Chemistry Reference
In-Depth Information
with its high average value is already quite indicative for the presence of ferrihydrite,
although, clear-cut evidence can only be obtained from the typical sextet spectrum
obtained at very low temperatures (\80 K). On the other hand, XRD may provide
more direct evidence through the characteristic broad-lines pattern which will be
discernible if ferrihydrite is present in sufficient amount in the sample. Unfortunately,
in many soils ferrihydrite occurs only as a minor fraction and no technique is able to
recognize it directly among the other interfering components such as poorly
crystalline goethite. In goethite-ferrihydrite associations, the doublet fraction which
persists down to 80 K may account for ferrihydrite as well as for very poorly crys-
talline and/or highly substituted goethite. However, if the 80 K spectrum exhibit a
goethite sextet with rather narrow lines, the remaining doublet can most likely be
attributed to ferrihydrite because goethite with two discrete and drastically different
crystallinities or substitutions are not commonly expected in the same sample. At
very low temperatures (4 K) the hyperfine fields of goethite and ferrihydrite are
comparable. Hence, these components cannot be separated merely by their difference
in quadrupole shift. Only the broader lines of the ferrihydrite sextet might be
indicative.
Another method which is helpful in the characterization of ferrihydrite is
selective dissolution in acid ammonium oxalate [
74
] followed by a so-called
differential X-ray diffraction (DXRD) [
75
], which consists of subtracting the
pattern of a treated sample from that of an untreated one, thus isolating and
enhancing the typical ferrihydrite diffraction pattern. In MS, this dissolution
applied to natural soil samples usually results in a decrease of the doublet at RT
and 80 K, and a narrowing of the sextet lines at lower temperatures. However, it
has been demonstrated that ammonium oxalate also dissolves organically bound
Fe, Fe from magnetite and from poorly crystalline lepidocrocite [
76
-
79
], and also
attacks vivianite, siderite [
66
] and even poorly crystalline goethite [
80
]. So, MS
applied in combination with such a treatment is not always decisive unless
information about the presence of the above interfering compounds is obtained
from other techniques.
3.3.6 Hematite (a-Fe
2
O
3
)
Hematite is the most abundant iron oxide in soils and sediments. In comparison to
other iron oxides and hydroxides, hematite (a-Fe
2
O
3
) exhibits a non-common
magnetic behavior. In addition to the normal magnetic-paramagnetic transition at
T
N
= 955 K, pure and well-crystallized hematite transforms at about 265 K from
a low-temperature antiferromagnetic (AF) to a high-temperature weakly ferro-
magnetic (WF) state, known as the Morin transition. This transition consists in fact
of a 90 spin reorientation from an antiferromagnetic spin configuration in the
c direction into the basal plane (perpendicular to the c-axis) in which the spins are
slightly canted resulting in a weak ferromagnetism (Fig.
3.9
). This magnetic
