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

A suite of Na-bearing type A–B carbonate hydroxyapatites {Ca10−yNay[(PO4)6−y(CO3)y][(OH)2−2x(CO3)x], x≈y} has been synthesized at 1200 °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) Å, space group P63/m, R=0.023, Rw=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° to the mirror plane. FTIR spectra have minimal ν3 absorption beyond 1500 cm−1 and dominant ν2 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.