Geoscience Reference
In-Depth Information
because the reactions consume H
+
ions. Actually it is necessary to
differentiate this hydrolysis from pedogenetic transformations that occur
in a strongly acid environment.
Table 3.3 gives some examples of hydrolysis: a feldspar of orthoclase
type is transformed to illite, to kaolinite and to gibbsite (aluminium
hydroxide) with release of silica into solution. The appearance of 2/1 clay
minerals is called
bisiallitization
, of 1/1 clay minerals
monosiallitization
and that of gibbsite, allitization. These terms were introduced in 1930
by the German scientist Harassowitz (Buol and Eswaran 2000) and were
later revived by Erhart, Millot and then Pédro (1966).
Table 3.3
Hydrolytic reactions.
Orthoclase → illite (example of bisiallitization)
3[
KAlSi
3
O
8
] + 2
H
+
+ 12
H
2
O
⇔ [
K
(
Si
3
Al
)
O
10
Al
2
(
OH
)
2
] + 6
Si
(
OH
)
4
+ 2
K
+
with
SiO
2
/
K
2
O
= 6/1 = 6 in drained waters
Orthoclase → kaolinite (example of monosiallitization)
2[
KAlSi
3
O
8
] + 2
H
+
+ 9
H
2
O
⇔ [(
OH
)
4
Al
2
Si
2
O
5
] + 4
Si
(
OH
)
4
+ 2
K
+
with
SiO
2
/
K
2
O
= 4/1 = 4 in drained waters
Orthoclase → gibbsite (example of allitization)
2[
KAlSi
3
O
8
] + 2
H
+
+ 14
H
2
O
⇔ 2
Al
(
OH
)
3
+ 6
Si
(
OH
)
4
+ 2
K
+
with
SiO
2
/
K
2
O
= 6/1 = 6 in drained waters
3.3.2 Crystallochemical View
Figure 3.9, one of the most important in the entire topic, displays the
evolution at the crystallochemical level when we start from a feldspar,
for example. The bases (Ca, Mg, K and Na) are eliminated at the start
of weathering, then pedogenesis through neutral hydrolysis results in
extensive desilication, a phenomenon to which we shall often return.
Clay minerals of 2/1 type, which first appear, are silica-rich with two
tetrahedral silica sheets. The 1/1 clay minerals that follow have but one
tetrahedral sheet and the last, gibbsite, has none at all.
But we must distinguish between temperate and tropical environ-
ments.
The kaolinite-gibbsite association is mostly characteristic of tropical
environments. When all the silicates disappear to the gain of gibbsite,
it is called
total hydrolysis
. This strong mobilization of silica shows up
in the composition of drainage waters as seen in Table 3.4 (Lucas
et al
.
1996, after Cornu 1995).
