The crystal structures of the hexagonal (P63/m) end-member apatites with the formula Ca5(PO4)3X (X = F, OH, Cl) were refined to R = 0.025, 0.016, and 0.020 for fluorapatite, hydroxylapatite, and chlorapatite, respectively. In accord with earlier studies, the F atoms in fluorapatite lie in the mirror plane in the 2a (0,0,1/4) special position. In hydroxylapatite, the OH species is disordered in (0,0,z) positions 0.35 Å above and below the mirror plane, and in chlorapatite the Cl is also disordered, in positions 1.2 Å above and below the mirror planes. In chlorapatite, the Cl anion is so far displaced from the mirror plane that an additional, weak bond develops between Ca(2) and a second Cl anion, thus increasing the Ca(2) coordination.
In the three end-members, differences in position of the column anions propagate throughout the structure, but have minor, secondary effects on the Ca(l)O9 polyhedron and the PO4 tetrahedron in terms of cation-oxygen bond lengths and polyhedron orientation; average P–O and average Ca(l)–O bond lengths for the three structures are identical within 0.005 Å. The Ca(2)O5X(O) polyhedron, however, is greatly affected by anion substitution. Individual Ca(2)-X bond lengths are significantly different for the three structures, with the Ca(2)-X bond length varying between 2.311 and 2.759 Å. The mean Ca(2)–O distance ranges between 2.461 and 2.493 Å.
The atomic arrangements of fluorapatite, hydroxylapatite, and chlorapatite suggest that these end-members are immiscible in solid solution. The 1.2-Å displacement of Cl from the mirror plane in chlorapatite prohibits the existence of F and OH as immediate neighbors at certain sites in the anion columns because of prohibitively short interatomic distances. Structural adjustments that enable solid solution to occur in Cl-bearing binary or ternary apatites may include (1) major shifts in column-anion positions compared to those in end-member structures, (2) reduction of symmetry from hexagonal to monoclinic in ternary apatites, and/or (3) ordering of anions within individual columns but disordering of columns throughout the apatite structure.