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

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 Al-[6](tot) similar to 0.55 atoms per formula unit (apfia), Ti-[6] similar to 0.45 apfia, and Fe-[6](3+) similar to 0.40 apfu. SIMS and SREF data show all samples to have an 03 anion composition of OH approximate to O2- approximate to 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: similar to 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 03 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 Ti-M1(4+) or Fe-M1(3+) Fe-M3(3+) are associated with O2- at both adjacent 03 sites, and that Al-M3 is focally associated with OH, confirming SRO models based on structure refinement results.