Rietveld refinement of the crystallographic structure of human dental enamel apatites

X-ray powder diffraction and chemical analyses show that mammalian dental enamel apatite is an impure form of hydroxylapatite, Ca10(PO4)6(OH)2 (structure described in Elliott 1994). The major difference of enamel apatite from hydroxylapatite is in the presence of about 3 wt% CO3. Details of the crystallographic structures of carbonate-containing apatites (including minerals) are important because CO3 ions increase their reactivity, both in thermal and aqueous systems. For example, enamel from a caries lesion contains less carbonate than normal enamel. However, it is not clear whether this is because the mineral has dissolved and some reprecipitated with less carbonate, or a more soluble, carbonate-rich fraction of the mineral has preferentially dissolved. These possibilities can only be distinguished if subtle structural changes can be characterized. Enamel apatite crystals are too small for single-crystal structure determination, but unit-cell contents have been deduced from chemical analyses assuming that tetrahedral sites are full (Table 1). The OH ion content is deduced from the requirement for charge balance assuming that CO3 ions only replace PO4 ions (except the last formula in which a small fraction also replaces OH ions). Compared to hydroxylapatite, these formulae show a deficiency of ions in Ca and OH ion sites. The loss of negative charge from the loss of OH ions and replacement of PO4 by CO3 ions is balanced by loss of positive ABSTRACT