Exploring the potential of Raman spectroscopy for crystallochemical analyses of complex hydrous silicates: II. Tourmalines

Abstract

Detailed Raman spectroscopic and electron microprobe studies of 33 amphibole (AB 2 C 5 T 8 O 22 W 2 , C 5 = M1 2 M2 2 M3) 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 Fe 2+ 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 Fe 2+ . (2) The coexistence of Mg and Fe 2+ 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 Fe 2+ . (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 T Al-poor amphiboles the M1M1M3-OH peaks shift toward higher wavenumbers by ~37 cm −1 in the case of W OH–A-cation– W F species and by 60 cm −1 in the case of W OH–A-cation– W OH species with respect to the peak position for W OH–A-vacancy– W OH species. (5) High content of C Al, i.e., M2 Al >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 T Al (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 TO 4 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 TO 4 -ring vibrations. A small value of η ( −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.