Apatite

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.

See also: list of minerals, thermal history modelling.