A contribution to the crystal chemistry of ellestadite and the silicate sulfate apatitest
A series of calcium silicate sulfate apatites from Crestmore, California, which contain the coupled substitution Silvsvl for 2Pv, has been investigated using electron microprobe, powder diffraction, and single-crystal diffraction methods. Chemical analysis of eighteen specimens of different phosphorus contents proves that the Si: S ratio is essentially I : I and yields the idealized general formula Caro(SiOn):-*(SO4)3-^@O4)2.(OH,F,CD2, where x : 0 to 3. The members of this series for which x : 0 and 3l2 have been labelled "ellestadite" and "wilkeite", respectively, by previous workers. "Ellestadite" is actually a solid solution involving the end-members Cale(SiO a,){SOq)tZz, where Z: OH (hydroxylellestadite), F (fluorellestadite), or Cl (chlorellestadite). The term ellestadite is redefined to make it a group name for all compositions having >(Si,S) > P. Wilkeite is not a valid mineral species, since it is only one of many solid solutions involving the six end-members fluorapatite, hydroxyapatite, chlorapatite, fluorellestadite, hydroxylellestadite, and chlorellestadite. Although natural hydroxylellestadite is monoclinic, precession photographs of type "ellestadite" and "wilkeite" show hexagonal symmetry and no evidence of Si-S ordering as suggested by the Si: S ratio of I : I . The silicate sulfate apatites from Crestmore show a strong linear relationship between their P and F contents, such that these two variables simultaneously go to zero. Linear relationships also exist between their unit cell parameters and their P, F, and (Si+S) contents. These correlations imply a convergence of the Crestmore apatite series towards a hypothetical memberof composition Caro(SiOa)r (SO4)3(OH,CI)z and cell constants a:9.543 and c = 6.9174.
The crystal chemistry of apatite: structure fields of fluor- and chlorapatite
tions were deduced. The size limits for substitutions in fluorapatite were 0.29 A<Re<0.60 A and 0.95 A<n"< t.Ss A where Ro and R" represent the radi'i of ions occupying the phosphorus and calcium positions respectively. The size limits for substitutions in chlorapatite were 0.29 A(Ro(0.60 A and 0.80 A<&< 1.35 A Gased on Ahrens'radii). The compounds, Cdro(POr)oFz, Caro(AsOr)oFz, and Caro(VOr)eFr did not have hexagonal sJ,,rnmetry, but may be regarded as distorted apatites. The solid solubility of divalent Mg and Ni and of pentavalent v in caro(Por)oFz was investigated. The lattice parameters of about thirty-five apatites were determined, some lor the first time.
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.