A range of synthetic carbonated apatites and human dental enamel have been studied at low temperatures (50 K) by laser Raman spectroscopy. The widths of the Raman spectral bands of hydroxyapatite were significantly reduced at low temperatures with the appearance of several peaks unobserved at room temperatures. The number of observed phosphate and hydroxyl ion Raman peaks was consistent with the space group P63. Spectra of carbonated apatites were not further resolved at low temperatures and Raman peak broadening due to loss of long-range translational order in the apatite structure was linearly correlated with the carbonate content of the sample. The same Raman peak broadening occurred in the spectrum of dental enamel. The number of carbonate bands in the Raman spectrum of carbonated apatites indicated that carbonate exists in two crystallographically distinct sites of different symmetry within the apatite structure.

The purpose of this study was to investigate the effect of trace metals and fluoride on structural disorder in carbonated apatites and to consider the relevance of the results to dental apatite (enamel). Synthetic carbonated apatites were precipitated in the presence of low levels of zinc, magnesium, strontium, iron and fluoride. Crystal imperfection, as indicated by X-ray peak half-widths, was altered markedly by the substituents. Fluoride alone was effective and zinc- fluoride was the most effective combination in compensating for disorder caused by carbonate inclusion. These results show that disorder induced by carbonate inclusion can be modified greatly by appropriate elements. Trace-element inclusions may therefore markedly affect dental enamel reactivity. Carbonate content alone cannot be used as an indicator of the reactivity of apatite or of the potential susceptibility of dental enamel to caries.

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