Coupled anion substitution in natural carbon-bearing apatites

Reliable wet chemical methods of minor element analysis (F, Cl, OH) and X-ray diffraction studies reveal that a coupled substitution of fluorine and carbonate is required in natural carbon-bearing apatites. Thus, CO ₃ ^2– · F^–replaces PO ₄ ^3– with tetravalent carbon substituting for pentavalent phosphorus and a halogen in excess replacing one O^2– to give: Ca₁₀ (PO₄)_6–x(CO₃·F)_x](F, OH, Cl)₂ The coupled substitution not only maintains the electrical neutrality of the structure, but also retains the tetrahedral coordination for the cations involved. The resulting variation in the unit cell parameter a_o corresponds well with the proposed geometrical model: partial replacement of P⁵⁺ (ionic radius 0.35Å) by the smaller C⁴⁺ (0.16 Å) ion reduces the lateral distance between the Ca-O trigonal prism columns, thus shortening the length of the a_o cell edge. The replacement of P by C does not have a significant effect on the cell dimension c_o which is mainly determined by the distance of the Ca- and O-triangles and is not influenced directly by minor substitutions of C for P. Our investigations suggest a carbonate apatite (francolite) with a stoichiometric chemical formula. The nature of the anion configuration is emphasized in this paper.