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 Ca
10(PO
4)
6(OH)
2, Ca
10(PO
4)
6(F)
2 and Ca
10(PO
4)
6(Cl)
2.