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CO₃ substitution in apatite

¹ Dipartimento di Scienze della Terra, Università di Perugia, Piazza Università, 06100 Perugia, Italy
² Wuhan Institute of Chemical Technology, Wuhan, 430074 Hubei, P.R. China

e-mail: comodip{at}unipg.it

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 (large cations) = 10, a cation content of (Ca_9.78Sr_0.05Fe_0.01REE_0.09Na_0.07) (P_4.38Si_0.52S_0.04) and an anion content of (F_0.54Cl_0.03). According to the structural refinement, the channel's anion occupancy is reduced, and the OH-content is about 1.2. The resulting (P+Si+S) < 6 indicates that carbonate anions enter the tetrahedra. The IR spectrum exhibits all the bands of PO₄, the ₃ and ₄ modes of SiO₄ as well as the ₂ and 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 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) Å vs 4.084(2) Å 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: (Ca_9.78Sr_0.05Fe_0.01REE_0.09Na_0.07) (P_4.38Si_0.52S_0.04C_1.23) O_23.45 (F_0.54OH_1.2Cl_0.03(CO₃)_0.23).

Key-words: apatite, crystal-chemistry, CO₃-substitution, microraman spectroscopy, infrared spectroscopy.

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