Exploring the potential of Raman spectroscopy for crystallochemical analyses of complex hydrous silicates: I. Amphiboles
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Lisa Leissner
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 siteoccupancy 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.
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
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- Hf and Nd isotope systematics of early Archean komatiites from surface sampling and ICDP drilling in the Barberton Greenstone Belt, South Africa
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