CO (sub 3) substitution in apatite; further insight from new crystal chemical data of Kasekere (Uganda) apatite

The vibrational and structural characteristics of a CO (sub 3) -rich apatite from an extrusive carbonatite in Kasekere, Uganda, were studied by infrared and Raman spectroscopy, single-crystal X-ray diffraction and electron microprobe analysis. Electron microprobe analysis provided, based on Sigma (large cations) = 10, a cation content of (Ca (sub 9.78) Sr (sub 0.05) Fe (sub 0.01) REE (sub 0.09) Na (sub 0.07) ) (P (sub 4.38) Si (sub 0.52) S (sub 0.04) ) and an anion content of (F (sub 0.54) Cl (sub 0.03) ). According to the structural refinement, the channel's anion occupancy is reduced, and the OH-content is about 1.2. The resulting Sigma (P+Si+S)<6 indicates that carbonate anions enter the tetrahedra. The IR spectrum exhibits all the bands of PO (sub 4) , the V (sub 3) and V (sub 4) modes of SiO (sub 4) as well as the V (sub 2) and V (sub 3) modes of the carbonate group. In addition the structural refinement is compatible with a partial replacement of PO (sub 4) by CO (sub 3) and also the broadening of the Raman V (sub 1) band of PO (sub 4) (15 cm (super -1) in Kasekere vs 5 cm (super -1) in F-apatite) correlates with a replacement of PO (sub 4) by CO (sub 3) . The lateral dimensions of the structural channel (Ca2-Ca2 = 4.105(2) Aa vs 4.084(2)Aa in pure OH-apatite) point to the presence of CO (sub 3) in the channels as well. Moreover in the infrared spectra, the curve fitting technique did show a wide band at 1525 cm (super -1) , implying an A-site carbonate in a B-type dominant carbonate Ap. Comparison of multimethod analyses suggests that the Kasekere apatite is the first occurrence of apatite from a natural environment in which, in addition to a larger substitution of CO (sub 3) for PO (sub 4) , few percents of CO (sub 3) enter the channel. Tentatively, the formula is: (Ca (sub 9.78) Sr (sub 0.05) Fe (sub 0.01) REE (sub 0.09) Na (sub 0.07) ) (P (sub 4.38) Si (sub 0.52) S (sub 0.04) C (sub 1.23) ) O (sub 23.45) (F (sub 0.54) OH (sub 1.2) Cl (sub 0.03) (CO (sub 3) ) (sub 0.23) ).