Brillouin spectroscopy of natural and chemically complex volcanic glasses: The role of divalent cations

This work investigates using Brillouin light scattering (BLS) spectroscopy the relationship between chemistry and longitudinal (or pressure) sound-wave propagation on a large dataset of alkaline to sub-alkaline silicate glasses. Results show that the frequency f_B of the Brillouin peak decreases with the silica content and the Silica vs. Calcia - Ferrous oxide - Magnesia (SCFM) parameter, while it increases with the degree of polymerization expressed by the ratio of nonbridging oxygens to tetrahedral cations (NBO/T). It is possible to infer that the values of both f_B and the real part of the longitudinal elastic modulus M' are tightly related to the content of divalent cations (M²⁺) participating in the silicate network. Our findings suggest that alkaline earth metals and Fe²⁺ linearly speed up the longitudinal acoustic waves in silicate glasses. This might open a new window on the possibility of using the BLS technique for rapid and accurate determinations of physical and chemical properties of natural glasses present on Earth and other planetary bodies.