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57Fe Mössbauer Spectroscopy
: a Tool for the Remote Characterization of Phyllosilicates? Enver Murad Marktredwitz, Germany Enver Murad Marktredwitz, Germany
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Basic principles of Mössbauer spectroscopy
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Free emitting and absorbing atoms
γ-ray energy Energy of recoil Mass of atom
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Emitting and absorbing atoms fixed in a lattice
Mössbauer spectroscopy is the recoil-free emission and absorption of gamma rays Mass of particle
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Appearance of Mössbauer spectra
Depending on the local environments of the Fe atoms and the magnetic properties, Mössbauer spectra of iron oxides can consist of a singlet, a doublet, or a sextet. Magnetic hyperfine field Quadrupole splitting Isomer shift δ Δ Bhf Symmetric charge No magnetic field Asymmetric charge No magnetic field Symmetric or asymmetric charge Magnetic field (internal or external)
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Fe3+ Fe2+
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Use of Mössbauer spectroscopy as a “fingerprinting” technique
Isomer shifts and quadrupole splittings of Fe-bearing phases vary systematically as a function of Fe oxidation, Fe spin states, and Fe coordination. Knowledge of the Mössbauer parameters can therefore be used to “fingerprint” an unknown phase.
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Hyperfine parameters of Fe3+ oxides
Mineral Ordering temperature (K) Magnetic hyperfine fields RT: Bhf (T) K: Bhf (T) Δ (mm/s) Hematite 950 51.8 / Magnetite 850 50.6 { + 36 – 52 } Maghemite ~ 950 ≤|0.02| Goethite 400 38.0 Akaganéite 299 – Lepidocrocite 77 – Feroxyhyte 450 41 ~0.0 Ferrihydrite 25 – 115 47 – – -0.07 Bernalite 427 41.5 56.2 ≤|0.01| ≤ ≤≤≤≤≤≤ ≤ ≤≤≤ ≤ ≤ ≤ ≤≤≤≤≤≤ # * ≤ ≤ * Magnetic blocking temperature # several B-site subspectra below 120 K
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Iron in phyllosilicates
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1:1 phyllosilicates 2:1 phyllosilicates Fe3+ Fe2+, Fe3+ Fe3+
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Classification of clay-sized phyllosilicates (clay minerals sensu stricto)
Layer type Octahedral occupancy Octahedral charge 1 Central cation(s) Group Common species 1 : 1 Di 2 Al Kaolin Kaolinite, halloysite Tri 3 Mg Serpentine Lizardite, chrysotile 2 : 1 Di < 0.2 Pyrophyllite Tri Talc Talc, minnesotaite Di / Tri 0.2 – 0.6 Al, Mg, Fe Smectite Montmorillonite, nontronite 0.6 – 0.9 Vermiculite > 0.9 Mica Illite, glauconite 2 : 1 (: 1) Chlorite Clinochlore, chamosite , (Fe) , (Fe) , (Fe) , (Fe) 1 Per formula unit [O10(OH)2], 2/3 Dioctahedral/Trioctahedral
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Mössbauer spectra of selected “simple” (pure) clay minerals
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The “simplest” clay mineral: kaolinite [Al2Si2O5(OH)4]
Kaolin / 295 K
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Kaolin / 4.2 K
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Mössbauer parameters of clay minerals
Temp Isomer shift (δ/Fe) Quadruple splitting (Δ) Kaolinite RT 0.35 0.51 * * * Average values. Isomer shift relative to αFe at room temperature. Only Fe3+ considered.
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Illite: (K,H3O)x+y(Al2-xMx)(Si4-yAly)O10(OH)2
Fe3+: 2 Δ Fe3+: P(Δ) Illite OECD #5
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Mössbauer parameters of clay minerals
Temp Isomer shift (δ/Fe) Quadruple splitting (Δ) Kaolinite RT 0.35 0.51 Illite 0.59 – 0.73 * * Average values for Fe-poor (≤ 3% Fe) and Fe-rich (> 5% Fe) samples, respectively
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Nontronite: MxFe2 (Si4-xFex)O10(OH)2
3+ 23.46 % Fe RT 1.44 % Fed → 2.3 % Gt From Asext → 1.4 % Gt 77 K Nontronite API H33a
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Nontronite: MxFe2 (Si4-xFex)O10(OH)2
3+ Fe2+/(Fe2++Fe3+) = 0.15 DCB Reoxidized 644 days in air → no Fe2+ Nontronite API H33a
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Mössbauer parameters of clay minerals
Temp Isomer shift (δ/Fe) Quadruple splitting (Δ) Kaolinite RT 0.35 0.51 Illite 0.59 – 0.73 Nontronite
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Complex natural clays : “The real world”
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Phyllosilicate with intercalated interlayer: “Nature’s trashcan”
H2O CH4 Fe3+ Fe2+ Phyllosilicate with intercalated interlayer: “Nature’s trashcan” Note: Fe-containing interlayer must be frozen to show Mössbauer Effect Phyllosilicate with intercalated interlayer Physics Today 61 (8)
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DCB-treated −0.11 % goethite Kaolin 4.2 K
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Bauxite
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Red soil
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Extraterrestrial Mössbauer spectroscopy
Lunar samples In situ Mössbauer spectroscopy on Mars
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Lunar “soil” 10084 S.S. Hafner, 1975: “The data should not ... be interpreted in an isolated form, but ... correlated with the results of other techniques ...”. For lunar samples, this is possible !
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——— Fe2+ sulfate ? ? ? “The data should not ... be interpreted in an isolated form, but ... correlated with the results of other techniques ...” (S.S. Hafner 1975) “… we assign the broad doublet present in Mössbauer spectra of [Mars] soils to be due to Fe2+ sulfates rather than olivine … ” (Bishop et al. 2004) NASA/JPL/University of Mainz
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Summar y
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Strengths and weaknesses of 57Fe Mössbauer spectroscopy
Sensitive only to 57Fe (no matrix effects) Sensitive to oxidation state Allows distinction of magnetic phases Very sensitive towards magnetic phases Non-destructive Resolution limited by uncertainty principle Sensitive only to 57Fe (“sees” only 57Fe) Coordination ? to ± Paramagnetic phase data often ambiguous Diamagnetic element substitution & relaxation Slow If possible, use other techniques as well Often a combination of Mössbauer spectroscopy with other techniques can help solve problems that cannot be resolved using Mössbauer spectroscopy alone.
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