Geoscience Reference
In-Depth Information
Microfissures and tubules appear in the upper part of the cuirass and
lead to its breakdown into cm-sized or mm-sized islands. Thus there
is a physical dismantling that results in a
gravelly
horizon (Maignien
1958; Leprun 1977). Mineralogically, there are three stages (Bitom and
Vol kof f 1993) :
Dismantling of the cuirass
zonewise goethitization of the largely haematitic cuirass (local
yellowing and loss of coherence) but preservation of nodules and
intact bridges between them;
progressive deferruginization of the bridges;
pedoturbation (organisms); this causes collapse of the original
structure; a mixture of kaolinite and goethite is left.
In northeastern wBrazil, the top kaolinite layer can be up to 10-m thick
(Truckenbrodt
et al.
1991). This is a sort of geochemical paradox. On the
one hand, between the base and the middle of the profile, the kaolinite
is destroyed and iron is concentrated. On the other hand, higher in the
profile, it is the opposite, at least apparently: the cuirass is decomposed
and kaolinite reappears, contradicting the idea of progressive desilication
presented earlier. Various hypotheses have been put forth that, perhaps,
should be considered to work in unison.
Return to kaolinite
(
1
)
Role of termites
. Termites bring up to the soil surface clays and
quartz grains extracted from the lithomarge. For these insects, it
is a matter of rebuilding the termitaria destroyed by rain. They
are capable of annually moving nearly 20 tonnes of soil per
ha (Lavelle
et al
. 1992). It has been calculated that the mass of
kaolinite found on the surface could have been brought up in a
few thousand years.
2
)
Change in pedoclimate
. In oxidizing medium, the oxides of
iron are more stable than quartz and kaolinite. In reducing
medium it is the opposite (Nahon 1976). Thus, to explain the
concentration of kaolinite on the surface it is enough if we
invoke the late appearance of a wetter climate. Then, haematite
will be transformed to goethite and finally evacuated whereas
the kaolinite still present in the cuirass will be concentrated in
place (Edou-Minko and Trescases 1990). This very spectacular
transformation is not quantitatively impossible (Table 5.3):
the cuirasses hide within themselves at least 10 per cent and
sometimes as much as 30 or 35 per cent kaolinite. What remains
