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still shows the typical Fe
2.5+
and Fe
3+
sextets at 100 K which is below the normal
Verwey transition for stoichiometric magnetite [
35
].
3.3.9 Maghemite
Maghemite, which is a fully oxidized form of magnetite, has the structural formula
Fe
3
þ
A
½
Fe
3
þ
5
=
3
h
1
=
3
B
O
4
where h represents again the vacancies on the octahedral
sites. The corresponding Mössbauer spectrum consists of a somewhat broad-lined
Fe
3+
sextet, which is in fact composed of two non-resolved sextets from Fe
3+
in
tetrahedral and octahedral sites, respectively (Fig.
3.15
a). Only by using an
external field, the hyperfine field and the isomer shift of both sextets could be
accurately determined [
125
] (see
Sect. 3.6.4
). Hyperfine parameter values are
given in Table
3.7
.
Maghemite is commonly formed from oxidation of fine-course lithogenic
magnetite, although its abundance in tropical and subtropical regions can also be
explained by the conversion of for instance goethite through fires under reducing
conditions. Also fine-particle magnetite produced by bacteria may lead to
maghemite after spontaneous oxidation. In most of those cases, maghemite pos-
sesses a rather small-particle morphology leading to a superparamagnetic beha-
vior. Maghemite can then appear in the Mössbauer spectrum as a doublet at RT.
From the effective anisotropy constant of about 10 Jm
-3
[
123
] a superparamag-
netic doublet is expected at RT for particle sizes smaller than about 5 nm. This
Fig. 3.15 Mössbauer spectra of maghemite: well-crystallized maghemite at RT (a) and at 8 K
(b) fitted with A-site and B-site sextets according to parameters derived from external-field
spectra; poorly crystallized maghemite at RT (c) and at 80 K (d) (after da Costa et al. [
122
])