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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


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0003-004X
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Volume 92, Issue 10

Issues

FTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O2- at the anionic O3 site in amphiboles

Giancarlo Della Ventura
  • Corresponding author
  • Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
  • CNR-Istituto di Geoscienze e Georisorse, Unità di Pavia, via Ferrata 1, I-27100 Pavia, Italy
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/ Roberta Oberti / Frank C. Hawthorne / Fabio Bellatreccia
  • Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
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Published Online: 2015-04-01 | DOI: https://doi.org/10.2138/am.2007.2199

Abstract

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]Altot ~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 ≈ O2- ≈ 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-O2- at the O3 site, thus showing that coupling with an O2- anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of O2- 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 M1Fe3+M3Fe3+ are associated with O2- at both adjacent O3 sites, and that M3Al is locally associated with OH, confirming SRO models based on structure refinement results.

Keywords: Ti-rich pargasite; Lherz (French Pyrenees); single-crystal FTIR spectroscopy; anion occupancy

About the article

Received: 2005-12-22

Accepted: 2007-05-09

Published Online: 2015-04-01

Published in Print: 2007-10-01


Citation Information: American Mineralogist, Volume 92, Issue 10, Pages 1645–1651, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2007.2199.

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