Rietveld refinement of the crystallographic structure of human dental enamel apatites

Rietveld refinements using 12 sets of X-ray diffraction powder data from mi lligram samples of human dental enamel provide detailed information about t he structure and composition of enamel apatite. The principal difference in atomic parameters between enamel apatite and Holly Springs hydroxylapatite is in O2, which is reflected in a reduction in the P-O2 bond length of 0.0 85 Angstrom and PO4 volume by 3.6%. Modeling the hexad axis scattering with a single OH ion gives a 0.089 Angstrom shift of the ion further away from the mirror plane at z = 1/4. The known distributed electron density along t he hexad axis in enamel has been confirmed by direct comparison with synthe tic hydroxylapatite. Although the CO32- ion position could not be determine d directly, evidence for partial replacement of PO43- by CO32- ions came fr om an 8% diminution of the P site occupancy compared with that in stoichiom etric hydroxylapatite. The observed reduction in the P-O2 bond length and P O4 volume in enamel is also consistent with this substitution. The loss of negative charge caused by CO32- replacing PO43- ions and loss of OH- ions i s compensated by loss of Ca2+ ions from Ca2 sites. The calculated density f rom the X-ray results is 3.021 g/cm(3), in agreement with deductions from p revious chemical analyses.