The crystallographic structure of dental enamel and related apatites
Synthetic, biological and mineralogical carbonate-containing apatites have been studied, principally by infra-red absorption spectroscopy (using polarised or unpolarised radiation), The substitution of a carbonate ion for two hydroxyl ions in the apatite lattice has been confirmed, by chemical analyses, and the variation of the lattice constants with the degree of substitution determined. The orientation of the carbonate ion has been determined by using polarised infra-red radiation; its plane is approximately parallel with the c-axis. The carbonate ions in dental enamel are in two different environments; the lane of the carbonate ion in these two environments is nearly parallel and perpendicular to the c-axis. One of these environments is very probably the substitution for hydroxyl ions discussed above; this accounts for only a small fraction of the carbonate ions, the remainder are probably adsorbed on the surface of the apatite crystals. A number of carbonate-containing apatites prepared in aqueous systems have been studied. These show similarities to enamel but with the carbonate ions in different relative proportions in the two different environments present in enamel. Some apatites have been prepared which have absorption bands due to molecular carbon dioxide. This has been confirmed by preparing apatites containing the heavy isotope of carbon, carbon-13. The orientation of the carbonate ion in francolite has been determined and is consistent with it occupying the sloping face (with respect to the basal plane) of the space left by a phosphate ion. The environment of the carbonate ion in francolite is different from that in enamel, The hydroxyl ion in enamel and francolite is oriented 'with the 0-H bond parallel with the c-axis of the apatite lattice. The assignment of the hydroxyl band in the infra-red spectrum of enamel has been confirmed by deuteration experiments.