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

Abstract

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 CO₂ displaces tunnel hydroxyl and framework phosphate (Ca²⁺ + 2PO₄³⁻→□_Ca + 2CO₃²⁻ and Ca²⁺ + OH⁻→ Fe³⁺ + CO₃²⁻), while Fe-CHAp (x = 0.2) is A type carbonate apatite-2M. For high iron loadings (x = 0.5), near the solubility limit, Fe³⁺ incorporation includes concomitant oxidation of hydroxyl groups (Ca²⁺ + OH⁻→ Fe³⁺ + O²⁻). The discontinuity in the lattice metric at x∼ 0.2 together with a progressive reduction of OH⁻ and CO₃²⁻, substantiates these incorporation mechanisms. The general formula of Fe-CHAp is [Ca_4−x^FFe_x][Ca_6−y^T□_y][(PO₄)_6−y(CO₃)_y][(OH₄)_2−x(CO₃)_x] (0 ≤x≤ 0.2, 0 ≤y≤ 0.2) and Fe-OAp is [Ca_3.8^FFe_0.2][Ca_6−x^TFe_x][(PO₄)][O_0.2+x(OH)_1.8−x] (0 < x≤ 0.3). These new data are placed in the context of earlier crystallographic studies, with the Ca^IO₆ metaprism twist angle (φ) shown to be a reliable indicator of symmetry, such that for φ_max > 27.₀°, monoclinic and triclinic structures are preferable.