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Apatite

The Phosphate Minerals

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Apatite (Ca5(PO4)3F) is a key part of the phosphorus cycle. It is widespread but uncommon in igneous and metamorphic rocks. (more below)
The main igneous and metamorphic phosphate
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
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 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.

Apatite 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 5 in 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.

Other Primary Minerals

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