Chemistry Reference
In-Depth Information
Fig. 3.17 Mössbauer spectrum of a Fougèrite sample at 78 K (a) compared with the spectrum of
a synthetic oxidized green rust sample with the same Fe
3+
molar fraction x (b) Mössbauer
spectrum of a green rust related sample from Trébeurden (c) and the spectrum of a similar sample
from the Mont Saint Michel (d) (adapted from Ref. [
307
])
the bay of Mont Saint Michel (Normandy, France) [
138
] as can be seen from the
example shown in Fig.
3.17
d.
Structural studies on synthetic samples [
133
] suggest that long ranger order
appears for x = 1/3, x = 2/3 and x = 1 which is confirmed by magnetic mea-
surements showing a ferrimagnetic behavior for x = 1/3, x = 2/3 with Néel
temperatures of 5 and 20 K respectively and being ferromagnetic for x = 1 with a
Curie temperature around 80 K [
139
]. From these findings together with the
appearance of typical doublets in the Mössbauer spectra Génin et al. [
138
] suggest
that the intermediate samples consist in fact of topotaxically mixed domains of the
ordered definite compounds. Therefore the natural green rust related minerals must
be considered as a mixture of three basic minerals:
Fe
2
þ
4
Fe
3
2
O
ð
12
CO
3
:
3H
2
O
ð
x
¼
1
=
3
Þ
;
Fe
2
2
Fe
3
4
O
ð
10
O
2
CO3
:
3H
2
O
ð
x
¼
2
=
3
Þ
and
Fe
3
þ
6
O
ð
8
O
4
CO
3
:
3H
2
O
ð
x
¼
1
Þ:
They suggest to redefine the first one as fougérite, whereas in the case of x = 2/3
and x = 1, the new mineral names ''trébeurdenite'' and ''mössbauerite'' are
respectively proposed [
138
].
