Chemistry Reference
In-Depth Information
Titanomaghemites originating from oxidation of lithogenic titanomagnetite
show similar features in their Mössbauer spectra, i.e. asymmetrically broadened
sextet lines accompanied by a central doublet [
127
,
128
]. These maghemites seem
to have vacancies on both sites [
127
,
129
].
3.3.10 Green Rust Related Minerals
Green rusts are mixed-valence iron hydroxysalts that have been extensively
studied for understanding the corrosion behavior of iron based materials and steels.
They are obtained by oxidizing metallic iron or Fe(OH)
2
and are intermediate
compounds comprising ferrous and ferric ions. Several types have been synthe-
sized incorporating various anions such as CO
3
2-
,Cl
-
or SO
4
2-
and were studied
by MS [
130
-
133
]. They all belong to the double layered hydroxide family (HDL)
where in the most stable one, i.e. with CO
3
2-
, positive charged brucite-like layers
Fe
2
4
Fe
3
2
O
ð
12
2
þ
alternate with (CO
3
.3H
2
O)
2-
interlayers, possessing a ferric
molar fraction x = Fe
3+
/Fe
total
= 1/3. Green rusts dissolve usually by oxidation
whereas a ferric oxyhydroxide precipitates such as ferrihydrite and/or goethite.
However, rapid oxidation under alkaline conditions (in situ deprotonation) keeps
the green rust structure essentially unchanged, leading finally to so-called ''ferric
green rust'' which has in fact an orange color [
134
].
A green rust related mineral in soils was first discovered in a so-called gleysol
at Fougères (Brittany, France) and has therefore been named fougèrite [
135
]. The
Mössbauer spectrum, usually measured at lower temperatures (78 K) to avoid
oxidation, is typical for green rust, consisting of one or two Fe
2+
doublets with
d & 1.25 mm/s and with D & 2.60-2.90 mm/s and a Fe
3+
doublet with
d & 0.48 mm/s and with D & 0.50-0.70 mm/s. The spectrum from the first
measured sample is shown in Fig.
3.17
a. For different fougèrite samples, the ferric
molar fraction x turned out to be between 1/3 and 2/3 [
136
]. A study in depth with
a miniaturized Mössbauer spectrometer showed that fougèrite is more ferric in the
upper horizons and variations are consistent with the fluctuations in the water table
and thus with aerobic and anaerobic conditions [
137
]. Those in situ experiments
yield
spectra
and
thus
hyperfine
values
close
to
RT
(283 K).
They
are
d & 1.05-1.08 mm/s
and
D & 2.7 mm/s,
d & 0.75-1.00 mm/s
and
Fe
2+
,
D & 2.2-2.7 mm/s
both
for
and
d & 0.21-0.30
mm/s
and
D &
0.6-0.8 mm/s for Fe
3+
.
In contrast to the permanently waterlogged soils of a continental aquifer with
green rust related minerals with values of x within the (1/3, 2/3) range as firstly
extracted in Fougères, similar minerals being still more ferric with x values within
the (2/3, 1) range occurs in gley from the schorre of a maritime marsh, as was
firstly extracted in Trébeurden (Brittany, France) [
138
]. The spectrum of a typical
Trébeurden sample with x & 0.75 measured at 78 K and fitted with 3 doublets is
shown in Fig.
3.17
c. Similar spectra were recorded from samples extracted from
