Chemistry Reference
In-Depth Information
3.2.2 Determination of the Oxidation State
and Coordination of Iron
The determination of the valence states of iron in minerals is of extreme impor-
tance in geology. In contrast to most of the other abundant elements, iron has in its
high-spin state predominantly two valence states which can easily transform from
one into another through oxidation or reduction. In this way the valence state may
be indicative for the geological history of the minerals (weathering, pressure and
temperature changes, …) and even the color can be associated to the valence states
or to transitions between them. For the determination of the oxidation states of
iron, MS is commonly used because most techniques are not able to distinguish
between Fe 2+ and Fe 3+ , and chemical analyses often result in unreliable results due
to oxidizing or reducing side effects. Among the hyperfine parameters, the isomer
shift is very sensitive to the valence and enables to discern readily the various
valence states of iron in minerals. Fe 3+ usually shows a relatively small isomer
shift d Fe in the range 0.3-0.6 mm/s whereas Fe 2+
covers the range 0.7-1.2 mm/s
(Fig. 3.1 ).
The quadrupole splitting, on the other hand, is generally large for divalent iron,
but depends also strongly on the coordination (Fig. 3.2 ). So, the combination of
both the isomer shift and quadrupole splitting values can give some idea about the
coordination of iron. Moreover, D in the case of Fe 3+ is merely determined by the
lattice contribution, and therefore the quadrupole splitting is also a good measure
for the local distortions in the lattice.
An important application in that respect is distinguishing cis and trans con-
figurations of an octahedral O 4 (OH) 2 (or (OH) 4 O 2 )Fe 3+ coordination, which quite
often occurs in mineralogical systems (Fig. 3.3 ). Simple point-charge calculations
show that the quadrupole splitting should follow the relation D trans = 2D cis .
Although the ratio is in practice never exactly 2, due to other effects such as the
influences of more distant charges, the measurement of the quadrupole splitting
enables the direct determination of those two types of isomers. In the case of Fe 2+ ,
the lattice contribution to the electric field gradient is usually opposite to the large
valence contribution of the iron cation itself yielding D trans \ D cis .
In the case of magnetic spectra, the magnetic hyperfine field B is generally also
a direct indication of the oxidation state in addition to the isomer shift. Far below
the magnetic transition temperature the hyperfine field of Fe 3+ amounts to 45-55 T
Fig. 3.1 Isomer shift (d Fe )
values at RT versus
coordination number for
low-spin (II, III) and
high-spin (2+, 3+) Fe in
compounds and minerals
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