Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite
and chlorapatite, named for high concentrations of OH−, F− and Cl− ions, respectively,
in the crystal. The formula of the admixture of the four most common endmembers is
written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual
minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2 and Ca10(PO4)6(Cl)2.