Geoscience Reference
In-Depth Information
addition to iron that is their principal constituent, these two forms of
accumulation include all the elements liable to reduction (Aide 2005).
We have mentioned this earlier.
For a concretion to grow, it is necessary that the groundmass leave
enough room for the purpose. Things are not simple… With time,
nodules and concretions can be concentrated by loss of other constituents,
till they sometimes become the major part of sand and gravel fractions
in the soil (tropical environment). They can also be transported and it
is necessary to examine their outlines: they have been formed
in situ
if
the outlines are diffuse and are, possibly, allochthonous if otherwise.
In the first analysis, the fundamental structure of oxides and
oxyhydroxides is rather simple (Fig. 12.4). The iron atoms are enclosed
in octahedral cavities defined by the close packing of two hexagonal
layers (contiguous spheres). The latter are composed of two sheets of
oxygen ions (for the oxides) and of a combination of a sheet of oxygen
ions and a sheet of hydroxyl ions (for the oxyhydroxides).
Iron oxyhydroxide
vertical section
Mineralogy and classification of iron oxides
Hexagonal close packing
seen from above
(the sheet that sits above the
iron atoms has been removed)
Iron oxide
vertical section
OH
-
Oxygen
Iron atom
Fig. 12.4
Structure of oxides and oxyhydroxides of iron.
But in actual fact, the stacking of successive atom layers takes place
in a complex and variable way from one oxide to another so that most
of them are considered to have a three-dimensional structure and not
layered. Also, there is no agreement in the way their chemical formulae
are written. Those given here (Table 12.3) are only examples!
Partial substitution of iron by aluminium is common. Also, oxidation
(Fe
2+
Æ Fe
3+
) results in an excess of positive charges often compensated
for by adsorption of anions. In sum, as listed in Table 12.3, there are
many subtypes of oxides…
Green rusts are mixed hydroxides of Fe
2+
and Fe
3+
. In them, the ratio
x
=
oxidized iron/total iron
ranges from about 1/3 to 2/3 (Feder 2001; Feder
et al
. 2005). It depends on the oxidation status of the medium in which
