5. Murad

57

Fe Mössbauer Spectroscopy :

a Tool for the Remote Characterization of Phyllosilicates?

Enver Murad Marktredwitz, Germany

Basic principles of Mössbauer spectroscopy

Free emitting and absorbing atoms

E m is s io n

A b s o r p tio n

R e c o il

Energy of recoil

2 γ

E ER = 2 2mc

γ-ray energy Mass of atom

Emitting and absorbing atoms fixed in a lattice

E m is s io n

A b s o r p tio n

N o r e c o il

2 Mössbauer spectroscopy is the γrecoil-free emission R and absorption of gamma rays 2

E E = 2Mc    

 

Mass of particle

Appearance of Mössbauer spectra

Symmetric charge No magnetic field

Asymmetric charge No magnetic field

Bhf

Magnetic hyperfine field

Δ

Quadrupole splitting

δ

Isomer shift

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.

Symmetric or asymmetric charge Magnetic field (internal or external)

R e la t iv e T r a n s m is s io n

∆

Fe3+

Fe2+

δ

-4

-2

0

V e lo c ity ( m m /s )

2

4

T r a n s m is s io n

∆ = 0 .4 1 m m /s [ ∆ = 0 .0 0 m m /s ] -1 0

-5

0

V e lo c ity ( m m /s )

5

10

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.

Hyperfine parameters of Fe3+ oxides Hematite

≤ 950

Magnetite

≤ 850

Magnetic hyperfine fields RT: Bhf (T) 4.2 K: Bhf (T) Δ (mm/s) ≤ 51.8 ≤ 53.5 -0.20 / 54.2 +0.41 ≤ 49.2 + 46.1 ≤ 50.6 { + 36 – 52 } #

Maghemite

≤ ~ 950

≤ 50.0 + 50.0 ≤ 52.0 + 53.0 ≤|0.02|

Goethite

≤ 400

≤ 38.0

Akaganéite

≤ 299

–

≤ 47.3 + 47.8 + 48.9

Lepidocrocite ≤ 77 Feroxyhyte ≤ 450

–

≤ 45.8

Mineral

Ferrihydrite Bernalite

Ordering temperature (K)

25 – 115 * ≤ 427

* Magnetic blocking temperature

0.02

≤ 53 + 52 ~0.0

≤ 41

–

47 – 50 -0.02 – -0.07

≤ 41.5 # several

≤ 50.6 -0.25

≤ 56.2 ≤|0.01|

B-site subspectra below 120 K

Iron in phyllosilicates

O

Fe3+

1:1 phyllosilicates

Si O H ,O

Fe2+, Fe3+

Al

O

Fe2+,

2:1 phyllosilicates

Si

Fe3+ Al

Fe3+

O H Si O

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

0

Al, (Fe)

Kaolin

Kaolinite, halloysite

1:1

Tri 3

0

Mg, (Fe)

Serpentine

Lizardite, chrysotile

2:1

Di

< 0.2

Al, (Fe)

Pyrophyllite

Pyrophyllite

2:1

Tri

< 0.2

Mg, (Fe)

Talc

Talc, minnesotaite

2:1

Di / Tri

0.2 – 0.6

Al, Mg, Fe

Smectite

Montmorillonite, nontronite

2:1

Di / Tri

0.6 – 0.9

Al, Mg, Fe

Vermiculite

Vermiculite

2:1

Di / Tri

> 0.9

Al, Mg, Fe

Mica

Illite, glauconite

2 : 1 (: 1)

Di / Tri

Al, Mg, Fe

Chlorite

Clinochlore, chamosite

1 Per

formula unit [O10(OH)2], 2/3 Dioctahedral/Trioctahedral

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

The “simplest” clay mineral: kaolinite [Al2Si2O5(OH)4] 100

T r a n s m is s io n ( % )

9 9 .5

O

99

Si 9 8 .5

O H ,O 98

Al -7 .5

-5

-2 .5

0

V e lo c ity ( m m /s )

2 .5

5

7 .5

Kaolin / Jari @ 295 K

T r a n s m is s io n ( % )

100

99

98

97 -1 0

-5

0

V e lo c ity ( m m /s )

5

10

Kaolin / Jari @ 4.2 K

Mössbauer parameters of clay minerals Mineral

Temp

Isomer shift Quadruple (δ/Fe) splitting (Δ)

Kaolinite

RT

0.35 *

0.51*

* Average values. Isomer shift relative to α-Fe at room temperature. Only Fe3+ considered.

Illite: (K,H3O)x+y(Al2-xMx)(Si4-yAly)O10(OH)2 T r a n sTmr ai sn ss i om n i s ( s%i o ) n ( % )

100 99

Fe3+: 2 Δ

98 97 100 96 99 95 9 8 -6

-3

0

-3

0

Fe3 3+: P(Δ) 6

97 96 95

(% )

16

-6

3

6

12

P

8 4 0 0

0 .2 5

0 .5

0 .7 5

1

1 .2 5

 (m m /s )

1 .5

1 .7 5

2

2 .2 5

Illite OECD #5

Mössbauer parameters of clay minerals Mineral

Temp

Isomer shift Quadruple (δ/Fe) splitting (Δ)

Kaolinite

RT

0.35

0.51

Illite

RT

0.35

0.59 – 0.73 *

* Average values for Fe-poor (≤ 3% Fe) and Fe-rich (> 5% Fe) samples, respectively

Nontronite: MxFe3+ 2 (Si4-xFex)O10(OH)2 T r a n s m T i rs as ni os nm (i s% s )i o n ( % )

100

RT

23.46 % Fe

90

80

70 100 -1 0

-5

95

0

V e lo c ity ( m m /s )

5

10

77 K

1.44 % Fed → 2.3 % Gt From Asext → 1.4 % Gt

90 85 -1 0

-5

0

V e lo c ity ( m m /s )

5

10

Nontronite API H33a

Nontronite: MxFe3+ 2 (Si4-xFex)O10(OH)2 Fe2+/(Fe2++Fe3+) = 0.15

100

DCB T ra n s m is s io n (% )

98

96 100 98

-4

-2

-4

-2

96

0

2

4

0

2

4

V e lo c it y ( m m / s )

Reoxidized 644 days in air → no Fe2+

94 92

V e lo c ity ( m m /s )

Nontronite API H33a

Mössbauer parameters of clay minerals Mineral

Temp

Isomer shift Quadruple (δ/Fe) splitting (Δ)

Kaolinite

RT

0.35

0.51

Illite

RT

0.35

0.59 – 0.73

Nontronite

RT

0.35

0.35

Complex natural clays : “The real world”

Na+ H2O

CH4 Fe3+

Fe2+

Phyllosilicate Note: Fe-containing with intercalated interlayer must interlayer interlayer: be frozen to show “Nature’s trashcan” Mössbauer Effect

Physics Today 61 (8)

1 0 0 .0

T ra n s m is s io n (% )

9 9 .8 9 9 .6

1 0 0 .0 9 9 .8 9 9 .6 DCB-treated

-1 0

-5

−0.11 % goethite

0

V e lo c ity ( m m /s )

5

10

Kaolin “Wolfka” @ 4.2 K

T r a n s m i t t a n c e ( %T r )a n s m i s s i o n ( % )

100

98

B X -N 96 100

-1 0

-5

0

V e lo c ity ( m m /s )

5

10

99

295 K

3 x D C B

98 -1 0

-5

0

V e lo c ity ( m m /s )

5

Bauxite 10

100 98

295 K

96 94

T r a n s m is s io n ( % )

92 100

120 K

98 96 100

77 K

98 96 100

4 .2 K

98

Red soil

96 -1 0

-5

0

V e lo c it y ( m m / s )

5

10

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

Lunar “soil” 10084 N a n o p h a s e ir o n

M e ta llic ir o n

R e s o n a n t a b s o r p tio n

F e 2 + in o liv in e F e 2 + in ilm e n ite F e 2 + in p y r o x e n e F e 2 + in g la s s

W a n g e t a l., 1 9 9 5 -5

0 V e lo c ity ( m m /s )

5

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 !

——— Fe2+ sulfate ?

?

?

“The shouldthe notbroad ... bedoublet interpreted in aninisolated form, “… data we assign present Mössbauer but ... correlated with thetoresults other 2+techniques ...” spectra of [Mars] soils be dueofto Fe sulfates rather NASA/JPL/University of Mainz (S.S. thanHafner olivine1975) … ” (Bishop et al. 2004)

Summar y

Strengths and weaknesses of 57Fe Mössbauer spectroscopy Strengths • 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

Weaknesses • 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.