Redox kinetics of iron in alkali silicate glasses exposed to ionizing beams: Examples with the electron microprobe

Fialin, Michel; Wagner, Christiane

doi:10.1016/j.jnoncrysol.2012.04.026

Cited by 9 publications

(7 citation statements)

References 39 publications

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“…The diffusivity of H 2 O in basaltic glasses depends on the total H 2 O content (Okumura and Nakashima 2006), thus the rate of oxidation increases with increasing H 2 O diffusivity. These results show that the migration of H + , in addition to Na + and K + as previously suggested by Fialin and Wagner (2012), leads to oxidation of Fe during electron beam irradiation. In fact, when considering the mobile cation responsible for Fe oxidation, H + plays a more important role than might be expected from its oxide wt% concentrations alone due to the low atomic mass of H.…”

Section: Rate Of Oxidationsupporting

confidence: 85%

“…Finally, no change in FeLβ f /FeLα f with time implies stable Fe 2+ /Fe T during analysis. The presence of predominantly magnetite nanolites after electron beam irradiation implies that oxidation proceeds via precipitation of FeO·Fe 2 O 3 , not just Fe 2 O 3 , as has been previously suggested (Fialin and Wagner 2012).…”

Section: During Electron Beam Irradiationsupporting

confidence: 57%

“…The FeL lines have low intensity and therefore high beam currents and/or long count times are required to record them. Silicate glasses are typically unstable under these conditions, leading to changes in Fe oxidation state during analysis (Fialin et al 2004(Fialin et al , 2011Fialin and Wagner 2012;Zhang et al 2018). Similar problems have also been observed for Fe in amphiboles (Wagner et al 2008;Lamb et al 2012) and S in silicate glasses and anhydrite (Wallace and Carmichael 1994;Rowe et al 2007;Klimm et al 2012).…”

Section: Introductionmentioning

confidence: 99%

“…Similar problems have also been observed for Fe in amphiboles (Wagner et al 2008;Lamb et al 2012) and S in silicate glasses and anhydrite (Wallace and Carmichael 1994;Rowe et al 2007;Klimm et al 2012). Fialin and Wagner (2012) observed two competing mechanisms of redox change during electron beam irradiation of alkali-bearing silicate glasses leading to either oxidation or reduction. As glasses are insulators, electrons are trapped within the subsurface during electron beam irradiation, causing a region of negative charge to buildup at depth in the sample, even with a conductive coat (e.g., Cazaux 1996).…”

Section: Introductionmentioning

confidence: 99%

“…FeLβ f /FeLα f is extrapolated to time zero to correct for changes over time, which we refer to as the Time-Dependent Ratio (TDR) correction, comparable to TDI corrections for alkalis. Due to the small sample size of silicate glasses analyzed by Fialin and Wagner (2012) and Zhang et al (2018), the controls on Fe redox processes during electron beam irradiation have not been explored and, crucially, few hydrous glasses have been analyzed. Therefore, we also investigate the compositional and analytical controls on Fe redox changes.…”

Section: Introductionmentioning

confidence: 99%

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High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe

Hughes

Buse

Kearns

et al. 2018

American Mineralogist

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The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβ f /FeLα f ) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO 2 , 0-10 wt% H 2 O, and 2-18 wt% FeO T , which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe 2+ /Fe T ) of hydrous (0-4 wt% H 2 O) basaltic (43-56 wt% SiO 2 ) and peralkaline (70-76 wt% SiO 2 ) glasses with FeO T > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeO T and 0.5 Fe 2+ /Fe T ) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (~20 and ~60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO 2 , Fe, and H 2 O content.

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Section: Rate Of Oxidationsupporting

confidence: 85%

Section: During Electron Beam Irradiationsupporting

confidence: 57%