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 Altot ~0.55 atoms per formula unit (apfu), Ti ~0.45 apfu, and 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.
DELLA VENTURA, G., Oberti, R., HAWTHORNE F., C., Bellatreccia, F. (2007). FTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O<sup>2-</sup> at the anionic O3 site in amphiboles. AMERICAN MINERALOGIST, 92, 1645-1651.