Abstract

Detailed Raman spectroscopic and electron microprobe studies of 33 amphibole (AB2C5T8O22W2, C5 = M12M22M3) crystals from 6 amphibole subgroups were performed to check whether the Raman scattering arising from OH bond stretching vibrations can be used for the identification and site-occupancy analysis of amphiboles. Several results can be gained from this study. (1) C-site Mg and Fe2+ on the M1M1M3 triplet linked to the W-site OH lead to two-mode behavior of the OH bond stretching mode: up to four Raman peaks separated by ~16 cm−1 from each other can be observed and their fractional intensities can be used to quantify the concentration of M1M1M3 chemical species as well as to estimate the overall C-site occupancy by Mg and Fe2+. (2) The coexistence of Mg and Fe2+ on the B site also leads to two-mode behavior but the peak splitting is only ~3 cm−1. The fractional intensities of these split components can be used to refine the B-site occupation factors of Mg and Fe2+. (3) The presence of B-site Li, Na, and Ca results in one-mode behavior of the OH Raman peaks, i.e., slight shift of the peak positions depending on the concentration of distinct chemical species. The total replacement of B-site Ca by Na should reduce the peak positions by 6.5 cm−1, whereas that of Li by ~13 cm−1. (4) A-site occupancy leads to strong broadening as well as to a strong shift of the OH peaks toward higher wavenumbers. The full-width at half maximum (FWHM) of OH-stretching peaks associated with filled A sites is ~22 vs. ~8 cm−1 for OH peaks associated with vacant A sites. For TAl-poor amphiboles the M1M1M3-OH peaks shift toward higher wavenumbers by ~37 cm−1 in the case of WOH–A-cation–WF species and by 60 cm−1 in the case of WOH–A-cation–WOH species with respect to the peak position for WOH–A-vacancy–WOH species. (5) High content of CAl, i.e., M2Al >1 atom per formula unit (apfu) seems to lead to one-mode behavior and an additional shift toward lower wavenumbers by ~4 cm−1. (6) OH peaks with FWHM ~30–40 cm−1 may indicate high content of TAl (1 or 2 apfu), which can be verified by analyzing the FWHM of the Raman peak near 670 cm−1, generated by vibrations of bridging oxygen atoms in TO4 rings. (7) The W site occupancy factor of the OH groups can be estimated using the intensity ratio η between the total Raman scattering generated by OH bond stretching and the Raman peak near 670 cm−1 generated by TO4-ring vibrations. A small value of η (<0.09) combined with the presence of strong Raman scattering near 750–780 cm−1 is indicative of oxo-amphiboles. Guidelines for non-specialists how to use the Raman scattering of OH bond stretching modes for “rough” but preparation-free, non-destructive, and easy-to-handle crystallochemical quantitative analyses of amphiboles are suggested.

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