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The substitution of carbonate in apatites has been voraciously explored for more than 60 yr. However, the characterization of the sites of carbonate substitution in apatite by the frequently used identification method, infrared spectroscopy, is still incompletely understood. In a significant departure from previous studies, a recent IR study concluded that most of the carbonate in apatites resides in the channels, at least in apatites prepared at high temperature and pressure. A series of A- and AB-carbonated calcium and strontium apatites have been prepared by aqueous precipitation and by carbonation with CO2 at 800 °C. The type of carbonate substitution—A-(substitution for hydroxide in the channel) or B- (substitution for phosphate)—was determined from the carbonate asymmetric stretch ( v 3) and out-of-plane bend ( v 2) regions in the IR spectra. The IR  v 3 and  v 2 regions were analyzed by peak-fitting using both four- and six-peak models for the  v 3 region. A correlation of the band position of the high-frequency A-type carbonate band frequency with weight percent carbonate was observed for the calcium apatites, whereas a correlation of the band positions of both the low- and high-frequency B-type carbonate bands with carbonate weight percent occurs for the strontium apatites. The high-frequency member of the A-type carbonate  v 3 region for the calcium apatites showed the greatest variation with a change in the composition (A- or B-type) of carbonate. The lower frequency observed for the A-type  v 3 band of those calcium apatites with the greatest carbonate content suggested the importance of carbonate clusters in the unit cell of highly carbonated apatites. Correlations of band frequencies with sodium content were weaker than those for carbonate, even though carbonate and sodium were correlated with each other in the calcium apatites. Analogous observations on the IR band frequencies in potassium-containing carbonated apatites, in which the potassium content is low, also suggest that carbonate content is the primary driver for the interactions that produce an overlap of A-type peaks with those in the B-type region. This conclusion is further strengthened by the lack of a correlation of the frequencies of A-type carbonate in strontium apatites in which the carbonate content is low.

The effect of incorporated carbonate and sodium on the IR spectra of A- and AB-type carbonated apatites