TURQUOISE MINERAL FACTS Nevada Turquoise gem stones
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Turquoise Mineral Facts:

Chemical Formula: CuAl6 (PO4)4 (OH)8:4H2O

Color: Blue, Bluish-green, green.

Hardness: 6
Note that turquoise nearly always occurs as a cryptocrystalline aggregate, and can vary from dense compact forms to porous chalky forms. The hardness given is the hardness for single crystals, which are rare. The hardness of the dense forms approaches the hardness of the single crystal, but the porous chalky forms may give an apparent hardness as low as 1. Therefore, field identification of turquoise should not be made based on hardness as the apparent readings can be misleading. 

Density: 2.6 to 2.8
For the reasons noted under hardness above, turquoise density readings may vary from readings in the range given here (for compact forms) to much lower apparent densities for the porous chalky forms.

Cleavage: One Perfect cleavage on 001 and one good cleavage on 010, these are rarely seen as visible crystals are uncommon.

Crystallography: Triclinic

Optics: (Refractive Index)  a = 1.61; b = 1.62; y = 1.65

Arizona Turquoise

Turquoise - Blue and green colors

Composition, Structure and Associated Minerals:
Turquoise occurs as a secondary mineral in rocks with significant copper and phosphate content. It normally appears filling spaces such as small cracks and other openings. It can also occur as botryoidal aggregations filling spaces in mud filled zones. Associated minerals are also secondary, and a complete solution exists between Chalcosiderite, CuFe6 (PO4)4 (OH)8:4H2O, and turquoise CuAl6 (PO4)4 (OH)8:4H2O. In places, variscite and turquoise also occur together. While no solid solution exists between the two, it is not uncommon that specimens may contain an aggregate of both crystals as a mix of both variscite and turquoise crystals can be precipitated from a solution if it is poor in copper.  Other secondary minerals such as Limonite (iron oxides) and secondary silica commonly occur mixed with the turquoise, both as a matrix and precipitated within the turquoise cryptocrystalline aggregate mix. 

Identification and Diagnostics
Turquoise is not often confused with Malachite or azurite, although some specimens of these two copper carbonates can take forms similar to turquoise. Chrysocolla is the most common mineral of similar appearance to turquoise. Both chrysocolla and turquoise form earthy to compact masses in light to medium blue tones, so hardness or density tests to differentiate between the two are not reliable. Far more useful is the fact that turquoise gives a positive test for phosphate, which chrysocolla will not.

In some deposits, both turquoise and variscite occur, which can lead to identification difficulties. Because of the typical green tones of variscite, it can be difficult to differentiate between the two by color alone. In comparison with variscite, turquoise will give a positive test for copper, which variscite will not. Green tones in turquoise usually indicate the presence of more or less iron in the material, and partial replacement of the turquoise by chalcosiderite. Another mineral of similar appearance and found in the same type of environment is Faustite, a copper zinc aluminum phosphate. Faustite tends to have a more yellow-green appearance.

Testing for the presence of Copper or Phosphates
FOR COPPER: The standard test for copper is to scratch or grind the mineral (if you are testing sulfides rather than turquoise they have to be roasted or oxidized first) and then put the powder in a few drops of Hydrochloric acid. Allow the mix to sit for an hour or so. Then put a drop of the powder-acid mix in a natural gas flame. The flame should turn a bright blue green from the presence of Copper Chloride if copper was present in the original mineral powder. If the Copper content is high, you will see the blue green color of the copper chloride in the acid solution even before it is put into the flame.  

FOR PHOSPHATES:  The mineral is crushed to a powder and dissolved in nitric acid (heating for digestion may be required). A few drops of the dissolved mineral solution are added to an Ammonium Molybdate solution. A precipitate of bright yellow ammonium phosphomolybdate indicates the presence of phosphorus.

SAFETY NOTE ON CHEMICAL TESTS: These tests are given as reference material only. Chemicals can be dangerous and those not well versed in chemistry and the potential hazards of these acids and any other chemical reactions that might occur, should not perform these tests.

If you would like to see some photos of our Nevada turquoise mines, and get a feel for what a turquoise mine looks like out in the field, then take a look at our TURQUOISE MINE TOUR PAGE. If you'd like to see our silver turquoise jewelry, or just get a piece of natural Nevada turquoise for yourself, you can take a look here: TURQUOISE JEWELRY    If you'd like to see some of the turquoise cabochons that have been produced from our mines, you can take a look at this page: TURQUOISE CABOCHONS

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Please note that the author, Chris Ralph, retains all copyrights to this entire document and it may not be reproduced, quoted or copied without permission.

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