Apatite Image needed General Category Mineral Chemical formula Ca5(PO4)3(F,Cl,OH) Identification Color Usually green, less often colorless, yellow, blue to violet. Crystal habit Tabular, prismatic crystals, massive, compact or granular. Crystal system Hexagonal Cleavage Poor Fracture Conchoidal to even Mohs Scale hardness 5 Luster Vitreous to subresinous Refractive index 1.632-1.646. Biref .002-.004 Pleochroism None Streak White Specific gravity 3.17-3.23 Major varieties None Apatite is a group of phosphate minerals, usually referring to: hydroxylapatite, fluorapatite, and chlorapatite, named for high concentrations of OH-, F-, or Cl- ions, respectively, in the crystal lattice. The formula of the admixture of the three most common species is Ca5(PO4)3(OH, F, Cl). Apatite is one of very few minerals which are produced and used by biological systems. Hydroxylapatite 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. Phosphorite is the name given to impure, massive apatite. External links Apatite on /mineral.galleries.com See also list of minerals thermal history modellingcs:Apatit Search Term: "Apatite" Categories: Phosphate minerals