Chemistry Reference
In-Depth Information
procedure is not always straightforward leading often after trial and error to
uncertain results. This is particularly true for complex spectra of minerals and soil
materials exhibiting in many cases strongly overlapping absorption lines or
showing distributed hyperfine interactions. It is therefore imperative that the
Mössbauer spectroscopist uses the appropriate fitting procedures, has a good
insight in the various spectra that can occur, and relies if necessary on the results of
complementary techniques. This tutorial aims to introduce the reader into those
various aspects of the application of MS in earth sciences. Together with a
description of the spectral behavior expected for the most relevant minerals, a
number of examples shall be given, which illustrates the analytical power of MS.
3.2 Mössbauer Spectroscopy Applied to Earth Sciences
As already mentioned in the introduction, MS leads to applications in earth sciences
and particularly in mineralogy, which are very important with respect to qualitative
and quantitative analysis of the samples and the determination of the oxidation state
and coordination of iron in the involved minerals. Moreover, this technique
additionally provides in some cases a crude insight in the morphological and
chemical features of the minerals.
3.2.1 Qualitative and Quantitative Analyzing Power of MS
For qualitative analyses of rocks, soils, sediments, ores, etc., MS consists of the
recognition of typical ''fingerprint'' spectra of the various iron-bearing species
present in the sample. The standard fingerprint spectra are usually obtained from
pure natural or synthetic samples. The appearance of the typical spectra, defined
by their specific hyperfine parameters, enables in many cases to assign immedi-
ately the unknown components in the sample. The doublet spectra of non-magnetic
(paramagnetic or superparamagnetic) materials are defined by two hyperfine
parameters, i.e. the isomer shift d
Fe
and the quadrupole splitting D. The sextet
spectra of magnetically ordered materials are defined by three hyperfine parame-
ters, i.e. the isomer shift d
Fe
, the quadrupole shift 2e and the magnetic hyperfine
field B. Such spectra are particularly obtained in the case of oxides at RT and of
(oxy)hydroxides at lower temperatures. The hyperfine field is then a welcome
extra parameter for qualitative phase characterization. The magnetic transition
temperature at which the sextet is expected to change into doublet is often also of
prominent value.
However, one may not overestimate the direct qualitative analyzing power of
Mössbauer spectroscopy. Indeed, many situations occur in which the spectra do
not give such a decisive qualitative information. This is particularly true in the
cases where merely paramagnetic doublet spectra are obtained. For instance, many
