Short-range order and chemical compositions of glasses along the basaltic-rhyolite sub-alkaline join by Raman and FTIR spectroscopies

Six sub-alkaline glasses with compositions progressively shifting from the tholeiitic basalt (B100) end-member to the rhyolite (R100) end-member were investigated and analysed in the same frequency domain by both Raman and FTIR. This approach highlights spectroscopic similarities and differences such as positions, widths and intensities of Raman and FTIR bands, which may also exhibit significant overlapping. Both the Raman and FTIR spectra show several peaks grouped in three main vibrational windows: 200-650 (low frequency region, named F-I in this study), 650-850 (intermediate region, F-II) and 850-1250 cm - 1 (high-frequency region, F-III). In line with previous investigations, the F-I interval can be ascribed to vibrational modes involving rings of tetrahedrally coordinated cations linked by bridging oxygens. It can be fitted with three components, whereas F-II involves the motion of Si atoms within its oxygen cage and is adequately represented by two components. Finally, F-III contains different T-O (T = tetrahedrally coordinated network-forming cation) stretching bands that can be tied to the overall degree of polymerization of the glass and are fitted with four components. In some glasses, the three and the two components within F-I and F-II are identifiable in both Raman and FTIR spectra; in cases of strong peak overlap, these peaks can be complementary to one another towards our interpretation of the molecular arrangement(s) in these glasses. Indeed, the positions of the four components in F-III are first constrained in the Raman spectra, which are more identifiable, then further refined using available Raman spectra for corresponding chemically simple silicate systems. The nine fitted components can reproduce the Raman and FTIR spectra extremely well. As a function of the amount of SiO2, the positions and intensities of the three low frequency components progressively shift in both Raman and FTIR. Similarly, the two bands in the F-II intermediate region exhibit a monotonic shifting of their positions. Indeed, the components at high frequency display less significant shifting of their positions as a function of SiO2, while their intensities change more markedly in the Raman spectrum compared to those for FTIR. The vibrational components measured in this study provide a referenced dataset of assignations of the most abundant natural volcanic glasses. Therefore, it provides a diagnostic tool based on the cross-validation of Raman and FTIR spectra to quickly identify the glass chemistry, offering the possibility to expand the applicability of remote investigations.