1. 57Fe Mössbauer Spectroscopy :
a Tool for the Remote Characterization of
Phyllosilicates?
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
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

10. Iron in phyllosilicates

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

12. 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

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

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

15. Kaolin / 4.2 K

16. 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.

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

18. 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

19. Nontronite: MxFe2 (Si4-xFex)O10(OH)23+
23.46 % Fe RT
1.44 % Fed
→ 2.3 % Gt
From Asext
→ 1.4 % Gt
77 K
Nontronite API H33a

20. Nontronite: MxFe2 (Si4-xFex)O10(OH)23+
Fe2+/(Fe2++Fe3+) = 0.15
DCB
Reoxidized 644 days in air → no Fe2+
Nontronite API H33a

21. 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

22. Complex natural clays :
“The real world”

23. 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)

24. DCB-treated
−0.11 % goethite
Kaolin 4.2 K

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. Summar y

32. 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.