Coupled anion substitution in natural carbon-bearing apatites
- Cite this article as:
- Binder, G. & Troll, G. Contr. Mineral. and Petrol. (1989) 101: 394. doi:10.1007/BF00372213
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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, CO32−· F−replaces PO43−with tetravalent carbon substituting for pentavalent phosphorus and a halogen in excess replacing one O2− 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 ao 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 ao cell edge. The replacement of P by C does not have a significant effect on the cell dimension co 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.