Chemistry Reference
In-Depth Information
that Al replaces Fe in the structure to a small extent. A reduction of the maximum-
probability hyperfine field B
m
with about 0.06 T per at %Al was observed.
3.3.4 Feroxyhite (d-FeOOH or d
0
-FeOOH)
d-FeOOH, which is isostructural with Fe(OH)
2,
is the fourth polymorph of iron
oxyhydroxide and had initially no mineralogical name because it was not found in
nature. On the other hand, d
0
-FeOOH, which can be considered as a poorly ordered
variant of d-FeOOH with a somewhat different Fe
3+
arrangement, has been
identified as a rare mineral known as feroxyhite (originally written 'feroxyhyte' by
Chukhrov et al. [
51
]). Showing a typical Fe
3+
doublet at RT with d
Fe
= 0.37 mm/s
and D = 0.69 mm/s this mineral cannot be distinguished from other oxyhydrox-
ides. This is particularly true in soils where biogenic d
0
-FeOOH is often associated
with ferrihydrite [
52
], the latter possessing a similar doublet. Feroxyhite orders
magnetically only at very low temperatures and shows at 4 K a broad-lined sextet
[
53
,
54
], which in fact is composed of two overlapping sextets (Table
3.3
).
Nowadays, both the delta-oxyhydroxides are being called feroxyhite, because
d
0
-FeOOH is considered to be nothing else than the poorly crystallized (super-
paramagnetic at RT) variant of d-FeOOH, the latter being magnetic at RT.
3.3.5 Ferrihydrite
Ferrihydrite is an iron oxyhydroxide with a high surface water content having an
approximate composition Fe
5
O
12
H
9
written as Fe
5
O
3
(OH)
9
or Fe
5
HO
8
.4H
2
O. The
most important feature of this mineral is its poor crystallinity, with particle sizes in
the range 2-7 nm. XRD patterns consist of 2-6 broad peaks which are not always
discernible due to the presence of broad lines of poorly crystalline goethite that
often accompanies ferrihydrite in soils. Different models have been proposed for
the structure [
55
-
62
] and the subject is still under discussion.
Both poor crystallinity and structural disorder lead to a very broad temperature
range in which the magnetic order develops and the nature of magnetic order is
still in doubt. For a so-called 6-XRD-line ferrihydrite sample the magnetic tran-
sition spans a temperature region from 35 to 110 K [
63
]. Ferrihydrites with less
than 6 XRD lines are still paramagnetic at 80 K. It is now believed that these low
magnetic transition temperatures result from a kind of superparamagnetic behavior
from interacting particles [
64
], whereas the real antiferromagnetic-paramagnetic
transition temperature of ferrihydrite is estimated to be of the order of 500 K [
65
].
It is obvious that the high degree of structural disorder in such a poorly crys-
talline system causes broad distributions of hyperfine parameters resulting in
broad-lined doublets and sextets. At RT a broad doublet is obtained (Fig.
3.7
)with
average D in the range 0.7-0.9 mm/s. Though the spectra reflect in fact a broad
