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In-Depth Information
sandwiched between two tetrahedral sheets (montmorillonite, for
example). But this weathering also can easily continue to the stage of
1/1 clay minerals, the monosiallites that exhibit one tetrahedral sheet
and one octahedral (kaolinite, for example). In both cases it is
partial
hydrolysis
. However, very many authors have observed the presence
of small quantities of gibbsite in the granitic saprolites of temperate
zone soils. The hypothesis of occurrence of a hot palaeoclimate at the
time of formation of aluminium hydroxide has been put forward. This
is possible, but we think mostly that the types of minerals and their
quantities obtained define the kind of weathering at the same time.
The tropical environment is the only one to accumulate large quantities
of gibbsite (Pédro
et al
. 1975a, b). The small amount of this mineral
present in soils of temperate climate only proves the uniqueness of the
weathering phenomena at the geochemical level.
Nowadays, the passage of smectites to kaolinite is studied with
refined means of investigation. Many papers on this subject will be found
in the journals
Clay Minerals
and
Clays
and
Clay Minerals
. We shall return
to the topic in Chapter 6 (§ 6.6). In the general case, the transformation
involves an intermediate mineral described as
interstratified
, in which we
see at the same time layers of the kaolinite type and of the smectite type.
The process of desilication corresponds to increase of the proportion
of kaolinite-type layers in these interstratified minerals. Thus we pass
gradually in the solid state from 0 to 100 per cent kaolinite (Cuadros
and Dudek 2006).
3.3.3 Geochemical View
The reactions presented in Figure 3.9 and others of the same type
correspond to equilibrium constants
K
that we can calculate. For
example,
The transformation of anorthite to kaolinite is written as:
CaAl
2
Si
2
O
8
[
anorthite
]
+ H
2
O +
2
H
+
¤ H
4
Al
2
Si
2
O
9
[
kaolinite
]
+ Ca
++
Thus: log
K
= log [
Ca
++
]/[
H
+
]
2
when the other two substances are in
the solid state. In other words, the equilibrium between anorthite and
kaolinite only depends on the Ca
++
/H
+
ratio in the medium.
The value of
K
is determined experimentally or on the basis of
thermodynamic considerations.
Presentation of an example
The transformation of kaolinite to gibbsite is represented by the
equation:
