Chemistry Reference
In-Depth Information
Fig. 3.34 Mössbauer spectra
of non-oxidized vivianite at
room temperature
paired unit the two other O
2-
anions are in a trans position. The two octahedra are
identical and are henceforward referred to as the Fe(2) sites. There are twice as
many Fe(2) sites in the vivianite structure than there are Fe(1) sites.
MS for a single crystal of non-oxidized vivianite were reported by [
257
-
259
].
The spectrum at RT as presented in the latter work is reproduced in Fig.
3.34
and
concerns a species from Anloua, Cameroon. It was recorded under a geometry
whereby the incident c-ray was perpendicular to the crystallographic ac plane and
had therefore to be analyzed by a superposition of two asymmetric quadrupole
doublets arising from Fe
2+
at the Fe(1) and Fe(2) sites respectively. The asym-
metry is due to texture effects as a result of the EFG's principal axis being non-
randomly oriented with respect to the incident c-ray direction. Such a situation
leads to a quadrupole doublet of which the two composing absorption lines have
unequal spectral areas. Fitting the spectrum of Fig.
3.34
yielded a line-area
asymmetry of 0.64 for both doublets, which is consistent with the conclusion of
Forsyth (1970) that for both Fe sites the EFG's principal axis is lying in the ac
plane. The hyperfine parameters are given in Table
3.19
. Both Gonser et al. [
257
]
and De Grave [
259
] observed a significant deviation of the area ratio of the two
subspectra from the ideal value of 1:2 and ascribed this feature to the pronounced
effective thickness of the single-crystal absorber.
Partly oxidized vivianites, commonly as powders, have been studied by
Mössbauer spectroscopy by De Grave et al. [
260
], McCammon and Burns [
261
],
and Dormann et al. [
262
]. Various oxidation degrees q have been considered. Two
room temperature spectra are reproduced in Fig.
3.35
referring to q = 0.31 and
q = 0.14, respectively, as derived from the analyses of the spectra using two
ferrous and two ferric doublets. The assignment of the various doublet components
to the Fe(1) and Fe(2) sites is specified in Table
3.19
, together with their respective
d
Fe
and D values. It was found that the quadrupole splitting for the Fe
2+
and the
Fe
3+
cations at the Fe(2) sites is not affected by the oxidation degree q. On the
other hand, for the Fe(1) sites both the ferrous and ferric D decrease with
increasing q. It was further noticed that the oxidation of Fe
2+
to Fe
3+
preferably
takes place at the Fe(1) octahedra so that the fraction of total iron as Fe
3+
at the
Fe(2) sites does not exceed 0.05 in the range q B 0.32.
