Apatite has the composition Ca5[PO4]3(OH,F,Cl) and is the major primary mineral source of phosphorous to most soils. It has a hexagonal crystal system and variable composition. Fluorine, hydroxyls, chlorine, and others may substite in the structure freely, thus forming fluorapatite, hydroxyapatite, chlorapatite, etc. Numerous other cations may substitute for calcium, though usually in small quantities.
The model shown is chlorapatite, in part because it's easier to identify the bright green chlorine atoms than the less highly colored fluorine atoms or hydroxyls.
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Apatite is widely distributed as an accessory mineral. It occurs in igneous, sedimentary, and metamorphic rocks, and can also be found in pegmatites and other veins, most likely occurring by hydorthermal origin.
Apatite is the most common primary mineral source of phosphorous in soils. It commonly occurs in the silt-sized fraction and may readily be identified in thin section. Apatite has been used extensively as a phosphorous fertilizer and is still mined for that purpose today.
Hughes, J.M., M. Cameron, and K.D. Crowley. 1991. Ordering of divalent cations in the apatite structure: Crystal structure refinements of natural Mn- and Sr-bearing apatite. American Mineralogist 76:1857-1862.