Analysis of Iron Oxidation in Garnets

1. Analysis of Iron Oxidation in Garnets By, Erica A. Emerson

2. Order of Events • Goals of this study • Brief introduction of research conducted • Major results • Mössbauer • XANES • Conclusions • Questions

3. Goals of this study • Measure recoil-free fractions for Fe2+ and Fe3+ and use those results to measure Fe3+ accurately on a suite of 20 garnets by Mössbauer • Use XANES on same samples to calculate the % Fe3+, and then compare it to Mössbauer results.

4. Introduction: Iron Oxidation and Relation to fo2 • Oxidation state is a description of how many electrons it has lost or gained from its original state • If the environment was abundant in oxygen, many of the minerals in the assemblage will contain oxidized iron, Fe3+. • If the environment is more reducing, there is likely to be more Fe2+. • Oxygen fugacity (fo2 ) is measure of the amount of free or uncombined oxygen available in an environment

5. The amount of each depends largely on the oxidation conditions, hence on the oxygen fugacity • Highfo2 => Fe3+ • Low fo2 => Fe2+ • Really Low fo2 => Fe0 • Presence of oxygen in a magma results in crystallization of different minerals

6. Effect of Oxidation State of Iron on Crystallization (basaltic magmas)

7. Mössbauer and XANES • Mössbauer spectroscopy • A technique based on the discovery of recoilless gamma ray emission and absorption discovered in 1957. • Radioactive isotope (57Co) breaks down into a stable isotope (57Fe). As the radioactive source breaks down it, releases gamma rays known as beta decay. • Used to identify and quantify Fe2+/Fe3+ ratios in garnets. • XANES (X-ray Absorption Near-edge Structure) • A synchrotron transmits photons into the samples (Calas et al., 1987). • The photon energy then excites electrons within the sample, resulting in low-probability, localized transitions of the K-level, 1s, to partially-filled or lowest-energy, empty, bound, excited states (Calas et al. 1987 and Dyar et al. 2002). • Measure the transmitted light and changes in energy

8. Mössbauer Results Mössbauer spectra displaying Fe3+ in andradite garnet. Andradite, pure garnet, courtesy of Val Malenko. Mössbauer spectrum displaying Fe2+ in a Fort Wrangell almandine garnet. MIT Teaching collection. • Error Analysis • Temperature error was  1K • Isomer shift and quadrupole splitting error was  0.02 mm/s

9. Temperature series of this sample was acquired (4-295K) • Calculated recoil-free fraction, f and C to obtain accurate Fe3+/Fe2+ percentages.

10. Mössbauer Spectroscopy • Recoil calculation • Corrects for energy loss due to recoil • Allows for accurate and true measurement of %Fe3+ • Calculation and significance of C • C value compares f (Fe3+) and f (Fe2+) • Used to calculate cations of Fe2+and Fe3+ per formula unit

11. XANES Results • Error Analysis • Pre-edge peak extraction in using X26A Data Plotter resulted in the error of  0.03 eV. • PAN: Peak Analysis and resulted in an error of  0.1 eV. • Error on peak position is thus at least  0.13 eV • Probably depends on extent of peak overlap • and peak multiplicity

12. Mössbauer Fe3+ vs. XANES Fe3+ Except for sample bbkg and 9b, results agree within ±5% absolute!

13. Mössbauer Fe3+ vs. XANES Fe3+ Best fit line to data 1:1 line

14. Conclusions • As a reminder: The goal of this study was to measure the oxidation states of garnets using the Mössbauer and XANES techniques. • The percentages of Fe3+ and Fe2+ according to Mössbauer and XANES, revealing that both techniques agree well within ±5%, with the exception of samples AK97-9b and the Kenyan melanite. • In conclusion, the Mössbauer spectroscopy and XANES results complement each other. Mössbauer and XANES data measure approximately the same percentage of Fe3+ content.