Chemistry Reference
In-Depth Information
Fig. 3.38 Mössbauer spectrum at RT and at 80 K of a hematite-rich sample containing hematite
and goethite
A first example concerns a sample from a weathered boulder in a granite
outcrop in Sungai Ringit (Malaysia), which is relatively rich in hematite. The
spectra at RT and at 80 K are displayed in Fig.
3.38
.
The RT spectrum is composed of a sextet with asymmetric lines having a
relatively high hyperfine field and a negative quadrupole shift, which is typical for
hematite. The hyperfine parameters are collected in Table
3.19
. The center of the
spectrum shows a doublet that corresponds to Fe
3+
, but can in principle not directly
be assigned to a particular mineral. Further, close inspection learns that the
spectrum exhibits a slightly curved bag-like shape, which has been accounted for
by including in the fit a B-distributed sextet component in the low hyperfine-field
region (5-35 T). Although the latter adjustment is not always theoretically correct
because of the comparable strength of the quadrupole and the magnetic interaction
[
268
], it adequately describes the overall spectral shape. This collapsed spectrum
can usually be considered as originating from that fraction of the goethite particles
that exhibit the highest degree of crystallinity.
At 80 K the doublet has almost completely disappeared in favor of a sextet with
strongly asymmetrical line shape. The hyperfine parameters of this sextet point to
goethite. This change is clearly the effect of super paramagnetism in which the
sextet increases at the expense of the doublet by lowering the temperature as a
result of a distribution of particle sizes. Because the hyperfine fields in the derived
goethite field distribution are tending to low values, it can be expected that the
remaining doublet at 80 K is due to that part of the goethite with the poorest
crystallinity. The moderate quadruple splitting of that doublet (0.57 mm/s) points
indeed in that direction because ferrihydrite, the presence of which cannot be
excluded a priori, has usually D C 0.6 mm/s. The sextet lines of hematite at 80 K
are sharp and only slightly asymmetric. However, the spectrum itself is asym-
metric in the sense that the first line is significantly deeper than the sixth one,
whereas the it is opposite is noticed for lines 2 and 4. This feature is typical for the
presence of the two magnetic hematite phases (cfr. Fig.
3.9
). Introducing a weak
AF hematite component indeed improved the fit significantly.
