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

Abstract

To evaluate the accuracy of Fe 3+ and Fe 2+ 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 Fe 3+ and Fe 2+ 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 Fe 3+ and Fe 2+ , C T , can then be used to adjust the observed area of the Mössbauer doublets into the Fe 3+ /SFe 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 Fe 3+ /SFe 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 Fe 3+ /SFe 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 Fe 3+ /SFe 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)Fe 2 O 4 oxide, but corrections for this component have negligible influence on the total Fe 3+ /SFe determined for the glass. For the VF3 glass, the Fe 3+ /SFe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2s)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2s). The Fe 3+ /SFe corrected for recoilless fraction contributions is 0.634 ± 0.078(2s), 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 Fe 3+ /SFe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe 3+ /SFe are always systematically higher compared to recoilless-fraction corrected ratios.

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

“…Dyar et al 434 (1987) also observed that wet chemical analyses yielded higher Fe 3+ , however, 435 their Fe 3+ contents were not directly from the fit of Mössbauer spectra. Similarly, 436 Jayasuriya et al (2004) showed that Fe 3+ /ΣFe in synthetic basaltic glasses 437 20 calculated from RT Mössbauer gave similar results to Fe 3+ /ΣFe calculated from 438Mössbauer spectra taken at 4.2 K, but both their RT and low temperature spectra 439 are complicated by strong non-paramagnetic components, which introduce 440 ambiguity to quantification of Fe 3+ /ΣFe(Zhang et al, 2015). Thus, there is no 441 consensus in the literature regarding whether Mössbauer and wet chemical 442 methods are consistent, or even in which direction the results may be biased.…”

“…The starting material of VF Series consists of a synthetic andesite similar in 92 5 composition to that used by O'Neill et al (2006) and Zhang et al (2015), and was 93 prepared from reagent oxides (SiO 2 , Al 2 O 3 , Fe 2 O 3 , TiO 2 , MgO), silicates 94 (CaSiO 3 ,Na 2 SiO 3 ) and stoichiometric sanidine glass. The Fe 2 O 3 consisted of 95 90% normal reagent oxide and 10% 57 Fe 2 O 3 (Isoflex, Inc) to obtain quality 96…”

“…The absorber thicknesses were adjusted for an absorber 183 density of ~2 mg/cm 2 Fe. 184 9 All Mössbauer spectra were fitted using the RECOIL software package 185 (Lagarec and Rancourt, 1998) with a 2D distribution extended Voigt-based fitting 186 (xVBF) method described in detail by Zhang et al (2015). Considerations of 187 precision and accuracy, including accounting for the effect of recoilless fraction, 188 are introduced briefly here.…”

“…The first detailed calibration for determination of Fe 3+ /∑Fe ratio for andesitic 586 glasses by XANES has been derived from a suite of samples spanning 8 to 84% 587 Mössbauer spectra collected at room temperature using xVBF fit methods and 849 33 corrected for recoilless fraction with a value of C=1.256 (Zhang et al, 2015). 850…”

“…Symbols are applied same as and Fe 2+ (Fig.1) and Mössbauer spectra collected at room temperature were fitted with Extended Voigt-based Fitting (xVBF) and Lorentzian Multiplet Analysis (LMA). *All experiments were performed at 1350 °C # equals (Fe 3+ / ΣFe )/[(Fe 3+ / ΣFe)+ C*(1-Fe 3+ /ΣFe)], where Fe 3+ /ΣFe is the ratio inferred from the relative areas of doublets from Mössbauer spectra collected from room temperature and C is the correction (1.256) for recoilless fraction for andesite glass quenched at ambient pressure (Zhang et al 2015). & Predicted from the model of Kress and Carmichael, 1991, using the observed composition of each glass (Table S1).…”