Geoscience Reference
In-Depth Information
Gibbsite +
goethite
Kaolinite +
goethite
Goethite
(surface
cuirasse)
Gibbsite
Kaolinite
Kaolinite +
goethite +
quartz
Kaolinite +
quartz
Montmorillonite
Nontronite
Fig. 3.21
Prediction of the morphological structure of profi les in environments favourable for
desilication (Fritz 1975). These examples have granite substratum.
The first profile, on the left, corresponds to the case where iron
is retained (aerated medium). Nontronite is actually a clay mineral
containing iron in its structure. In this case, at the final stage, when
the silicates and gibbsite have been totally eliminated, iron alone is left
behind and can form a cuirasse at the very top of the soil. The second
profile, on the right, represents the situation in which iron disappears
(water-saturated medium).
We should note that the horizons with 2/1 clay minerals have been
truncated at the bottom to reduce the height of the diagrams. Actually,
simulation predicts horizons with thickness reducing towards the top
(residues of residues).
3.5.5 Variability within the Horizon
Figure 3.22 (Tardy 1969) is an example of what is likely to happen at the
centimetre scale in a horizon of clayey nature. Water flows slowly in the
fine pores and is loaded with ions. The system is closed and smectites
can be preserved. On the contrary, water flows fast in large pores. There
is no time for it to be saturated with silica. This favours geochemical
depletion and hence the occurrence of kaolinite.
In other words, we sometimes notice at the aggregate level what is
seen at the level of the toposequence or of the profile: the differentiation
and spatial separation of different clay minerals. This explains in part
why the clay minerals in the same soil sample obtained by grinding
are often composed of mineralogical mixtures whereas the processes
