Apatite is a mineral, or rather an admixture of three minerals: hydroxyapatite, fluorapatite, and chlorapatite, named for high concentrations of OH-, F-, or Cl- ions, respectively, in the crystal lattice. The formula of this admixture is Ca5(PO4)3(OH, F, Cl).
Mohs hardness is 5. Refractive index is 1.632-1.646. Specific gravity is 3.17-3.23. Birefringence is .002-.004. Crystal structure is hexagonal.
Apatite is one of very few minerals which are produced and used by biological systems. Hydroxyapatite is the major component of tooth enamel, and a large component of bone material. Fluorapatite is slightly stronger than hydroxyapatite; thus, fluoridated water, which will allow exchange in the teeth of hydroxyl ions for fluoride ions, slightly strengthens the teeth.
Fission tracks in apatite are commonly used to estimate the thermal history of sediments in sedimentary basins.