Geoscience Reference
In-Depth Information
The concentration of the solid being 1 by convention, we obtain,
taking logarithms,
log
[
Ca
2+
]
+ log
[
SO
4
2 -
]
= log
K
= - 4.85
This straight line passes through the point log [Ca
2+
] = -4.85 for
log [SO
4
2-
] = 0 and reciprocally. When saturation is attained, the point
representing the water will split off at a right angle, towards the ordinate
axis because gypsum precipitates and calcium remains in excess. If we
were to start from point B (sulphate in excess), the change of direction
would have been towards the abscissa. This is the 'T' law (Tardy and
Gac 1979).
Thus, such a diagram allows us to visualize things correctly. For
example, on the same Figure 13.14, the group marked C corresponds to
waters that have reached saturation with gypsum, precipitated it and
then are progressively enriched in residual sulphate.
13.5
EVOLUTION AND TYPOLOGY OF SALINE SOILS
13.5.1
Evolution of Minerals
When the medium is base-rich and the pH is not very far from neutral
(5<pH<9.6), formation of various 2/1 clay minerals is promoted. The
feldspar orthoclase, for example, will give a beidellite whereas it will
evolve to kaolinite or even yield gibbsite in a more dilute medium
(Pédro 1979). Albite will evolve similarly (Fig. 13.15).
Clay genesis
Beidellite
Albite
Gibbsite
Kaolinite
log Si OH
[(
)
4
-5
-4
-3
Fig. 13.15
Transformation of albite in saline medium (diagram from Garrels).
NaSi
3
AlO
8
Æ
Si
3.7
Al
0.3
O
10
Al
2
(
OH
)
2
Na
0.3
+ 3.8
SiO
3
H
-
+ 0.2
Al
(
OH
)
4
-
+ 2.2
Na
+
+ 2.2(
OH
)
-
