Geoscience Reference
In-Depth Information
3.2.2 Dissolution Phenomena
The ions released by weathering either remain in place or are exported
in quantities that largely depend on their solubility in water. The curves
reproduced here were established at 20° C, for normal pressures and in
the absence of other ions. They only have indicative value but are very
important for understanding what happens in nature. In the rest of the
topic, we will often refer to them. The diagrams are complicated by the
fact that, when the pH changes, the ionic species containing the element
considered also changes.
All things considered, silica is rather soluble (Fig. 3.6; Table 3.1).
Solubility of silica
2
Total
Si
0
-2
-
0
4
H
4
SiO
-4
-6
4
H
3
SiO
-8
4
4
SiO
-10
-12
-14
2
4
H
2
SiO
3
4
HSiO
pH
- 16
4
567 89 011
1 2
13
14
Fig. 3.6
Solubility curves of silica.
Table 3.1
Content of silica H
4
SiO
4
(silicic acid, the soluble form) in spring
waters from the Fougères forest in Brittany (Bourrié 1981).
mg l
-1
Min 60 5.76
Max 470 450
The solubility of quartz is 6 mg l
-1
below pH 8. Therefore the
minimum found in the table corresponds to a water in equilibrium
with this mineral. The higher values (cf. the maximum observed) prove
the dissolution of silica from silicates less resistant than quartz.
m
moles l
-1
Ferric iron is nearly 10,000 times less soluble than its reduced homologue
(Fig. 3.7). Thus, iron in soils can only move if it is in the ferrous form. The
Solubility of iron
