Micro-Raman determination of iron redox state in dry natural glasses: Application to peralkaline rhyolites and basalts

The position, scaled intensity and topology of the Raman stretching envelope at ~1000 cm−1 (HF) have been established in complex anhydrous glasses having variable Fe3+/ΣFe ratios and contrasting compositions of peralkaline rhyolite (pantellerite; FeOT: 7.8 wt.%), iron-poor and iron-rich basalts (FeOT: 7.6 and 10.1 wt.%). Sensitivity of glass structure to iron redox decreases with increasing glass basicity. Raman spectra of pantellerite glasses (Fe3+/ΣFe: 0.04–0.97) evidence the sharp increase of a band at ~970 cm−1 with the progressive entry of 4-fold Fe3+ in alkali-bonded Q3 units. The spectral evolution of basalts with increasing iron oxidation (Fe3+/ΣFe: 0.05–0.87) is more subtle and is likely determined by formation of less polymerized units. The moderate to low influence of redox state changes on the structure of natural basalt glass, compared with the high sensitivity of their synthetic analogues, can be attributed to strong competition between networkforming cations (Al, Fe) in the absence of an excess of alkalis.We demonstrate that Raman calibration against reference glasses allows precise determination of Fe2+/Fe3+ratio but is highly composition dependent. As a consequence of the variable link existing between Fe2+/Fe3+ and glass polymerization, specific calibration procedures based on deconvolution, shift or intensity of HF band are required for compositions with high (acid glasses), moderate or poor (basic glasses) sensitivity to iron redox changes, respectively.