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Fig. 3.37 Mössbauer
spectrum at room temperature
of synthetic leucophosphite
dimer (hereafter called Fe(2) sites) which further links by corner sharing to two
other octahedrons (Fe(1) sites). The edge-sharing octahedrons possess two OH -
groups in cis position along the shared edge. The two other octahedrons each have
a water molecule at the corner that is in cis position with respect to the shared
corner. The K ions and one water molecule per formula unit are located in
channels that run parallel to the [010] and [001] crystallographic axes.
The Mössbauer spectrum for a synthetic leucophosphite at RT is reproduced in
Fig. 3.37 [ 267 ]. It is adequately fitted with two ferric quadrupole doublets, which
are assigned to the Fe(1) and Fe(2) sites (see Table 3.19 ) on the basis that the Fe(1)
are expected to exhibit the largest DE Q value since the ferric species in these sites
experience the largest charge asymmetry for their closest co-ordination shell.
Within error limits the area ratio of the two doublets is 1:1, but the Fe(1) doublet
has a slightly broader linewidth.
3.7 Examples
3.7.1 Goethite-Hematite-Ferrihydrite Associations
Most weathered mineral or soil-related samples contain simultaneously goethite,
hematite and ferrihydrite. As a general rule, hematite occurs more frequently in
places below 40 latitude whereas ferrihydrite is mostly found above that latitude
[ 124 ]. In such composite samples, hematite exhibits nearly always a sextet with
asymmetric lines at RT and remains predominantly weakly ferromagnetic down to
80 K. On the other hand, the spectrum of goethite is strongly dependent on the
crystallinity and at RT can vary from a doublet for poorly crystallized goethite
over a largely collapsed spectrum to a sextet with strongly asymmetric lines for
relatively well-crystallized goethite. Due to the asymmetry of the absorption lines
in the latter case, the involved subspectrum was often fitted with a few sextets with
consecutively decreasing intensity. However, an adequate fit is only obtained if a
distribution of hyperfine fields is considered.
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