Apatite

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). Fluorapatite makes up the framework of teeth and bones, which accounts for our dietary need for fluorine, phosphorus and calcium.

Apatite is a key part of the Earth's phosphorus cycle. It is widespread in igneous and metamorphic rocks, usually in small amounts. It 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. Here it is a white or brownish earthy mass, and the mineral must be detected by chemical tests.

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