analysis of iron oxidation in garnets
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Analysis of Iron Oxidation in Garnets. By, Erica A. Emerson. Order of Events. Goals of this study Brief introduction of research conducted Major results M össbauer XANES Conclusions Questions. Goals of this study. - PowerPoint PPT PresentationTRANSCRIPT
Analysis of Iron Oxidation in Garnets
Analysis of Iron Oxidation in Garnets
By,
Erica A. Emerson
By,
Erica A. Emerson
Order of EventsOrder of EventsGoals of this studyBrief introduction of research conductedMajor results
Mössbauer XANES
ConclusionsQuestions
Goals of this studyBrief introduction of research conductedMajor results
Mössbauer XANES
ConclusionsQuestions
Goals of this studyGoals 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.
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.
Introduction: Iron Oxidation and Relation to fo2
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
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
The amount of each depends largely on the oxidation conditions, hence on the oxygen fugacityHigh fo2 => Fe3+
Low fo2 => Fe2+
Really Low fo2 => Fe0
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 fugacityHigh fo2 => Fe3+
Low fo2 => Fe2+
Really Low fo2 => Fe0
Presence of oxygen in a magma results in crystallization of different minerals
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Effect of Oxidation State of Iron on Crystallization (basaltic magmas)
Effect of Oxidation State of Iron on Crystallization (basaltic magmas)
Mössbauer and XANESMössbauer and XANESMössbauer 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
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
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.
Mössbauer ResultsMössbauer Results
Error AnalysisTemperature error was 1KIsomer shift and quadrupole splitting error was
0.02 mm/s
Error AnalysisTemperature error was 1KIsomer 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 Fe3+/Fe2+ percentages.
Temperature series of this sample was acquired (4-295K)
Calculated recoil-free fraction, f and C to obtain accurate Fe3+/Fe2+ percentages.
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Recoil calculation
Corrects for energy loss due to recoilAllows 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
Recoil calculation
Corrects for energy loss due to recoilAllows 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
Mössbauer SpectroscopyMössbauer Spectroscopy
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⎟⎠
⎞⎜⎝
⎛⎟⎠⎞
⎜⎝⎛ −×+
×=
++
++
33
33
100
100
FeFe
FeFe
ACA
AN
+
+
=2
3
f
fC
XANES ResultsXANES 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
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 Fe3+ vs. XANES Fe3+Mössbauer Fe3+ vs. XANES Fe3+
True Amount of Fe3+ for Mössbauer and XANES Peaks
Samples Mössbauer True
% Area Fe3+
XANES % Area Fe3+
alm 0 0
ak972a 0 0 ak978a 0 0 ak9729 0 0
he1 2 0 ak9723 5 0
2b 5 0 10c 14 18 129 3 11 1251 8 7 9b 46 11 g17 43 41 g89 40 49 bbkg 92 96 5183g 86 93 a32w 76 79 ahun 92 100 hrm1 95 100 and 100 100
Except for sample bbkg and 9b, results agree within ±5% absolute!
y = 1.0559x - 2.2299
R2 = 0.973
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Percentage of Mossbauer Fe 3+
Percentage of XANES Fe
3+
y = 1.0559x - 2.2299
R2 = 0.973
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Percentage of Mossbauer Fe 3+
Percentage of XANES Fe
3+
Mössbauer Fe3+ vs. XANES Fe3+Mössbauer Fe3+ vs. XANES Fe3+
Best fit line to data
1:1 line
ConclusionsConclusions 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.
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.
