Apatite is a phosphate mineral (and is the most abundant phosphorous-bearing mineral). The name actually covers three different minerals (fluorapatite, chlorapatite and hydroxylapatite) depending on the predominance of either fluorine, chlorine or the hydroxyl group. These ions can freely substitute in the crystal lattice and all three are usually present in every specimen, although some specimens have close to 100% in one or other. The three are usually considered together due to the difficulty in distinguishing them in hand samples using ordinary methods.
Apatite is widely distributed in all rock types ( igneous, sedimentary and metamorphic), but usually as small disseminated grains, or cryptocrystalline fragments. Large, well-formed crystals can be found in certain contact metamorphic rocks.
Chemical composition - Ca 5(PO4) 3(OH, F, Cl)
Hardness - 5
Specific gravity - 3.1-3.2
Transparency - Transparent to translucent
Colour - Typically green but also yellow, blue, reddish brown and purple
Streak - White
Lustre - Vitreous to greasy
Cleavage/fracture - Poor / conchoidal
Crystal habit/mode of occurrence - Prismatic (hexagonal prism with hexagonal pyramid or pinacoid or both as termination), acicular / granular, massive.
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