Geoscience Reference
In-Depth Information
Circles have been drawn in the diagram enclosing the compositions
of the waters found in different climates according to the analyses done
by Tardy (1969). All of them show that:
The waters are never in natural equilibrium with anorthite and
other plagioclases. These minerals are all destroyed on the sur-
face of the earth. This destruction happens when Ca
++
/H
+
dimin-
ishes, that is when the medium is acidified.
Starting from a feldspar, according to the environmental condi-
tions, montmorillonite, kaolinite or gibbsite might be obtained.
This is what we had stated earlier, without proof.
When the quantity of silica in the water diminishes, it is accom-
panied by desilication of the solid phase. The sequence goes from
montmorillonite to kaolinite to gibbsite.
The case of anorthite is just an example. The decomposition of
minerals of all kinds can be studied in similar diagrams.
Let us consider a mineral being solubilized in water:
Si
a
Al
b
Ca
c
… + nH
2
O
¤
eH
4
SiO
4
+ bAl
3+
+ cCa
2
+
…
We can write the law of mass action and proceed to the logarithms.
Then a linear equation will be obtained corresponding to the hyperplane
in
n
-dimensional space of the chemical equilibrium in question, where
n
is the number of parameters. Taking into account the constraints
(electrical neutrality of the solution, no negative coefficients and
conservation of matter), we can calculate:
the distances between a point representing a water and the
hyperplanes corresponding to the weathering of reference
minerals for finding out which of these minerals 'controls'
the solution, that is to say is in equilibrium with it (= zero
distance = no precipitation and no dissolution of that mineral),
the mineral in equilibrium with a series of given waters (more
water samples are necessary than parameters). A possible error
persists: a slight modification of the content in such or such
water of such or such ion is enough to change the nature of the
mineral found.
For visualizing all this, the point representing the water and the
hyperplanes representing the minerals are projected in a special two-
dimensional space. Figure 3.11 gives an example.
Generalization (Aurousseau and Pagès 1985)
