ABSTRACT
The apatite-type structure is adopted by numerous minerals and
inorganic compounds of general formula A(1)4A(2)6(BO4)6X 2, where A(1) and A(2) are large cations (Ca2+, Sr2+, Ba2+, Na+, . . .), B is a metal or metalloid element forming a strongly bonded oxycomplex (PO3−4 , AsO
3− 4 , VO
3− 4 , SO
2− 4 , . . .), and X is a halide, oxyanion
or small polar molecule (OH−, F−, Cl−, Br−, O2−, H2O, . . .).23,24
The common apatite minerals are calcium orthophosphates
(Ca(1)4Ca(2)6(PO4)6X 2) belonging to the apatite group in the 2010 classification of apatite supergroup minerals (Appendix I).25 They
are fluorapatite, hydroxylapatite, and chlorapatite with end-member
compositions of Ca10(PO4)6F2 (FAP), Ca10(PO4)6(OH)2 (HAP), and
Ca10(PO4)6Cl2 (CLAP), respectively. These three minerals all have
the hexagonal crystal symmetry of the apatite-type structure (space
group P63/m; No. 176 in Aroyo and Hahn26). Their unit-cell parameters vary somewhat with composition: those of fluorapatite
are about a = 9.397, c = 6.878 A˚, V = 526.0 A˚3 (Table 2.1). Ideal, end-member compositions do not occur in nature because the com-
mon apatite minerals generally form in compositionally complex
environments: thus, the minerals fluorapatite, hydroxylapatite, and
Table 2.1 Atomic coordinates and unit-cell parameters for fluorapatite, hydroxylapatite, and chlorapatite1
chlorapatite are solid solutions (mixed crystals) predominantly of
the end-member compositions FAP, HAP, and CLAP.
