Chemistry Reference
In-Depth Information
because of the same amount of octahedral Fe
2.5+
in both cases, the formula for
x will not be altered.
However, caution should be taken in using the equation because part of the
deviation from the ideal ratio R = 1.8 might also be due to isomorphic substitution
for Fe by small quantities of other elements such as Al and especially Ti.
Titanomagnetite is the common name for the minerals with general formula
Fe
3
x
Ti
x
O
4
arising from the solid solution between magnetite and the ulvöspinel,
Fe
2
2
TiO
4
:
These Ti-magnetites are very common in igneous rocks such as basalts
and have been of particular interest in connection with magnetism of the earth. In
these magnetites Ti
4
þ
substitutes Fe on the octahedral sites creating more Fe
2
þ
.
Many models have been proposed for the cation distribution (e.g. Pearce et al.
[
115
] and references therein). In most of the models the tetrahedral sites are fully
occupied with Fe
3
þ
up to x = 0.2. This means that, apart from the Fe
2
þ
Fe3
þ
pairs giving Fe
2
:
5
þ
there is an excess of octahedral Fe
2
þ
:
This results in a two-
sextet magnetite-like spectrum with an additional inner shoulder on the Fe
2
:
5
þ
sextet belonging to a Fe
2
þ
sextet [
116
,
117
]. However, natural samples which are
chemically inhomogeneous and frequently non-stoichiometric show often more
complex Mössbauer spectra.
Natural magnetite may also occur with a small particle morphology yielding a
Mössbauer spectrum with asymmetrically shaped lines. Due to the increased
overlap of the lines of both sextets in that case, it becomes difficult to determine
the ratio R = S(Fe
2.5+
)/S(Fe
3+
) accurately.
At low temperatures, the spectrum of pure magnetite is very complex
(Fig.
3.14
a) and may be described by at least five subspectra [
118
]. This is due to
the 3d electron localization below the so-called Verwey transition at about 125 K
leading to discrete Fe
2+
and Fe
3+
spectral contributions of the B sites. However,
this transition temperature is lowered in the case of substitution or partial oxidation
[
119
-
121
]. This is illustrated in Fig.
3.14
b where oxidized magnetite (Fe
2.944
O
4
)
Fig. 3.14 Spectra below the Verwey transition: a spectrum of stoichiometric magnetite at 100 K
with visible Fe
2+
lines (indicated by arrows), and b spectrum of non-stoichiometric magnetite
Fe
2.944
O
4
at 100 K with the two typical Fe
3+
and Fe
2.5+
sextets
