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.