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

ARIATIONS in the Ca:P:C ratios among analyses of teeth and bones are well known and it is generally conceded that these variations cannot be attributed to analytical error. Because it has been difficult to interpret these variations in the absence of an adequate theory for the crystal structure of carbonate apatite, certain calculations have been based upon the hypothetical compound, 3Ca3(PO4)2 -CaCO3. Inasmuch as the Ca:P:C ratios of this hypothetical compound are fixed, these calculations cannot be consistent with the analytical data, so the assumption is frequently made that more than one solid phase is present and contributes the elements Ca, P, or C. Gruner and McConnell"1 were able to show that the mineral francolite cannot contain any component with the composition 3Ca3(PO4)2 -CaCO3, but it has since been suggested that submicroscopic particles of CaCO3 can exist in francolite. This question requires further scrutiny. To state the matter briefly, the question resolves itself into whether carbonate apatites represent (1) a single crystalline phase or (2) a heterogeneous mixture, the units of which are below the resolving power of optical microscopes. Inasmuch as dental enamel and other calcified tissues contain organic matter which merely serves to confuse the question of the composition of the "mineral phase, " it seems reasonable to direct attention to the crystalline substances having related compositions. Although the complete solution of the more complex problem of calcified tissues will probably not result from detailed consideration of the crystalline minerals, it seems probable, nevertheless, that a thorough knowledge of the crystalline minerals will greatly augment an understanding of calcified animal tissues. The question of the crystal structure of carbonate apatites is not new. Two fairly complete summaries, which emphasize chemical and mineralogical approaches, have appeared within recent years.9' 23 Armstrong2 has summarized those aspects dealing with calcified tissues. In order to consider certain types of structural configurations which were virtually unknown in 1937 and in order to obtain complete reconciliation with more recent analyses, the hypothesis of Gruner and McConnell requires minor modification. Initially, however, it seems advisable to demonstrate that carbonate apatite, in the form of francolite at least, is a single phase and, therefore, is a structural variant of apatite.