Geoscience Reference
In-Depth Information
2
Na
+
+
CO
3
2 -
+ 2
H
+
+ 2
OH
-
Æ 2
Na
+
+ 2
OH
-
(+
H
2
CO
3
)
but,
H
2
CO
3
Æ
CO
2
+
H
2
O
This is very prejudicial to plants and soils. This is the
alkaline pathway
or
alkalization
.
The limitations of the approach are many:
Temperature plays a role. If it is cooler or hotter, the sequence
of crystallization is modified. Salts with solubility that varies
considerably with temperature in the range 10-40 °C are
Na
2
SO
4
,10H
2
O and Na
2
CO
3
,10H
2
O.
Ions when mixed do not behave like the pure species in solution.
For example, the precipitation of CaCO
3
is slowed down when
Mg is present.
The scheme ignores some cations present in the solution (iron,
potassium and aluminium) and anions (NO
3
); they must be
taken into account in some cases.
There are many more mineral species liable to be precipitated
than those listed in the diagram (see the table of most common
salts; and there are many others!). For example, blödite, abundant
in the oases of Tunisia, has the formula MgSO
4
,Na
2
SO
4
,4H
2
O (Job
1992). Each salt has a specific solubility!
The diagram also skips the fact that it is necessary to reason
more meticulously and to consider activities that, in concentrated
medium, are much lower than unity.
It proposes an all-or-nothing reasoning. And yet these are
equilibrium reactions. Although globally gypsum precipitates
after calcite, this is not wholly true as much as wrong. There is
concomitant precipitation of all salts.
The phenomena are greatly simplified by taking only relative
solubilities into account. For example, a solution very rich in Ca
and SO
4
will precipitate gypsum (CaSO
4
) before calcite (CaCO
3
)
even though the latter is less soluble. But it is true that many
waters are nearly saturated with calcite so that CaCO
3
generally
precipitates first.
Let us consider the case where 7 anions and cations are present.
As each one can be more abundant or less so than another, we
theoretically have thousands of alternatives to consider and not
just four as examined in Figure 13.11.
The role of the soil is ignored! Yet, if there is water and organic
matter, the medium is anaerobic. In such a case the quantity of
