Second derivative infrared spectra of hydroxyapatite

Infrared absorption spectra of polycrystalline fluor- and hydroxyapatite are transformed to their second derivatives for resolution enhancement. It is observed that although the symmetry of the hydroxyapatite lattice is lower than that of fluorapatite, the spectra are nearly congruent. The data of polycrystalline samples are analysed on the basis of single crystal Raman and infrared dichroism spectra. The number of vibrational bands that are observed in the spectra of polycrystalline fluor- and hydroxyapatite exceed the number predicted by factor group analysis. The "excess" bands are assigned to vibrations that are expected to be Raman active previously reported in the single crystal Raman data of these two minerals. This can be accounted for by a breakdown of factor group selection rules as applied to the infrared data of polycrystalline apatites.