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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.