Preparation, analysis, and characterization of carbonated apatites

A range of synthetic carbonated apatites were characterized using infrared and Raman spectroscopy, X-ray diffraction, and chemical analysis and compared to stoichiometric hydroxyapatite and dental enamel. Synthetic apatites were prepared by aqueous precipitation and by solid-state reaction at high temperatures. A method is described for the carbonate determination of apatite samples using gas chromatography. This study demonstrates that carbonate exists in two crystallographically distinct sites in the apatite crystal structure and is consistent with a carbonate-for-phosphate substitution mechanism, where four carbonate groups replace three phosphate groups. Sodium and hydroxyl ions are shown to be involved with the substitution of carbonate ions in the apatite structure. Observed sodium/carbonate and calcium/phosphorus molar ratios confirm this mechanism. Substitution of this type may explain the predominant mode of carbonate substitution in biological apatites.