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

Andesitic glasses equilibrated at 1350 degrees C over a range of oxygen fugacities (log fO(2) from -8.63 to -0.68) were examinedwith Fe K-edge X-ray absorption near-edge structure (XANES) and Mossbauer spectra. XANES spectral features were then calibrated as a function of Mossbauer-derived Fe3+/Sigma Fe ratios. Additionally, both methods help characterize the local structure of iron ions in andesitic glasses. Fe3+/Sigma Fe ratios were determined from Mossbauer spectra collected at room temperature but corrected with recoilless fractions obtained from previously reported Mossbauer 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+/Sigma 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+/Sigma Fe ratio, rather than to intrinsic differences in the spectra as a function of Fe3+/Sigma Fe ratio for mafic glasses. The ratios of intensities of pre-edge sub-peaks and Fe3+/Sigma Fe ratios for andesitic, basaltic, and rhyolitic glasses plot along a common trend, indicating that these measures provide a XANES calibration for Fe3+/Sigma 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+/SFe ratio from Mossbauer 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 Mossbauer spectra. The mean coordination number of Fe3+ inferred from XANES increases from similar to 4.5 to similar to 5 as andesitic glasses vary from reduced to oxidized; Mossbauer hyperfine parameters also suggest network-forming behavior of Fe3+, but with higher coordination for more reduced glasses. (C) 2016 Elsevier B.V. All rights reserved.