Geoscience Reference
In-Depth Information
Si
2
Al
2
O
5
(
OH
)
4
[
kaolinite
]
+
5
H
2
O
¤
2
Al
(
OH
)
3
[
gibbsite
]
+
2
Si
(
OH
)
4
[
orthosilicic acid
]
with
log
K
=
2log
[
Si
(
OH
)
4
]
.
The equilibrium only involves the silica content. Specialists early
knew this kind of transformation occurs in Ferralsols in tropical
environment (Sieffermann
et
al
. 1968).
The direct transformation of anorthite to gibbsite is described
by:
CaAl
2
Si
2
O
8
[
anorthite
]
+
6
H
2
O
+
2
H
+
¤
2
Al
(
OH
)
3
[
gibbsite
]
+
2
H
4
SiO
4
[
orthosilicic acid
]
+
Ca
++
giving log
K
= log [
Ca
++
] - 2log [
H
+
] + 2log [
H
4
SiO
4
]
or again log
K
= log [
Ca
++
]/[
H
+
]
2
+ 2log [
H
4
SiO
4
]
In the latter type of equation, the
Ca
++
/
H
+
ratio and the silica content
are simultaneously involved.
When all calculations have been done, Figure 3.10 is obtained.
log
[
Ca
++
[]
]
H
+2
20
Anorthite
Ca-Montmorillonite
15
Gibbsite
Kaolinite
Waters
of
dry zones
10
Waters of
the temperate
zone
Waters
of hot
countries
5
ppm of SiO
2
1
10
100
-5
- 4
-3
log [H SiO ]
4
4
Fig. 3.10
Diagram of the montmorillonite-kaolinite-gibbsite equilibrium (Tardy 1969).
Actually, considering what has been said above, the equilibrium
boundary between anorthite and kaolinite in a diagram with silica
on the abscissa and Ca
++
/H
+
on the ordinate, is a horizontal line. The
gibbsite/kaolinite boundary is a vertical line. The anorthite/gibbsite
boundary is oblique.
