Chemistry Reference
In-Depth Information
minerals have Fe
3+
in octahedral environment which all yield similar doublets with
a quadrupole splitting in the range 0.4-0.8 mm/s and therefore they cannot be
unambiguously identified. Moreover, the hyperfine parameters can slightly be
altered by morphological and chemical influences or may show a distributive
behavior, which prevents a clear-cut assignment of the spectral components. As
will be shown further, many minerals with poor crystallinity or small-particle
morphology exhibit a range of hyperfine field values and the sextet spectra often
consist of asymmetrically broadened absorption lines. For all those cases it is
obvious that a more elaborated Mössbauer analysis combined with results of other
techniques are necessary.
Quantitative information is obtained from the spectra through the relative area
of each subspectrum. This area is essentially proportional to the concentration of
each kind of iron with its valence in a specific environment. So, the distribution of
various types of iron among different sites in a mineral or the concentration of
different iron-bearing compounds in a multi-phase assemblage can in principle be
determined.
The area ratio A
A
/A
B
of the spectra of two Fe species A and B is given by
A
A
A
B
¼
C
A
RT
ð
T
A
C
B
RT
ð
T
B
ð
3
:
1
Þ
where C is the width at half maximum of the absorption peaks and R(T)isa
thickness reduction function due to saturation effects which depends on the
absorber thickness T. The absorber thickness is not a real physical thickness, but a
scalar defined by T = nfr
0
, where n is the number of Mössbauer active atoms per
cm
2
in the absorber, f the Mössbauer fraction (recoilless fraction) and r
0
the cross
section at resonance equal to 2.35 9 10
-18
cm
2
for
57
Fe. If we set C
A
= C
B
and in
a first approximation R(T
A
) = R(T
B
) the ratio N
A
/N
B
of the amount of iron atoms
of both types A an B can then be calculated from
N
A
N
B
¼
n
A
n
B
¼
f
B
f
A
A
A
A
B
ð
3
:
2
Þ
The Mössbauer fraction f of each kind of iron species is mainly governed by the
lattice dynamics in the crystal. Therefore, f is dependent on the coordination and
can differ slightly from mineral to mineral. Moreover, the Mössbauer fraction is
particularly very sensitive to the valence state of iron and is for Fe
2+
considerably
lower than for Fe
3+
. Because of the relationship with lattice vibrations, f is also
strongly temperature dependent. This means that at RT a large difference in
f values is observed and only to a lesser extent at 80 K. The f values for some iron-
containing minerals, determined from the temperature dependence of the isomer
shift (second order Doppler shift), are listed in Table
3.1
.
So far, we did not take into account any thickness effects. The aforementioned
formulas are only valid in the case of small thicknesses, because only in that case
the transmission integral function, which mathematically describes the spectrum,
can be replaced by a sum of Lorentzian lines. The latter profile can be considered
