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FTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O^2– at the anionic O3 site in amphiboles

¹ Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
² CNR-Istituto di Geoscienze e Georisorse, Unità di Pavia, via Ferrata 1, I-27100 Pavia, Italy
³ Department of Geological Sciences, University of Manitoba, Winnipeg, R3T 2N2 Canada

Correspondence: ^* E-mail: dellaven{at}uniroma3.it

This paper reports a single-crystal unpolarized-light FTIR study in the OH-stretching region of a suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied have fairly constant M-site composition, with ^[6]Al_tot ~0.55 atoms per formula unit (apfu), ^[6]Ti ~0.45 apfu, and ^[6]Fe³⁺ ~0.40 apfu. SIMS and SREF data show all samples to have an O3 anion composition of OH O^2– 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption consisting of several overlapping bands; three main components can be recognized: ~3710, 3686, and 3660 cm^–1, respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites allows five of these components to be assigned to local configurations involving OH-O^2– at the O3 site, thus showing that coupling with an O^2– anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of O^2– in amphiboles in chemically favorable cases, i.e., in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving ^M1Ti⁴⁺ or ^M1Fe^{3+ M3}Fe³⁺ are associated with O^2– at both adjacent O3 sites, and that ^M3Al is locally associated with OH, confirming SRO models based on structure refinement results.

Key Words: Ti-rich pargasite • Lherz (French Pyrenees) • single-crystal FTIR spectroscopy • anion occupancy

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