Chemistry Reference
In-Depth Information
Table 3.20
Hyperfine parameters for the goethite-hematite sample of example 1
T
B
av
(T)
B
p
(T)
2e or D (mm/s)
d
Fe
(mm/s)
RA (%)
Assignment
RT
48.3
49.7
-0.20
0.36
63
Hematite (WF)
(21)
-
-0.2
0.36
10
Goethite (coll.)
-
-
0.57
0.35
27
Goethite (SP)
80 K
52.4
52.9
-0.20
0.47
52
Hematite (WF)
53.5
53.9
0.20
0.47
6
Hematite (AF)
39.7
48.0
-0.24
0.48
39
Goethite (AF)
-
-
0.56
0.46
3
Goethite (SP)
The relative spectral areas RA (see Table
3.20
) provide an indication of the
amount of Fe species in the different components. However, the relative area of
hematite derived from the RT spectrum (63 %) is not the same as that observed at
80 K (52 % ? 6 %). This quite strong reduction (about 8 %) cannot be attributed
to the relative change in the Mössbauer fraction of hematite with respect to that of
goethite on decreasing the temperature from RT to 80 K. This difference is rather
an artifact of the distribution fitting applied to sextets where there is strong overlap
of the broad goethite sextet with the low-field tail of the hematite sextet. So, the
relative area is most accurately determined from the RT spectrum in which the
goethite appears as a doublet and consequently the sextet of hematite is well
resolved.
In a second example it is demonstrated that Mössbauer spectroscopy is helpful
in determining the relative change in the amounts of the different Fe phases present
in samples taken at different depths from a so-called Griffin Farmhill soil profile in
South Africa [
269
]. The RT spectra show a dominant doublet together with a
hematite sextet of which the intensity increases in the sequence C1 to C6, i.e. with
increasing depth along the profile (Fig.
3.39
). The 80 K spectra reveal a similar
increase of goethite, leaving only a small amount of doublet for C6. It was found
that the remaining doublet at 80 K for sample C1 possessed a large quadrupole
splitting of 0.68 mm/s (Table
3.21
) pointing to ferrihydrite. In order to verify the
latter, spectra of sample C1 have been taken at lower temperatures.
The spectrum at 4 K shows three sextets: one of hematite, one of goethite and
one of ferrihydrite (Fig.
3.40
). However, there is still a doublet present at 4 K. At
15 K the spectrum shows that the contribution of this doublet has increased at the
expense of the ferrihydrite sextet. This means that the doublet arises from
ferrihydrite species behaves superparamagnetically at temperatures as low as 4 K.
This feature can be explained by the presence of so-called DOM ferrihydrite [
73
]
in view of the high carbon content in the topsoil samples.
The evolution of the relative spectral areas for the different iron-bearing phases
as a function of depth is represented in Fig.
3.41
, which thus provides an idea of
the transformation of ferrihydrite to goethite and hematite in the different horizons.
From this picture is clear that in the deeper layers ferrihydrite is nearly completely
transformed to goethite and hematite.
