Geoscience Reference
In-Depth Information
Based on a careful study of the sediments of Lake Chad (Carmouze
et al
. 1977), when the pH rises and the medium is concentrated,
precipitation takes place in succession of nontronites, alumino-ferriferous
montmorillonites and, lastly, magnesian montmorillonites. When 2/1
clay minerals are originally present in the parent material, they are
preserved or even find their proportion increasing.
At the same time, dioctahedral micas can precipitate in alkaline
lakes, as in Brazil for example (Furquim
et al
. 2010). In certain cases
Na
+
replaces
K
+
in the micas. The layer thickness goes from 10 to 14 Å. A
mineral resembling vermiculite is obtained. In fact, as the replacement
does not take place in all the layers, an interstratified
mica/Na-vermiculite
is the product.
To summarize, the neutral saline path and the alkaline path generally
result in formation or preservation of 2/1 phyllites.
At pH higher than 9.6, silica is ionized as follows (H. Chamayou, pers.
comm.):
Si
(
OH
)
4
¤
SiO
3
H
-
+
H
2
O
+
H
+
with log
K
= -9.71
On the other hand, at pH higher than 11.3, alumina gives:
Al
(
OH
)
3
+
OH
-
¤
Al
(
OH
)
4
-
with log
K
= -2.67
Alkalinolysis (Pédro 1979), that is, destruction of clay minerals in
alkaline medium, therefore, takes place easily at pH above 11.3, since
the oxides formed by hydrolysis can all be eliminated. In the case of
albite, for example:
NaSi
3
AlO
8
+ 6
H
2
O
¤ 3
SiO
3
H
-
+
Al
(
OH
)
4
-
+
Na
+
+
OH
-
+ 4
H
+
The Si/Al ratio is 3 in the destroyed mineral as also in the drainage
water. In other words, the dissolution is congruent. Everything is
eliminated.
These phenomena are very important.
For the one part, they enable us to understand why saline soils could
be degraded (decomposition of clay minerals in the upper part of the
profile) when alkalization is very marked. Then the soils obtained have
surface horizons impoverished in clay and are grouped in the Solonetz
(see below). Admittedly the mechanism is specific, but the result is
there: some saline soils evolve in conformity with the general view
that underlies this topic: clays are formed at the bottom of the soil and
disappear in its upper part.
Alkalinolysis
