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

Universita di Perugia, Dipartimento di Scienze della Terra, Perugia, Italy
Wuhan Institute of Chemical Technology, China
Universita G. d'Annunzio, Italy
Natural History Museum, United Kingdom

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 atom/a.p.f.u.) and a systematic parallel increase in REE, S and Si indicate two substitution mechanisms, i.e. REE (super 3+) +Si (super 4+) = Ca (super 2+) +P (super 5+) and Si (super 4+) +S (super 6+) = 2 P (super 5+) . 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 Ca1 site was observed. Tetrahedral distances (from 1.535 to 1.541 Aa) reflect the substitution of Si (super 4+) and S (super 6+) for P (super 5+) , which is also confirmed by vibrational spectra. As (SiO ₄ ) (super 4-) and (SO ₄ ) (super 2-) substitute for (PO ₄ ) (super 3-) , the relative intensity of nu ₁ Raman bands of (SO ₄ ) (super 2-) (at 1007 cm (super -1) ) and (SiO ₄ ) (super 4-) (at 865 cm (super -1) ) increase systematically, while that of phosphate decreases and the five components of phosphate nu ₃ modes disappear. Moreover, the (PO ₄ ) (super 3-) Raman peak broadening is linearly correlated with the Si and S concentrations. Apatite crystals are sometimes zoned with compositions varying from SiO ₂ = 1.15-2.07 wt.%, Sigma (LREE ₂ O ₃ ) = 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.

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