Geoscience Reference
In-Depth Information
iron is omnipresent in soils and often in appreciable quantity
(3% on average); the ferrous iron content in soils as well as in
the water is closely related to the redox potential (Vizier 1970,
1974),
change in colour because of oxidation or reduction of iron is
rarely long-lasting; some soil samples, after being drawn from
the groundwater, are oxidized in air within a few minutes;
others undergo spectacular changes in colour in a fortnight
within the soil itself, from an intense blue to bright orange (Feder
et al.
2005),
the properties of the soil solutions (pH, redox potential) often
place them at the boundary between Fe
2+
and Fe
3+
. Thus, the iron
reacts to slight changes in the oxygen content of the medium;
but this is not so for highly calcareous soils, in which the Fe
3+
is stable even at low redox potentials (see Fig. 12.1).
The other redox-sensitive elements also should be considered: V,
Cr, As, Se, Mn, Ce and Co. Manganese particularly has a behaviour
similar to that of iron. But all these elements are not abundant and are
not morphological indicators.
Fig. 12.3
Classic structure of redox mottles. a. bleached zone (reduction of iron, its
dissolution and partial evacuation); b. intermediate zones enriched by movement and
precipitation of iron-containing solutions; c. brown starting material. Here the size of mottles
is exceptional (see knife); it is generally limited to a few mm. Massif Central, France.
Photo
:
author.
Nodules
are often seen in soils affected by excess water. They do
not have special internal structure. On the contrary,
concretions
are
organized in concentric layers as electron-microscopic studies show. In
Formation of nodules and concretions
