The carbonate presence

The presence of carbonate in fluorapatite is accepted as the product of a substitution 

Even given the excess of F in the structure the theory of the distorted tetrahedron is not universally accepted. Regnier et al. (1994) used FTIR spectroscopy, NMR, and quantum mechanical calculations on synthetic and natural samples to demonstrate that the repulsion between the F anion and the carbonate anionic complex is too great to form the distorted tetrahedron. They do not dismiss the possibility that F couples with COs to balance the charge, but that if they do both enter the structure, the F enters independently and fills a vacancy in the structure. Nathan (1996) however points out that all of the samples used in these experiments are actually fluorine-deficient apatites and thus unlikely to have the COsF pseudo-tetrahedron, so clearly more research is needed before final judgments can be made. 

McClellan (1980) suggested that the degree of substitution of Na and Mg is linked to carbonate substitution (Fig. 3) and thus is also linked to the a-cell parameter shift by the following equations  

On this basis McClellan (1980) gave the general formula for carbonate fluorapatite as Caio-x-Y Nax Mgy (P04)6-z (C03)z Po.4z f 2- hlote that this generalized formula is similar but not identical to the one given previously in that it does not account for Ca site 

Gulbrandsen (1970) also proposed a method for determination of carbonate content using 74 of the samples of McClellan and Lehr (1969), in which he used the shift in the a-cell parameter and relative stability of the c-cell parameter to create a method that does not require an internal standard. He compared the (410) and (004) diffraction peaks, and found that the distance between them (in °20) could be related to carbonate substitution by  

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