Chemistry Reference
In-Depth Information
concentration or a measure for the crystallinity can in principle be determined if
the other parameter is known from complementary experiments. Although it has
been shown that these equations could be successfully applied to certain series of
well-characterized natural goethite samples [
22
,
24
], they completely failed for
many other natural and synthetic sample series [
28
]. Friedl and Schwertmann [
29
]
investigated 33 natural goethite samples from different origins, which according to
their formation conditions could be divided into 24 samples from tropical and
subtropical soils and 9 samples from lake ores. Comparison of the observed
hyperfine fields at 4 K with the respective values calculated from the correlation
equations resulted in significant deviations, with a somewhat different general
behavior for the goethites occurring in the tropical soils than for those occurring in
the lake ores. This was corroborated by the linear regressions separately obtained
for the two kinds of samples, resulting in different coefficients for both the Al
content C and the crystallinity as reflected by the inverse MCD. From a thorough
study on well-defined samples of Al goethite, it has been shown that other
structural parameters, such as water content, excess hydroxyl DOH and structural
defects, all having some influence on the lattice parameters [
30
,
31
], play also a
substantial role in the magnitude of the magnetic and electrical hyperfine
parameters [
32
-
36
]. These additional structural parameters are mainly determined
by goethite formation factors such as crystallization rate, temperature, OH con-
centration, etc., and are to some extent related to each other [
30
].
The situation becomes even more complicated if other elements are involved in
the goethite formation. In view of the similar structure of a-MnOOH (groutite),
Mn has also been found to substitute for Fe in goethite to a large extent [
37
,
38
].
Because Mn
3+
is a paramagnetic ion the hyperfine field is less reduced than in the
case of Al [
38
]. Other elements such as Si and P show the tendency to adsorb to the
goethite crystallites rather than to substitute for iron in the structure [
39
-
41
]. This
surface ''poisoning'' not only opposes the crystal growth during the formation but
may also reduce B due to surface effects [
28
].
It can be concluded that MS is a very suitable tool with respect to the qualitative
and to some extent quantitative analysis of goethite in soils and sediments.
However, it is clear that MS is still far from being a ''magic'' analytical technique
that provides an in-depth knowledge of the morphological properties of goethite.
For the moment, if one compares goethites from a same soil profile, MS can yield
some crude indications with respect to crystallinity if equal isomorphous substi-
tutions are expected. Also, other techniques are not so powerful in that respect
because natural soil samples mostly contain goethite in more or less close asso-
ciation with other mineral species, likewise hampering to obtain accurate results.
3.3.2 Akaganéite (b-FeOOH)
Akaganéite as the second polymorph of iron oxyhydroxide is by far less abundant in
nature in comparison with goethite. In fact, akaganéite requires a small amount of
