Geology Reference
In-Depth Information
can be represented by equilibria, where the equilibrium constant is
related only to the activities of the aqueous solutes, i.e. an assumption
is made that the activities of solid phases have the value of unity.
Examples of weathering of primary minerals, those formed at the same
time as the parent rock, are shown in (3.19)-(3.22):
Congruent dissolution of quartz
SiO
2
ð
s
Þ
þ
2H
2
O
Ð
Si
ð
OH
Þ
4
ð
aq
Þ
quartz
ð
3
:
19
Þ
silicic acid
K
¼
{Si(OH)
4
0
}
(3.20)
At pH
o
9, the equilibrium between quartz and silicic acid can be
represented as in (3.19) and the value of K at 298 K is 1.05
10
4
mol L
1
,
30
indicating that the solubility of quartz is low. At higher pH
values, the dissociation of silicic acid results in the increased solubility of
quartz (see Section 3.2.4.1).
Incongruent dissolution of Na-feldspar
þ
2H
þ
þ
9H
2
O
Ð
Al
2
Si
2
O
5
ð
OH
Þ
4
ð
s
Þ
kaolinite
þ
2Na
ð
aq
Þ
2NaAlSi
3
O
8
ð
s
Þ
Na
-
feldspar
ð
3
:
21
Þ
þ
4Si
ð
OH
Þ
4
ð
aq
Þ
silicic acid
K
¼f
Si
ð
OH
Þ
4
g
4
f
Na
þ
g
2
=f
H
þ
g
2
ð
3
:
22
Þ
Alteration of primary minerals such as the feldspars gives rise to
secondary minerals such as kaolinite (Al
2
Si
2
O
5
(OH)
4
), smectites (e.g.
Na
0.33
Al
2.33
Si
3.67
O
10
(OH)
2
), and gibbsite (Al(OH)
3
). As the composition
of the new mineral phases and the potential for further alteration of
secondary minerals are a function of the prevailing geochemical condi-
tions, many equilibrium expressions must be used to fully describe
chemical weathering processes in natural systems. For example, reac-
tions leading to the formation of kaolinite from the primary mineral,
Na-feldspar, as well as alteration of the secondary mineral, e.g. to
gibbsite, must be considered. The composition of the aqueous phase is
of major importance in determining the nature of the solid products
formed during chemical weathering. In particular, the solution activities
of silicic acid, metal ions, and hydrogen ions are key parameters
influencing the formation and alteration processes of new solid phases
(Example 3.4).
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