For this reason, it is often associated with copper deposits as a secondary mineral, most often in copper deposits in arid, semiarid, or desert environments.
How Is Turquoise Formed?
As a secondary mineral, turquoise apparently forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of pre-existing minerals. For example, the copper may come from primary copper sulfides such as chalcopyrite or from the secondary carbonates malachite or azurite; the aluminium may derive from feldspar; and the phosphorus from apatite. Climate factors appear to play an important role as turquoise is typically found in arid regions, filling or encrusting cavities and fractures in typically highly altered volcanic rocks, often with associated limonite and other iron oxides. In the American southwest turquoise is almost invariably associated with the weathering products of copper sulfide deposits in or around potassium feldspar bearing porphyritic intrusives.
In some occurrences alunite, potassium aluminium sulfate, is a prominent secondary mineral. Typically turquoise mineralization is restricted to a relatively shallow depth of less than 20 m, although it does occur along deeper fracture zones where secondary solutions have greater penetration.
Although the features of turquoise occurrences are consistent with a secondary or supergene origin, some sources refer to a hypogene origin. The hypogene hypothesis, which holds that the aqueous solutions originate at significant depth, from hydrothermal processes. Initially at high temperature, these solutions rise upward to surface layers, interacting with and leaching essential elements from pre-existing minerals in the process. As the solutions cool, turquoise precipitates, lining cavities and fractures within the surrounding rock. This hypogene process is applicable to the original copper sulfide deposition; however, it is difficult to account for the many features of turquoise occurrences by a hypogene process. That said, there are reports of two phase fluid inclusions within turquoise grains that give elevated homogenization temperatures of 90 to 190 °C that require explanation.
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| Rough Spiderweb Turquoise |
Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are exceedingly rare. Typically the form is vein or fracture filling, nodular, or botryoidal in habit. Stalactite forms have been reported. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Odontolite is fossil bone or ivory that has been traditionally thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Intergrowth with other secondary copper minerals such as chrysocolla is also common.
The increased acceptance of turquoise resulted in higher prices, some of the most desirable materials going for as much as $2,500 per kg. The increased demand could not be met through production of acceptable mine run materials. Therefore, an industry emerged--the business of turquoise stabilization, reconstitution, and the manufacture of synthetic and simulated turquoise.
Turquoise Group
- Chalcosiderite-Turquoise Series.
- Planerite-Turquoise Series.
- The copper analogue of Faustite.
Spiderweb Turquoise
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| Spiderweb Turquoise |
Spiderweb Turquoise is rare Turquoise that has matrix that is in the form of spiderweb's.
Most American Turquoise that has spiderwebing comes from Nevada, however, it is found in Arizona and Colorado Turquoise as well. Spiderweb Turquoise Jewelry is considered to be some of the most valuable Jewelry made in the Southwest by collectors and Turquoise Connoisseurs.
When other materials appear within turquoise, those materials which often look like veins, are referred to as matrix or part of the “mother rock”. Matrix can range in color from honey gold (rhyolite, a volcanic rock) or brown (iron oxide) to jet black (iron pyrite aka iron sulfide) and many other color variations.
The material can be solid colored or spiderwebbed with either brown or black webbing; the spiderwebbing may occur in any of the different colors or shades. Some of the blue material is represented as the finest pure-blue turquoise produced. It can occur in thin veins or seams or as nodules, with single nodules reported as large as 150 pounds.