Geoscience Reference
In-Depth Information
iron becomes easier during the winter period of saturation with water.
Each time this reduction happens, even occasionally and fortuitously,
haematite becomes unstable. Part of it is solubilized and exported, prior
to goethite (Jeanroy
et al
. 1991). Another part of the iron of haematite,
also solubilized, remains in place; during the following dry period it
precipitates in the form of large concretions and plays no further role in
the physical or chemical evolution of the soil. These two phenomena are
thus irreversible since the iron is totally withdrawn from the reaction
medium. Analysis of the ferruginous concretions of soils of Italy shows
that they contain haematite as well as oxides of manganese. As Mn
becomes more abundant the concretions become darker (Boero and
Schwertmann 1987). The black nodules will correspond to the late phase
of derubefaction.
The clay minerals being gradually acidified, structure collapses and
the soil shrinks, leading to longer and longer periods of saturation. The
soil 'auto-asphyxiates' (Bornand 1978). Goethite becomes more dominant
the more it also originates from the residual haematite by hydration.
The soil becomes yellow. This is
derubefaction
(yellowing, xanthization).
If the process continues, even goethite itself is eliminated and the
colour becomes grey or white. The above explains the morphological
organization of the final stages of the sequence in the Rhône valley,
that is, in the order of increasing weathering: yellow colour at the base
of the B, mottled colour in the top part of the B, white colour of the
E horizon.
Time in thousands of years
250
500
800
2000
0
0
Front of physical fragmentation
of siliceous elements
Front of complete disintegration
of igneous rocks
5
10
Front of complete dissolution
of limestones
15
Derubef ied Acrisol
Gleyic
Development stages
20
Fig. 9.8
The fronts of weathering of coarse fragments in soils of the Rhône valley (Bornand
1978). For example, in 0.5-0.8 million years, dissolution removed the limestone gravel to a
depth of about 10 m.
