Learn About Phosphate Minerals

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The main igneous and metamorphic phosphate
The Phosphate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

The element phosphorus is very important for many facets of life. Thus phosphate minerals, in which phosphorus is oxidized in the phosphate group, PO4, are part of a tight geochemical cycle that includes the biosphere, rather like the carbon cycle.

Apatite (Ca5(PO4)3F) is a key part of the phosphorus cycle. It is widespread but uncommon in igneous and metamorphic rocks.

Apatite is a family of minerals centered around fluorapatite, or calcium phosphate with a bit of fluorine, with the formula Ca5(PO4)3F. Other members of the apatite group have chlorine or hydroxyl that take the place of the fluorine; silicon, arsenic or vanadium replace the phosphorus (and carbonate replace the phosphate group); and strontium, lead and other elements substitute for the calcium. The general formula for the apatite group is thus (Ca,Sr,Pb)5[(P,As,V,Si)O4]3(F,Cl,OH). Because fluorapatite makes up the framework of teeth and bones, we have a dietary need for fluorine, phosphorus and calcium.

This element is usually green to blue, but its colors and crystal forms vary, and apatite can be mistaken for beryl, tourmaline and other minerals (its name comes from the Greek "apate," deceit). It is most noticeable in pegmatites, where large crystals of even rare minerals are found. The main test of apatite is by its hardness, which is a 5 on the Mohs scale. Apatite can be cut as a gemstone, but it is relatively soft.

Apatite also makes up sedimentary beds of phosphate rock. There it is a white or brownish earthy mass, and the mineral must be detected by chemical tests.

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The Phosphate Minerals Lazulite. Wikimedia image

Lazulite, MgAl2(PO4)2(OH)2, is found in pegmatites, high-temperature veins and metamorphic rocks.

The color of lazulite ranges from azure- to violet-blue and bluish-green. It's the magnesium end member of a series with the iron-bearing scorzalite, which is very dark blue. Crystals are rare and wedge-shaped; gemmy specimens are even rarer. Typically you'll see small bits without good crystal form. Its Mohs hardness rating is 5.5 to 6.

Lazulite can be confused with lazurite, but that mineral is associated with pyrite and occurs in metamorphosed limestones. It is the official gemstone of the Yukon.

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A lead-bearing phosphate
The Phosphate Minerals. Photo courtesy Aram Dulyam of Wikimedia Commons

Pyromorphite is a lead phosphate, Pb5(PO4)3Cl, found around the oxidized edges of lead deposits. It is occasionally an ore of lead. 

Pyromorphite is part of the apatite group of minerals. It forms hexagonal crystals and ranges in color from white to gray through yellow and brown but is usually green. It is soft (Mohs hardness 3) and very dense, like most lead-bearing minerals. This specimen is from the classic Broken Hill mine in New South Wales, Australia, and was photographed at the Natural History Museum in London.

Other Diagenetic Minerals

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The most precious phosphate
The Phosphate Minerals. Photo courtesy Bryant Olsen of flickr under Creative Commons license

Turquoise is a hydrous copper-aluminum phosphate, CuAl6(PO4)4(OH)8·4H2O, that forms by near-surface alteration of igneous rocks rich in aluminum. 

Turquoise (TUR-kwoyze) comes from the French word for Turkish, and it is also sometimes called Turkey stone. Its color ranges from yellowish green to sky blue. Blue turquoise is second only to jade in value among the nontransparent gemstones. This specimen displays the botryoidal habit that turquoise commonly has. Turquoise is the state gem of Arizona, Nevada and New Mexico, where the Native Americans revere it.

Other Diagenetic Minerals

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A surface-dwelling phosphate
The Phosphate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Variscite is a hydrous aluminum phosphate, Al(H2O)2(PO4), with a Mohs hardness of around 4. 

It forms as a secondary mineral, near the surface, in places where clay minerals and phosphate minerals occur together. As these minerals break down, variscite forms in massive veins or crusts. Crystals are small and very rare. Variscite is a popular specimen in rock shops.

This variscite specimen comes from Utah, probably the Lucin locality. You might see it called lucinite or possibly utahlite. It looks like turquoise and is used the same way in jewelry, as cabochons or carved figures. It has what's called a porcelaneous luster, which is somewhere between waxy and vitreous.

Variscite has a sister mineral called strengite, which has iron where variscite has aluminum. You might expect there to be intermediate mixtures, but only one such locality is known, in Brazil. Usually strengite occurs in iron mines or in pegmatites, which are very different settings from the altered phosphate beds where variscite is found.

Other Diagenetic Minerals