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

2. Order of Events  Goals of this study  Brief introduction of research conducted  Major results  Mössbauer  XANES  Conclusions  Questions  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 Fe 2+ and Fe 3+ and use those results to measure Fe 3+ accurately on a suite of 20 garnets by Mössbauer  Use XANES on same samples to calculate the % Fe 3+, and then compare it to Mössbauer results.  Measure recoil-free fractions for Fe 2+ and Fe 3+ and use those results to measure Fe 3+ accurately on a suite of 20 garnets by Mössbauer  Use XANES on same samples to calculate the % Fe 3+, and then compare it to Mössbauer results.

4. Introduction: Iron Oxidation and Relation to fo 2  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, Fe 3+.  If the environment is more reducing, there is likely to be more Fe 2+.  Oxygen fugacity (fo 2 ) is measure of the amount of free or uncombined oxygen available in an environment  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, Fe 3+.  If the environment is more reducing, there is likely to be more Fe 2+.  Oxygen fugacity (fo 2 ) 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  High fo 2 => Fe 3+  Low fo 2 => Fe 2+  Really Low fo 2 => Fe 0  Presence of oxygen in a magma results in crystallization of different minerals  The amount of each depends largely on the oxidation conditions, hence on the oxygen fugacity  High fo 2 => Fe 3+  Low fo 2 => Fe 2+  Really Low fo 2 => Fe 0  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 ( 57 Co) breaks down into a stable isotope ( 57 Fe). As the radioactive source breaks down it, releases gamma rays known as beta decay.  Used to identify and quantify Fe 2+ /Fe 3+ 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  Mössbauer spectroscopy  A technique based on the discovery of recoilless gamma ray emission and absorption discovered in 1957.  Radioactive isotope ( 57 Co) breaks down into a stable isotope ( 57 Fe). As the radioactive source breaks down it, releases gamma rays known as beta decay.  Used to identify and quantify Fe 2+ /Fe 3+ 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 spectra displaying Fe 3+ in andradite garnet. Andradite, pure garnet, courtesy of Val Malenko. Mössbauer spectrum displaying Fe 2+ in a Fort Wrangell almandine garnet. MIT Teaching collection. Mössbauer Results  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 Fe 3+ /Fe 2+ percentages.

10.  Recoil calculation  Corrects for energy loss due to recoil  Allows for accurate and true measurement of %Fe 3+  Calculation and significance of C  C value compares f (Fe 3+ ) and f (Fe 2+ )  Used to calculate cations of Fe 2+ and Fe 3+ per formula unit  Recoil calculation  Corrects for energy loss due to recoil  Allows for accurate and true measurement of %Fe 3+  Calculation and significance of C  C value compares f (Fe 3+ ) and f (Fe 2+ )  Used to calculate cations of Fe 2+ and Fe 3+ per formula unit Mössbauer Spectroscopy

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 Fe 3+ vs. XANES Fe 3+ Except for sample bbkg and 9b, results agree within ±5% absolute!

13. Mössbauer Fe 3+ vs. XANES Fe 3+ 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 Fe 3+ and Fe 2+ 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 Fe 3+ content.  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 Fe 3+ and Fe 2+ 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 Fe 3+ content.