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

Published online by Cambridge University Press:  05 July 2018

P. Comodi ,
Y. Liu ,
F. Stoppa  and
A.R. Woolley
P. Comodi
Affiliation:
Dipartimento di Scienze della Terra, Università di Perugia, 06100-Pemgia, Italy
Y. Liu
Affiliation:
Wuhan Institute of Chemical Technology, 430073-Wuhan, P.R. China
F. Stoppa
Affiliation:
Dipartimento di Scienze della Terra, Università G. d'Annunzio, 66013-Chieti Scalo, Italy
A.R. Woolley
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
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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 chemistryvibrational spectrastructural refinementsulphur-rich apatite
Type
Research Article
Information
Mineralogical Magazine , Volume 63 , Issue 5 , October 1999 , pp. 661 - 672
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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