Rietveld enamel

Michel, V., Ildefonse, P. and Morin, G. 1995 Chemical and structural changes in Cervus elephus tooth enamels during fossilization (Lazaret Cave) a combined IR and XRD Rietveld analysis. Applied Geochemistry 10 145-159. 

Wilson, R.M., Elliott, J.C. and Dowker, S.E.P. (1999) Rietveld refinement of the crystallographic structure of human dental enamel apatites. American Mineralogist, 84, 1406-1414. 

It is not always necessary that the fit be as good as that in Fig. 3 for useful results to be obtained. Fig. 4 shows the patterns for Rietveld refinement of a modified hydroxyapatite model for human tooth enamel (17) with both x-ray and neutron data. Here the difference curves show poorer fits than are shown in Fig. 3 and RWp is larger, being 26% for both the x-ray and the neutron cases. Nonetheless, the positions of the principal atoms were verified to ML 01 A, some of their site occupancies suggested the presence of expected substitutions, and information about the location of structurally incorporated water was revealed. 

Figure 4. Rietveld refinement patterns for human tooth enamel (18,). X-ray data were taken at 29 intervals of 0.375° and neutron data at 0.10° intervals. R p = 25.5% and 26.7% respectively.

It has been known for many years (see Miles 1967) that the lattice parameters of dental enamel, particularly a, differ significantly from those of stoichiometric HAP. For example, for human dental enamel, a = 9.441(2) A and c = 6.878(1) A for a fraction with density > 2.95 g cm (Young and Mackie 1980), and a = 9.4555(76) A and c = 6.8809(47) A (Wilson et al. 1999) for pooled enamel from permanent teeth have been reported. Both sets of measurements used Rietveld methods applied to XRD data. These values compare with a = 9.4243(55) A and c = 6.8856(35) A (Morgan et al. 2000) for stoichiometric HAP, also determined from Rietveld analysis. Thus the a-axis parameter for dental enamel is about 0.3% larger than for HAP. 

Rietveld structure refinements have also been undertaken on synthetic precipitated COsAps as model systems for biological apatites. The reduction in apparent PO4 volume, and P and Ca2 occupancies seen in dental enamel were also seen in Na-free precipitated COsAps with CO3 contents of 4 wt % produced by the reaction between CaCOs and CaHP04 at 100°C (Morgan et al. 2000). COsAps precipitated in the presence of Na ions... 

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