Geoscience Reference
In-Depth Information
7.4 Water-solid reactions
Water has the effect on the minerals of igneous or metamorphic rocks of putting the
more soluble cations into solution and producing residual clay minerals. A notable excep-
tion is quartz. A first type of reaction controlling interaction between the lithosphere and
hydrosphere essentially involves reactions of proton and cation exchange such as:
2H
+
2NaAlSi
3
O
8
+
+
H
2
O
⇔
(albite) (solution)
Al
2
Si
2
O
5
(OH)
4
2Na
+
+
4SiO
2
+
(kaolinite)
(silica)
(solution)
(7.22)
Since albite is a very common constituent of crustal rocks, this reaction can also be seen
as limiting the solubility of kaolinite, one of the most important clay minerals in soils. In
the absence of feldspar, kaolinite solubility is controlled by a different proton-exchange
reaction:
1
7
Al(OH)
4
H
+
2
Al
2
Si
2
O
5
(OH)
4
+
2
H
2
O
⇔
H
4
SiO
4
+
+
(kaolinite)
(solution)
(solution)
(solution)
(solution)
(7.23)
The complexation of ions in solution makes the calculation of mineral solubility more
complicated. Ferric iron hydroxide Fe(OH)
3
, one of the important minerals of soils, is a
good example. Ferric iron speciation in near-neutral fresh water (chlorine would produce
strong complexes) is controlled by the species Fe
3
+
, Fe(OH)
2
, and Fe(OH)
4
. The con-
centration of the dissolved species Fe(OH)
3
is essentially zero. We can write the following
dissolution reactions for the Fe(OH)
3
mineral:
3H
+
⇔
Fe
3
+
+
Fe(OH)
3
(s)
+
3H
2
O
(7.24)
H
+
⇔
Fe(OH)
2
Fe(OH)
3
(s)
+
+
H
2
O
(7.25)
Fe(OH)
4
H
+
Fe(OH)
3
(s)
+
H
2
O
⇔
+
(7.26)
where Fe(OH)
3
(s) stands for solid hydroxide and the other species are in solution.
Using the equilibrium constants suitable for these two equations and considering that the
concentration of a pure solid phase is unity, we obtain:
Fe
3
+
H
+
3
10
3.2
=
(7.27)
Fe
)
2
(
OH
10
−
2.5
H
+
=
(7.28)
Fe
)
4
H
+
=
10
−
18.4
(
OH
(7.29)
