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Electron microprobe technique for the determination of iron oxidation state in silicate glasses

  • Chao Zhang , Renat R. Almeev EMAIL logo , Ery C. Hughes , Alexander A. Borisov , Eric P. Wolff , Heidi E. Höfer , Roman E. Botcharnikov and Jürgen Koepke
Published/Copyright: August 28, 2018
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American Mineralogist
From the journal American Mineralogist Volume 103 Issue 9

Abstract

We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe microanalysis (EPMA) with the “flank method.” This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe2+ content. Provided that this correlation is calibrated on reference materials, the Fe2+/ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Fe-rich ferric and ferrous garnet end-members, i.e., andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe2+/ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe2+content (in wt%). A single linear trend can be established for both garnets and silicate glasses with 4–18 wt% FeOT (total iron expressed as FeO). In glasses with up to 18 wt% FeOT and 15 wt% TiO2, no systematic compositional (matrix) effects were observed. A possible influence of Ti on the Fe2+ determination has only been observed in one high-Ti glass with ~25 wt% TiO2, a content that is not typical for natural terrestrial silicate melts. The accuracy of the Fe2+/ΣFe determination, which depends on both the Fe2+ content determined with the flank method and on the total Fe content, is estimated to be within ±0.1 for silicate glasses with FeOT > 5 wt% and within ±0.3 for silicate glasses with low FeOT ≤ 5 wt%. The application of the flank method on silicate glasses requires minimization of the EPMA beam damage that can be successfully achieved by continuous movement of the sample stage under the electron beam during analysis, e.g., with a speed of 2 μm/s.

Keywords: Microprobe; ferric-ferrous ratio; silicate glasses; redox state; flank method; pillow glasses

Acknowledgments

We thank Marius Stranghöner and Florian Pohl for the help with the wet-chemistry colorimetric method and Julian Feige for sample preparation. We appreciate editorial work of Maurizio Petrelli as well as detailed and insightful comments from John Fournelle and two anonymous reviewers. R.A. thanks Anette von der Handt and Eric Hellebrandt for support of the project. This study was funded by DFG projects AL1189/6-1, AL1189/8-1, AL1189/9-1, and HO1337/35-1.

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Received: 2017-12-17
Accepted: 2018-04-27
Published Online: 2018-08-28
Published in Print: 2018-09-25

© 2018 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2018-6437
Keywords for this article
Microprobe; ferric-ferrous ratio; silicate glasses; redox state; flank method; pillow glasses

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. The tales of disequilibrium and equilibrium crystallization of rare metal minerals: Data from new experiments
  3. Pressure, temperature, water content, and oxygen fugacity dependence of the Mg grain-boundary diffusion coefficient in forsterite
  4. Questioning the biogenicity of Neoproterozoic superheavy pyrite by SIMS
  5. The effect of disequilibrium crystallization on Nb-Ta fractionation in pegmatites: Constraints from crystallization experiments of tantalite-tapiolite
  6. Titanite major and trace element compositions as petrogenetic and metallogenic indicators of Mo ore deposits: Examples from four granite plutons in the southern Yidun arc, SW China
  7. Kuliginite, a new hydroxychloride mineral from the Udachnaya kimberlite pipe, Yakutia: Implications for low-temperature hydrothermal alteration of the kimberlites
  8. Electron microprobe technique for the determination of iron oxidation state in silicate glasses
  9. Experimental investigation of F and Cl partitioning between apatite and Fe-rich basaltic melt at 0 GPa and 950–1050 °C: Evidence for steric controls on apatite-melt exchange equilibria in OH-poor apatite
  10. Carbonic acid monohydrate
  11. High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe
  12. Disturbance of the Sm-Nd isotopic system by metasomatic alteration: A case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam
  13. Segerstromite, Ca3(As5+O4)2[As3+(OH)3]2, the first mineral containing As3+(OH)3, the arsenite molecule, from the Cobriza mine in the Atacama Region, Chile
  14. Vestaite, (Ti4+Fe2+) Ti34+ O9, a new mineral in the shocked eucrite Northwest Africa 8003
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  16. Letter
  17. Making tissintite: Mimicking meteorites in the multi-anvil
  18. Book Review
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