Mechanism of CO32- substitution in carbonate-fluorapatite: Evidence from FTIR spectroscopy, 13C NMR and quantum mechanical calculations
Regnier, P.; Lasaga, A. C.; Berner, R. A.; Han, O. H.; Zilm, K. W. (1994). Mechanism of CO32- substitution in carbonate-fluorapatite: Evidence from FTIR spectroscopy, 13C NMR and quantum mechanical calculations. Am. Mineral. 79: 809-818
In: American Mineralogist. Mineralogical Society of America: Washington. ISSN 0003-004X; e-ISSN 1945-3027, meer
The substitution of CO32- in carbonate-fluorapatite (“francolite”) has a destabilizing effect on the apatite structure that results in increased solubility. A commonly proposed substitution mechanism involves the replacement of a PO43- ion by CO3F3-, in which C is at the center of a tetrahedron and is surrounded by three O atoms and one F atom at the corners. However, no spectroscopic evidence exists for this hypothetical anion.We have investigated the microenvironment of CO32- in 13C-enriched type B synthetic and natural sedimentary carbonate-fluorapatite samples by 13C static and MAS NMR and FTIR spectroscopies. (True CO32- substitution in the apatite structure was ascertained by X-ray diffraction and FTIR.) The CO32- ion itself consists of an sp2-hybridized planar arrangement of three O atoms about the central C atom. If the CO32- geometry were changed to an sp3 hybrid with F- ate one corner, the NMR chemical isotropic shift would change considerably, owing to the altered environment about the central C atom. Also, there would be an internuclear dipolar interaction between 13C and 19F.Simulated NMR spectra for 13C-19F distances of 1.38 and 2.00 Å based on the sp3 model are very different from those obtained on our apatite samples, which are instead characteristic of C in a planar sp2 arrangement and no 13C-19F dipolar coupling. No evidence from direct bonding between CO32- and F was found. This result is confirmed by ab-initio quantum mechanical calculations, which show that the hypothetical CO3F3- ion is unstable in an apatitic environment. Ab-initio results, as well as FTIR data, favor the existence of an interstitial position for F- if nonstoichiometric amounts of this element are present in apatite.
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