Accurate determination of Fe3+/∑Fe of andesitic glass by Mössbauer spectroscopy

To evaluate the accuracy of Fe3+ and Fe2+ ratios in silicate glasses determined by Mossbauer spectroscopy, we examine in detail the temperature (47-293 K) of Mossbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 degrees C (VF3) and the other at 3.5 GPa, 1600 degrees C (M544). Variable-temperature Mossbauer spectra of these two glasses are used to characterize the recoilless fraction, f, by two different methods a relative method (RM) based on the temperature dependence of the ratios of Fe3+ and Fe2+ Mossbauer doublets and the second based on the temperature dependence of the center shift (CS) of the doublets. The ratio of the recoilless fractions for Fe3+ and Fe2+, CT, can then be used to adjust the observed area of the Mossbauer doublets into the Fe3+/Sigma Fe ratio in the sample. We also evaluated the contributions of non-paramagnetic components to the Fe in the glasses by determining the influence of applied magnetic field on sample magnetization. Finally, for the VF3 glass, we determined the Fe3+/Sigma Fe independently by wet chemical determination of the FeO content combined with careful electron microprobe analyses of total Fe. Recoilless fractions determined with the CS method (CSM) are significantly smaller than those determined with the relative method and suggest larger corrections to room-temperature Fe3+/Sigma Fe ratios. However, the RM determinations are believed to be more accurate because they depend less on the assumption of the Debye harmonic model and because they produce more nearly temperature-independent estimates of Fe3+/Sigma Fe ratios. Non-linear responses of sample magnetizations to applied magnetic fields indicate that the glasses contain a small (0.4-1.1% for VF3) superparamagnetic component that is most likely to be nanophase precipitates of (Fe,Mg)Fe2O4 oxide, but corrections for this component have negligible influence on the total Fe3+/Sigma Fe determined for the glass. For the VF3 glass, the Fe3+/Sigma Fe produced by uncorrected room-temperature Mossbauer spectroscopy [0.685 +/- 0.014 in two standard deviation (2 sigma)] agrees within 3% of that determined by wet chemistry (0.666 +/- 0.030 in 2 sigma). The Fe3+/Sigma Fe corrected for recoilless fraction contributions is 0.634 +/- 0.078(2 sigma), which is 7.5% lower than the uncorrected room-temperature ratio, but also agrees within 5% of wet chemical ratio. At least for this andesitic glass, the room-temperature determination of Fe3+/Sigma Fe is accurate within analytical uncertainty, but room-temperature Mossbauer determinations of Fe3+/Sigma Fe are always systematically higher compared to recoilless-fraction corrected ratios.