Chemistry Reference
In-Depth Information
associated oxides from lattice iron. At 4 K kaolinite still shows a doublet with
d
Fe
= 0.48 mm/s and D = 0.52 mm/s. In some cases a small percentage (\10 at %)
of Fe
2+
can be detected with d
Fe
= 1.1-1.3 mm/s and D = 2.4-2.9 mm/s at RT.
The trioctahedral analogue of kaolinite is the group of serpentine minerals
based on antigorite Mg
3
Si
2
O
5
(OH)
4
with polymorphs chrystolite and lizardite.
Small amounts of iron in those serpentines yield hyperfine parameters of
d
Fe
= 1.14 mm/s, D = 2.68-2.76 mm/s for Fe
2+
and d
Fe
= 0.37-0.42 mm/s, and
D = 0.65-0.85 mm/s for Fe
3+
[
235
,
236
]. The iron-rich serpentine is cronstedtite
with formula Fe
2
2
Fe
3
þ
SiFe
3
þ
O
5
O
ð
4
:
The RT spectrum of cronstedtite is
rather complicated due to the electron hopping on the octahedral sites [
237
].
Tetrahedral and octahedral Fe
3+
are best distinguished in the magnetically split
spectrum at 4 K for which hyperfine fields of 40.6 and 46.7 T are found respec-
tively. The ferrous end member of the serpentines is greenalite with formula
Fe
2
3
Si
2
O
5
O
ð
4
:
Its ferrous doublet is well defined with d
Fe
= 1.15 mm/s,
D = 2.75 mm/s [
238
]. For the intermediate members, the berthierines (not to be
confused with chamosite, which is a 2:1 silicate), the quadrupole splitting D falls
within the range 2.62-2.68 mm/s [
239
].
3.5.6.2 2:1 Layer Silicates
The basic 2:1 dioctahedral silicate is pyrophyllite with formula Al
2
Si
4
O
10
(OH)
2
.In
muscovite, KAl
2
(Si
3
Al)O
10
(OH)
2
, interlayer K compensates electrically for the Al
replacement for Si. Illites have a more variable composition with a general formula
(H
3
0,K)
x
Al
2
(Si
4-x
Al
x
)O
10
(OH)
2
. Nontronite is the iron-rich dioctahedral silicate
with general formula M
þ
Fe
3
2
Si
4
x
Al
ð Þ
O
10
O
ð
2
:
Although, Mössbauer spectra of the various silicates turned out so far to be
reasonably analyzable in a relatively unambiguous way, this is surely not the case
for 2:1 layer silicates. In all these silicates Al
3+
is partly replaced by Fe
3+
resulting
in one or two ferric doublets, which can be interpreted by the Fe-for-Al substi-
tution on trans and cis sites. This is the basic spectrum for all the 2:1 dioctahedral
silicates. The quadrupole splitting of the inner doublet (cis) is found to increase in
the sequence ferripyrophyllite, nontronite, glauconite, montmorillonite, illite,
muscovite (Table
3.18
), which in some sense corresponds to increasing distortion
of the octahedral [
240
]. The presence of small amounts of Fe
2+
possibly introduces
two additional doublets similarly attributed to cis and trans arrangements. A third
Fe
3+
doublet with very small quadrupole splitting can be assigned to iron in the
tetrahedral sites. (see Johnston and Cardile [
241
] and references therein). How-
ever, in general there is still a lack of common approach for the analysis of the
spectra. This is clearly illustrated in the published Mössbauer results for illites and
nontronites [
242
]. Hence, only a fit with two broad-lined doublets or by quadru-
pole splitting distributions, one for each Fe
2+
and Fe
3+
, can provide some
straightforward insight in the iron behavior in the structure of these minerals [
243
].
The iron-rich variants of illite are glauconite and celadonite. In glauconite there is
