57 Fe Mössbauer Spectroscopy

1. : a Tool for the Remote Characterization of Phyllosilicates? 57Fe Mössbauer Spectroscopy Enver Murad Marktredwitz, Germany Enver Murad Marktredwitz, Germany

2. Basic principles of Mössbauer spectroscopy

3. Free emitting and absorbing atoms γ-ray energy Energy of recoil Mass of atom

4. Emitting and absorbing atoms fixed in a lattice Mössbauer spectroscopy is the recoil-free emission and absorption of gamma rays Mass of particle

5. 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)

6. Fe3+ Fe2+

8. 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.

9. Hyperfine parameters of Fe3+ oxides ≤ ≤≤≤≤≤≤ ≤ ≤≤≤≤ ≤ ≤ ≤≤≤≤≤≤ # * ≤ ≤ *Magnetic blocking temperature #several B-site subspectra below 120 K

10. Iron in phyllosilicates

11. 1:1 phyllosilicates Fe3+ Fe2+, Fe3+ 2:1 phyllosilicates Fe3+ Fe2+, Fe3+

12. Classification of clay-sized phyllosilicates (clay minerals sensu stricto) , (Fe) , (Fe) , (Fe) , (Fe) 1Per formula unit [O10(OH)2], 2/3 Dioctahedral/Trioctahedral

13. Mössbauer spectra of selected “simple” (pure) clay minerals

14. The “simplest” clay mineral: kaolinite [Al2Si2O5(OH)4] Kaolin / Jari @ 295K

15. Kaolin / Jari @ 4.2K

16. Mössbauer parameters of clay minerals * * *Average values. Isomer shift relative to α­Fe at room temperature. Only Fe3+ considered.

17. Illite: (K,H3O)x+y(Al2-xMx)(Si4-yAly)O10(OH)2 Fe3+: 2Δ Fe3+: P(Δ) Illite OECD #5

18. Mössbauer parameters of clay minerals * *Average values for Fe-poor (≤ 3% Fe) and Fe-rich (> 5% Fe) samples, respectively

19. Nontronite: MxFe2 (Si4-xFex)O10(OH)2 3+ 23.46 % Fe RT 1.44 % Fed → 2.3 % Gt From Asext → 1.4 % Gt 77K Nontronite API H33a

20. Nontronite: MxFe2 (Si4-xFex)O10(OH)2 3+ Fe2+/(Fe2++Fe3+)=0.15 DCB Reoxidized644 days in air → no Fe2+ Nontronite API H33a

21. Mössbauer parameters of clay minerals

22. Complex natural clays : “The real world”

23. Na+ H2O CH4 Fe3+ Fe2+ Phyllosilicate with intercalated interlayer Note: Fe-containing interlayer must be frozen to show Mössbauer Effect Phyllosilicate with intercalated interlayer: “Nature’s trashcan” Physics Today 61(8)

24. DCB-treated −0.11% goethite Kaolin “Wolfka” @ 4.2K

25. Bauxite

26. Red soil

27. Extraterrestrial Mössbauer spectroscopy  Lunar samples In situ Mössbauer spectroscopy on Mars

28. 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 !

30. ———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

31. Summary

32. 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 Strengths and weaknesses of 57Fe Mössbauer spectroscopy Strengths Weaknesses Often a combination of Mössbauer spectroscopy with other techniques can help solve problems that cannot beresolved using Mössbauer spectroscopy alone.