Apatite is a group of calcium 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.