Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, with 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.