The Crystal Chemistry of Carbonate Apatites and their Relationship to the Composition of Calcified Tissues

Abstract

A modification of the Gruner-McConnell structural hypothesis for the mineral, francolite, is presented in which 4 CO3 groups substitute for 3 PO4 groups and H ions substitute for P as H4O4 groups. Dental enamel, for example, probably contains 3 types of structural water or hydroxyl ions in the "mineral phase." Variations of the CaP:C ratios can be explained by the new hypothesis, and thus a direct applicability to the crystal chemistry of the "mineral phase" of dental enamel, dentin, and bone is indicated. Since 3Ca3(PO4)2-CaCO3 does not exist, attempts to calculate the composition of the "mineral phase" cannot be correct if this formula is used. It is not possible to account for the properties of carbonate apatites by assuming the presence of CaCO3, whether crystalline or noncrystalline, microscopic or submicro-scopic. In a qualitative manner the proposed structure for francolite will explain observed differences between the optical and X-ray diffraction data for fluorapatite and francolite.