CO3 substitution in apatite:: further insight from new crystal-chemical data of Kasekere (Uganda) apatite

The vibrational and structural characteristics of a CO3-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 (Ca9.78Sr0.05Fe0.01REE0.09Na0.07) (P4.38Si0.52S0.04) and an anion content of (F-0.54 Cl-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 PO4, the nu(3) and nu(4) modes of SiO4 as well as the nu(2) and nu(3) modes of the carbonate group. In addition the structural refinement is compatible with a partial replacement of PO4 by CO3 and also the broadening of the Raman nu(1) band of PO4 (15 cm(-1) in Kasekere vs 5 cm(-1) in F-apatite) correlates with a replacement of PO4 by CO3. The lateral dimensions of the structural channel (Ca2-Ca2 = 4.105(2) Angstrom vs 4.084(2) Angstrom in pure OH-apatite) point to the presence of CO3 in the channels as well. Moreover in the infrared spectra, the curve fitting technique did show a wide band at 1525 cm(-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 CO3 for PO4, few percents of CO3 enter the channel. Tentatively, the formula is: (Ca9.78Sr0.05Fe0.01REE0.09Na0.07) (P4.38Si0.52S0.04C1.23)O-23.45 (F0.54OH1.2Cl0.03 (CO3)(0.23)).