Chemistry Reference
In-Depth Information
Fig. 3.47
Spectra at 130 K of magnetically separated topsoil and underlying loess
The samples were also measured at low temperature i.e. at 130 K in order to
remain above the Verwey transition of magnetite (Fig.
3.47
). In these spectra the
AF and WF phase of hematite were simultaneously present demonstrating a better
crystallinity for this hematite phase than for the hematite observed in the total soil
and loess sample, for which at 80 K only the WF state was revealed.
This procedure of separation has also been applied to a series of samples from a
paleosol-loess sequence section in Huangling (China). From the corresponding
Mössbauer spectra and particularly from the relative spectral area ratios, interesting
conclusions could be drawn with respect to the origin of the enhanced magnetic
susceptibility of the soils in comparison with that of the loess [
275
]. A similar method
was applied and concurrent results were obtained in a series of samples from a section
in the north-eastern area of the Buenos Aires province [
276
].
A second example of magnetic separation concerns a study of fresh dolerite
samples from Berg en Dal, 90 km north of Paramaribo in Suriname, in which MS
was utilized as an additional characterization technique of the samples [
277
].
Apart from several doublets, the RT spectrum shows two weak sextets that can be
assigned to magnetite. In order to study more carefully the magnetic components
the sample has been subjected to a magnetic separation yielding a magnetic and a
non-magnetic fraction. The RT spectra of both fractions are depicted in Fig.
3.48
.
The spectrum of the magnetic fraction shows clearly the two sextets of mag-
netite and no other sextets were visible at first sight. In order to fit the spectrum
adequately in the complex central part of the spectrum, those hyperfine parameters
were used that were obtained from the fit of the spectrum of the non-magnetic
fraction, which is not disturbed by inner sextet lines. The ratio of the relative areas
for magnetite, S(Fe
2.5+
)/S(Fe
3+
), turned out to be 2.2 which is by far larger than 1.8
as expected for pure magnetite. Because oxidation or substitution lowers the
amount of Fe
2.5+
pairs and thus the relative area of the Fe
2.5+
sextet, there must be
another sextet component present, which overlaps to some extent with the latter.
This can hardly be another Fe
3+
oxide component because this low field would
correspond to a small particle morphology, which would lead to a rather bag-like
