Infrared spectra of carbonate apatites: ν2-Region bands

The proportions of A and B carbonate ions in a selection of AB carbonate apatites, including hydroxyapatite (CHAP), chlorapatite (CCLAP) and fluorapatite (CFAP), have been obtained using the out-of-plane bend (ν2) bands of Fourier transform infrared (FTIR) spectra. Band area ratios (B/A) are in very good agreement with site occupancies from single-crystal X-ray structure refinement; the correlation is linear (1:1) for B/A values ranging up to three. Most compositions have ν2 spectra with one band for A carbonate (at 878–880 cm−1) and one for B (at 870–872 cm−1). Na-free AB CHAP has a third prominent band at 862 cm−1, which is assigned to the stuffed channel species (A2), and Na-bearing CFAP has a third band at 864 cm−1, which is assigned to a second B carbonate environment (B2). The A2 and B2 assignments are based largely on spectral changes in annealed samples.