Structural environment of iron and accurate determination of Fe3 +/ΣFe ratios in andesitic glasses by XANES and Mössbauer spectroscopy

- New calibrations between XANES pre-edge centroids and Fe3 +/∑Fe for andesitic glasses.
- Pre-edge intensity ratios and Fe3 +/∑Fe give less composition-dependent relation.
- XANES and Mössbauer give new constraints on Fe3 +, Fe2 + structure in andesitic glass.

Andesitic glasses equilibrated at 1350 °C over a range of oxygen fugacities (log fO2 from − 8.63 to − 0.68) were examined with Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectra. XANES spectral features were then calibrated as a function of Mössbauer-derived Fe3+/∑Fe ratios. Additionally, both methods help characterize the local structure of iron ions in andesitic glasses. Fe3+/∑Fe ratios were determined from Mössbauer spectra collected at room temperature but corrected with recoilless fractions obtained from previously reported Mössbauer data collected on one of the glasses from 47 to 293 K. An empirical model was derived for the correlation between the pre-edge centroid energy and Fe3+/∑Fe ratio for andesitic glasses. This trend is intermediate between those previously determined for rhyolitic and basaltic glasses, but the distinction from basaltic compositions may be owing chiefly to differences in calibrations for Fe3+/∑Fe ratio, rather than to intrinsic differences in the spectra as a function of Fe3+/∑Fe ratio for mafic glasses. The ratios of intensities of pre-edge sub-peaks and Fe3+/∑Fe ratios for andesitic, basaltic, and rhyolitic glasses plot along a common trend, indicating that these measures provide a XANES calibration for Fe3+/∑Fe ratio that is less dependent on silicate composition. The coordination numbers of Fe3+ and Fe2+ ions in andesitic glass can be calculated from observations of pre-edge centroid energies and total intensities, combined with independent constraints on Fe3+/∑Fe ratio from Mössbauer spectra. The mean coordination of Fe2+ ions calculated this way is close to 5.5 for reduced and oxidized compositions, and this is consistent with inferences from hyperfine features of the Mössbauer spectra. The mean coordination number of Fe3+ inferred from XANES increases from ~ 4.5 to ~ 5 as andesitic glasses vary from reduced to oxidized; Mössbauer hyperfine parameters also suggest network-forming behavior of Fe3+, but with higher coordination for more reduced glasses.