Geoscience Reference
In-Depth Information
are oxidized in 24 hours when exposed to the atmosphere. They are
then transformed to lepidocrocite and goethite. Their disappearance is
accompanied by change in colour to yellows or greys.
In most soils, iron is found in the form of oxyhydroxides or, as was
mentioned earlier, hydrated oxides. In soils of dry zones, on the contrary,
the iron is in the form of true oxides.
In addition to these oxyhydroxides, soils have
a certain quantity of iron contained in the primary ferriferous
minerals;
exchangeable iron adsorbed on the clays;
structural iron of the clay minerals; except in particular cases,
this represents a small amount of iron;
iron complexed by organic matter;
dissolved iron.
Many soils contain 2-3 per cent iron in their horizons. But if the
parent material is devoid of this element, the soil can be anoxic without
exhibiting the least sign of reduction visible to the naked eye.
Figure 12.5 presents the possible transformations between the
different forms of iron present in soils.
Evolution of iron oxides in soils
Iron of mineral-organic
complexes
Sources of iron:
- ferriferous silicates of
the rocks,
- sulphides,
- haematites
- magnetites
Soil
solution
Iron of clay
minerals: structural
or exchangeable
Fougerite
Lepidocrocite
MAGHEMITE
HAEMATITE
Oxides and
hydroxide
Ferrihydrite
GOETHITE
Fig. 12.5
Forms of iron in soils. In bold, the forms found in soils with excess water; in
capitals, the forms found mostly in tropical soils (goethite forms part of both groups). The
transformations possible in the natural environment are shown by continuous lines if they
occur in solution and by broken lines if they occur in the solid state, according to Trolard
(1955) and G. Bourrié (pers. comm.). Poorly crystallized ferric gels are not shown; they are
the precursors of oxides and hydroxides.
