Chemistry Reference
In-Depth Information
expected. Therefore, for characterization purposes, it is mostly recommended to
measure at different temperatures. For practical reasons, Mössbauer spectra are
usually collected at two specific temperatures, i.e. room temperature (RT) and liquid
nitrogen temperature (*80 K) from which already valuable results can be obtained.
The more expensive low-temperature measurements down to 4 K or the time-
consuming detailed temperature scanning of the spectra are only necessary in those
cases where important additional information can be expected.
Secondly, the magnetic sextet spectra of these materials at lower temperature
exhibit usually asymmetrically broadened lines. Several mechanisms have been
suggested to explain this behavior, but that discussion would lead us far beyond
the scope of this paper. Fortunately, for characterization purposes, such spectra can
be adequately fitted by a few sextets or even more accurately by considering a
distribution of hyperfine fields [
6
-
8
]. In this case the spectra are no longer fitted
with a single sextet of Lorentzian lines but by a set of such sextets. A versatile
distribution fitting procedure which uses several tens of elemental sextets with the
necessary smoothing constraint, has been developed by Hesse and Rübartch [
9
]
and was further improved by Le Caër and Dubois [
10
] or extended by Wivel and
Mørup [
11
] and has been combined in one analyzing method [
12
]. In this way a
much more accurate description can be obtained of partly overlapping sextets,
such as those occurring for goethite-hematite associations as for instance dem-
onstrated by the analysis of a Tunisian soil profile [
8
].
For characterization purposes, MS studies on natural soil samples usually rely
on the results of a variety of systematic studies on pure natural or synthetic
compounds. Such systematic studies provide a description of the spectral behavior
of the different Fe-bearing oxides and hydroxides and a determination of the
hyperfine parameters in relation to morphological and chemical features. Many
reviews on this subject are available in literature [
13
-
19
]. In what follows a brief
survey will be given of the spectral features of the various iron oxides and
(oxy)hydroxides in relation to their identification and characterization in natural
soil samples.
3.3.1 Goethite (a-FOOH)
Goethite is by far the most encountered Fe-bearing compound in soils, sediments
and clays, and has therefore been intensively studied in the past forty years. In its
most ideal mineral form it is antiferromagnetic with a Néel temperature
T
N
= 400 K [
20
,
21
]. The magnetic hyperfine field amounts to 38.1 T at RT,
50.0 T at 80 K and saturates to 50.7 T at 4 K. However, the well-crystallized form
is of rare occurrence and has only been found in particular sites such as the Harz
Mountains (Germany) and Lostwithiel (Cornwall, UK) due to the presence of the
required extreme hydrothermal formation conditions.
In soils goethite is usually obtained as a weathering product of Fe
2+
silicates and to
a lesser extent of sulfides, carbonates, oxides, etc. This results in a poorly crystalline
