Analysis of Iron Oxidation in Garnets By, Erica A. Emerson By, Erica A. Emerson
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
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.
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
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
Effect of Oxidation State of Iron on Crystallization (basaltic magmas)
Mössbauer and XANES Mössbauer spectroscopy A technique based on the discovery of recoilless gamma ray emission and absorption discovered in 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 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 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 and Dyar et al. 2002). Measure the transmitted light and changes in energy
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
Temperature series of this sample was acquired (4-295K) Calculated recoil-free fraction, f and C to obtain accurate Fe 3+ /Fe 2+ percentages.
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
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
Mössbauer Fe 3+ vs. XANES Fe 3+ Except for sample bbkg and 9b, results agree within ±5% absolute!
Mössbauer Fe 3+ vs. XANES Fe 3+ Best fit line to data 1:1 line
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.