A Multi-method Analysis of Si-, S- and REE-rich Apatite from a New Find of Kalsilite-bearing Leucitite (Abruzzi, Italy)

P. Comodi1, Y. Liu2, F. Stoppa3 and A. R. Woolley4
1 Dipartimento di Scienze della Terra, Università di Perugia, 06100-Perugia, Italy
2 Wuhan Institute of Chemical Technology, 430073-Wuhan, P.R. China
3 Dipartimento di Scienze della Terra, Università G. d'Annunzio, 66013-Chieti Scalo, Italy
4 Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Abstract: 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. 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 Å.) 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 v1 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 v3 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.%, Σ(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.

Keywords: crystal chemistry • vibrational spectra • structural refinement • sulphur-rich apatite

Mineralogical Magazine; October 1999 v. 63; no. 5; p. 661-672; DOI: 10.1180/002646199548826
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)