Variations in atomic disorder in biogenic carbonate hydroxyapatite using the infrared spectrum grinding curve method

Biogenic carbonate hydroxyapatite crystals are inherently disordered at the atomic level due mainly to the substitutions of various ions in the crystal structure, and, in the case of the bone family of materials, to the fact that these very small crystals have a very large surface-to-bulk ratio. Characterization of the extent of disorder is of much interest, as this relates to the stability and hence solubility of the crystals. Here the infrared spectrometry grinding curve approach developed for calcite, is adapted to carbonate hydroxyapatites. The infrared splitting factor is plotted against the full width at half height of the strong phosphate absorption peak as a function of increased grinding of the sample. By doing so, the contribution of particle size to the shape of the peaks is better separated from the contribution of atomic disorder to peak shape. It is shown that differences in disorder exist between dentine, cementum, and bone crystals which could reflect crystal size and/or atomic defects within the crystal. It is als shown that systematic differences exist between enamel samples from different taxa, which we assume only reflects atomic disorder differences within these large crystals. The method can be used to characterize atomic disorder in natural hydroxyapatites, as well as in the many different types of synthetic hydroxyapatites used for biomedical implants. Carbonated hydroxyapatite is widely used in biomedical applications. Biogenic carbonate hydroxyapatite crystals are inherently disordered at the atomic level. Using the grinding curve approach based on peak broadening in infrared spectrometry, systematic disorder differences between enamel samples from different taxa, and differences in disorder between dentine, cementum and bone crystals are shown.