Accurate determination of Fe³⁺/ΣFe of andesitic glass by Mössbauer spectroscopy

To evaluate the accuracy of Fe³⁺ and Fe²⁺ ratios in silicate glasses determined by Mössbauer spectroscopy, we examine in detail the temperature (47-293 K) of Mössbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 °C (VF3) and the other at 3.5 GPa, 1600 °C (M544). Variabletemperature Mössbauer 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⁺ Mössbauer 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⁺, C_T, can then be used to adjust the observed area of the Mössbauer doublets into the Fe³⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe determined for the glass. For the VF3 glass, the Fe³⁺/ΣFe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2σ)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2σ). The Fe³⁺/ΣFe corrected for recoilless fraction contributions is 0.634 ± 0.078(2σ), 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⁺/ΣFe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe³⁺/ΣFe are always systematically higher compared to recoilless-fraction corrected ratios.