Chemistry Reference
In-Depth Information
Fig. 3.42 RT Mössbauer spectrum of Cameroun soil sample (a) full spectrum, b central part—
the shaded doublet represents the ilmenite subspectrum
non-resolved doublets of superparamagnetic goethite and ferrihydrite. However,
the central part of that spectrum has been analyzed more carefully with conven-
tional doublets (Fig.
3.42
b). The overlapping long tails of the distributed lines of
the goethite and hematite sextets can be considered as contributing to a constant
non-resonant background in the central part. It was thus attempted to fit this part
with two Lorentzian lines to account for the inner lines of the hematite sextet and
two doublets for goethite. It was found, however, that an additional doublet was
needed to obtain a reasonable fit. This extra doublet with d
Fe
= 1.02 mm/s and
D = 0.8 mm/s corresponds to ilmenite which was not detected by any other
technique. So, it clear that a closer inspection of the central part of the spectra
might reveal the presence of some minor amounts of Fe-bearing minerals which
would not have been found by a quick standard fit.
Another example concerns the Mössbauer measurement of iron-containing
nodules in a planosol from the S-W Ethiopian highlands [
270
]. The RT Mössbauer
spectrum of the nodules in the vertic horizon as well as that of the accumulated
nodules consists of a single doublet at RT, which converts partly into a goethite
sextet at 80 K (Fig.
3.43
). The remaining Fe
3+
doublet with a quadrupole splitting
of 0.60 mm/s might be attributed to ferrihydrite.
At this point, one might conclude that no other iron-bearing phases are present
unless another spectral component in addition to the one of goethite/ferrihydrite
would be hidden in the Fe
3+
doublet. However, if the doublet at RT is analyzed with a
quadrupole-splitting distribution an extra maximum is found around 1.1 mm/s in
addition to the expected one for goethite/ferrihydrite at about 0.5-0.6 mm/s
(Fig.
3.44
). The second sample showed the same features. The RT spectra were then
accordingly reanalyzed with two doublets. The second doublet with d
Fe
= 0.37 mm/
s and D = 1.1 mm/s appeared indeed in the fit (Fig.
3.44
) and could be identified as
the main doublet of bixbyite (FeMnO
3
)[
271
,
272
], the presence of which was
detected by XRD. The other bixbyite doublet with a lower quadrupole splitting of 0.6
mm/s coincides with the main goethite/ferrihydrite doublet.
