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Similarly, as for most of the ferrihydrites, the spectrum of oxykerchenite remains a
doublet down to 80 K [
66
].
At very low temperatures the sextet of ferrihydrite exhibits broad lines with a
rather symmetrical lineshape (Fig.
3.7
). At 4 K the average hyperfine field amounts
to between 46 and 50 T [
67
], with a small quadrupole shift 2e between -0.02 and
-0.1 mm/s. The spectra are usually slightly asymmetric in which in particular the
6th line is less deep than the first one. This has been attributed to the presence of
additional tetrahedral iron sites [
68
], although this is still in doubt and correlation
effects between B in the distribution on the one hand, and D and/or d on the other
hand, which are normally expected in largely disordered structures, may similarly
produce such an asymmetry.
As in other iron oxyhydroxides, isomorphous substitution for Fe by Al is
expected in natural samples. A study of synthetic samples with Al substitutions by
[
69
] revealed an increasing asymmetry of the doublet lineshape at RT, pointing to
an increased average quadrupole splitting. At low temperatures, a decrease of the
average hyperfine field and a lowering of the magnetic transition temperature
region with increasing Al content is reported. All these features are quite similar to
crystallinity effects and are therefore not of practical use for characterization
purposes. Also silicon seems to play an important role in ferrihydrite. As in the
case of goethite, Si species can easily adsorb on ferrihydrite and thus prevents its
further growth [
70
]. Childs [
57
] claimed that ferrihydrite can contain up to 9 %at
Si, but the question remained if it is adsorbed or incorporated in the structure.
Campbell et al. [
71
] suggested that Si is structurally incorporated and demon-
strated the ability of Si to inhibit a transformation to more stable Fe
3+
oxides or
oxyhydroxides.
However, several more recent works have resulted in a complete change in the
earlier, more-or-less contradictory ideas about ferrihydrite and its XRD and
Mössbauer behavior. Berquó et al. [
72
] report the possibility of synthesizing
Si-ferrihydrites with much better crystallinity. One of the authors' ferrihydrites
shows even relatively sharp lines in the XRD pattern and its Mössbauer spectrum
even consists of a somewhat collapsed sextet at RT, but with no doublet contri-
bution. The spectrum of a natural sample, showing similarly seven, but somewhat
broadened lines in the XRD pattern, exhibits a collapsed sextet at 130 K, but
remains exclusively a doublet at RT.
On the other hand, it has recently been shown that some ferrihydrite species still
exhibit a magnetic-superparamagnetic transition at very low temperatures. In
particular, this seems to happen when ferrihydrite is closely associated to organic
carbon [
73
]. These so-called DOM (dissolved organic matter) ferrihydrites have a
lower hyperfine field and are even not completely magnetically ordered at 4 K (see
also
Sect. 3.5.1
).
In conclusion, the very poor crystallinity and the low superparamagnetic blocking
temperature in natural ferrihydrites hamper to some extent the characterization of
ferrihydrite with MS at standard measuring temperatures (RT, 80 K). From the point
of view of identification, ferrihydrite can be recognized by MS as long as it represents
the main constituent in soil samples. The broad quadrupole distribution (see Fig.
3.8
)
