Coupled substitution of type A and B carbonate in sodium-bearing apatite.

A suite of Na-bearing type A-B carbonate hydroxyapatites {Ca(10-y)Na(y)[(PO4)(6-y)(CO3)y][(OH)(2-2x)(CO3)x], x approximately = y} has been synthesized at 1200 degrees C and 0.5-1.0 GPa, and investigated by single-crystal X-ray structure and FTIR spectroscopy. Crystal data for the maximum content of carbonate (11.1 wt%) are a = 9.3855(7), c = 6.9142(4) A, space group P6(3)/m, R = 0.023, R(w) = 0.014. Structural accommodation of the substitutions requires local coupling of Na and channel (type A) and phosphate (type B) carbonate ion defects. The type B carbonate ion is located on the sloping faces of the substituted phosphate group, but is inclined at an angle of 53 degrees to the mirror plane. FTIR spectra have minimal nu3 absorption beyond 1500 cm(-1) and dominant nu2 absorption at 873 cm(-1). Synthetic Na-bearing type A-B apatites (with a high content of type A carbonate) are thus similar in both chemical composition and infrared spectra to biological apatites. The latter are reinterpreted as Na-bearing type A-B carbonate apatites with channel carbonate up to 50% of total carbonate.