Chemistry Reference
In-Depth Information
are joined by oxygen sharing into chains or sheets. Apart from the silicon, they all
further contain monovalent, divalent and trivalent metal ions of which a substantial
amount of Fe
2+
and Fe
3+
can be present. Therefore, Mössbauer studies in silicate
mineralogy have already been started in the early days of the application of the
Mössbauer effect. The power of the Mössbauer technique resides in the ability to
determine qualitatively and quantitatively the iron in the different lattice sites with
their specific valence and the distortion of their environments. This has been well
demonstrated in the numerous publications of G.M. Bancroft, Roger.G. Burns, M.
Darby Dyar, Stefan S. Haffner, Georg Amthauer, Victor A. Drits… and their
respective co-workers. Dyar et al. [
150
] have composed a comprehensive list of
the hyperfine parameters of a large variety of silicates.
Commonly, silicates are only magnetic at very low temperatures and hence
their MS recorded at RT and at 80 K generally consist of doublets. Moreover, the
divalent and trivalent iron cations are in the high spin state yielding comparable
hyperfine parameters for a given Fe
2
þ
or Fe
3
þ
site in various minerals. This means
that distinct doublets due to a particular valence, if present in a given spectrum,
may strongly overlap and are sometimes difficult to resolve. Furthermore,
absorbers from silicates that are crystallized in chains or sheets may be subject to
texture effects resulting in a different intensity of the two lines of a given doublet
component. All this makes MS not so powerful as far as direct identification of
silicates is concerned and one has to rely on the results of other techniques such as
X-ray diffraction. Nevertheless, a better resolution for the Fe
2+
doublets in par-
ticular can often be obtained from measurements at several temperatures because
of the divergent variation of the quadrupole splitting with temperature for the
different ferrous sites. Further, asymmetry in the doublets can be avoided by a
suitable absorber preparation eliminating texture effects to a large extent. If nec-
essary, measurements under the so-called ''magic angle'', i.e. with the absorber at
54 degrees with respect to the direction of the c-ray, may also be helpful in that
respect.
From the spectral data of silicates some general rules can be put forward:
• the Fe
2+
and Fe
3+
oxidation states are usually easily distinguished
• the isomer shift for Fe
2+
ions in silicates depends both on coordination number
and symmetry in the following order: d (square planar)
d (tetrahedral)
d
(octahedral)
d (dodecahedral); d shows a linear relation with bond distance
and bond strength
• the quadrupole splitting of octahedral Fe
2+
is very sensitive to site symmetry and
generally decreases with increasing distortion, due to the lattice contribution
being opposite to the dominant valence contribution.
According to the way of stacking the SiO
4
tetrahedra, the silicates are usually
divided into different classes (Fig.
3.22
): nesosilicates (single tetrahedra), sorosi-
licates (double tetrahedra), cyclosilicates (tetrahedra joined to rings), inosilicates
(tetrahedra joined to single or double chains), phyllosilicates (sheets of tetrahedra)
and tektosilicates (three dimensional stacking of tetrahedra). This classification
