A multi-method analysis of Si-, S- and REE-rich apatite from a new find of kalsilite-bearing leucitite (Abruzzi, Italy)

The crystal chemistry characteristics of a hydroxyl-fluor apatite from a recently discovered kalsilite-bearing leucitite from Abruzzi, Italy, were investigated by electron microprobe, single crystal X-ray diffraction, IR, Raman and micro-Raman spectroscopy. The apatite has exceptionally high S and relatively high Si, Sr and LREE, whereas the HREE content is negligible. The IR spectra confirm the presence of OH calculated from formula difference. A high positive correlation between Ca-site Substitution Index (CSI = 100(10-Ca)/Ca) and Tetrahedral Substitution Index (TSI = 100 (Si+C+S)/P atoml/a.p.f.u.) and a systematic parallel increase in REE, S and Si indicate two substitution mechanisms, i.e. REE3+ + Si4+ = Ca2+ + P5+ and Si4+ + S6+ = 2 P5+. Site occupancy data and bond lengths, determined from structural refinements on selected samples, demonstrate that LREE and Sr show a marked preference for the Ca2 site, even though in the LREE-rich samples a partial substitution of LREE for Ca in the Cal site was observed. Tetrahedral distances (from 1.535 to 1.541 Angstrom) reflect the substitution of Si4+ and S6+ for P5+, which is also confirmed by vibrational spectra. As (SiO4)(4-) and (SO4)(2-) substitute for (PO4)(3-), the relative intensity of vl Raman bands of (SO4)(2-) (at 1007 cm(-1)) and (SiO4)(4-) (at 865 cm(-1)) increase systematically, while that of phosphate decreases and the five components of phosphate nu(3), modes disappear. Moreover, the (PO4)3- Raman peak broadening is linearly correlated with the Si and S concentrations. Apatite crystals are sometimes zoned with compositions varying from SiO2 = 1.15-2.07 wt.%, Sigma(LREE2O3) = 0.56-1.08 wt.% and SrO = 0.58-1.02 wt.% in the core to 3.98-5.03, 4.14-6.73 and 1.97-2.17, respectively, in the rim. A sharp, strong enrichment in Sr and LREE in the rim indicate that the apatite suddenly became an acceptor of these elements in the late stages of crystallization.