Crystal structure refinements of the 2H and 2M pseudomorphs of ferric carbonate-hydroxyapatite.

The crystal structures of the ferric carbonate-hydroxyapatite (Fe-CHAp) and oxyapatite (Fe-OAp) pseudomorphs were investigated by powder neutron diffraction and Fourier transform infrared spectroscopy. At low iron loadings, Fe-CHAp (x = 0.1) is A-B type carbonate apatite-2H, where atmospheric CO(2) displaces tunnel hydroxyl and framework phosphate (Ca(2+) + 2PO(4)(3-) --> square(Ca) + 2CO(3)(2-) and Ca(2+) + OH(-) --> Fe(3+) + CO(3)(2-)), while Fe-CHAp (x = 0.2) is A type carbonate apatite-2M. For high iron loadings (x = 0.5), near the solubility limit, Fe(3+) incorporation includes concomitant oxidation of hydroxyl groups (Ca(2+) + OH(-) --> Fe(3+) + O(2-)). The discontinuity in the lattice metric at x approximately 0.2 together with a progressive reduction of OH(-) and CO(3)(2-), substantiates these incorporation mechanisms. The general formula of Fe-CHAp is [Ca(4-x)(F)Fe(x)][Ca(6-y)(T) square(y)][(PO(4))(6-y)(CO(3))(y)][(OH(4))(2-x)(CO(3))(x)] (0 < or = x < or = 0.2, 0 < or = y < or = 0.2) and Fe-OAp is [Ca(3.8)(F)Fe(0.2)][Ca(6-x)(T)Fe(x)][(PO(4))][O(0.2+x)(OH)(1.8-x)] (0 < x < or = 0.3). These new data are placed in the context of earlier crystallographic studies, with the Ca(I)O(6) metaprism twist angle (phi) shown to be a reliable indicator of symmetry, such that for phi(max) > 27.(0) degrees, monoclinic and triclinic structures are preferable.