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model might be considered defective because the error is consubstantial
with the scientific approach, rather it is that pedogenesis could accept
such a great exception to its usual mode of functioning!
Bocquier's hypothesis effectively relies on precise and indisputable
facts: endorheism of the basin of Lake Chad, then non-evacuation of the
silica liberated, micromorphological arguments suggesting that the clay
mineral was allochthonous and had coated the skeleton grains, lime
concretions whose age appears to diminish upslope as if they had been
formed of more recent materials.
Later, observations have been made that oppose the model proposed
by Bocquier: (i) it is physically impossible to find on the top the amount
of silica required to compose all the smectite preserved on the footslope,
as the relevant area is very large (Boulet 1974); (ii) in dry environment,
pedological weathering leads directly and
in situ
to the appearance of
smectites as attested by the vast areas of Vertisols observed in various
parts of the world; thus there is no need to make a special case for
Kosselili and to believe that lateral movement of the clay mineral or its
constituent ions takes place there; (iii) the age of the lime concretions
has no great significance, as already indicated.
The progress made in mineralogy over the past three decades enables a
decision. Scientists are now in agreement (Bühmann and Schoeman 1995;
Vingiani
et al.
2004; Cuadros and Dudek 2006): in these toposequences
smectites are weathered
in situ
to kaolinite. The transformation proceeds
through the intermediate stage of smectite-kaolinite, the layers of smectite
type giving place slowly to kaolinite-type layers. Two mechanisms are
compatible with the observations (Ryan and Huertas 2009):
Stripping of the tetrahedral sheet
. Within a clay particle, a layer of
smectite loses a tetrahedral sheet and directly gives a kaolinite
layer (see Chap. 2, Fig. 2.4 and related explanations).
Inversion of tetrahedra.
The 2/1 layers first fix aluminium in inter-
layer position
(gibbsite layer); locally a 2/1/1 mineral is obtained;
it is then necessary to invert the tetrahedral sheet adjacent to the
gibbsite sheet so that a 2/1/1 clay mineral layer gives two 1/1
clay mineral layers. This seems possible…
The smectites under consideration are iron-rich. Their destruction
releases Fe
3+
ions that crystallize as goethite and also as haematite,
whence the possible occurrence of a red-coloured soil in well-drained
The modern view
