Geoscience Reference
In-Depth Information
falls on the ground usually encounters basic cations, although in small
quantities. The weak acid of the rainwater is easily neutralized. The
reaction medium, therefore, attains a pH of 6 or 7. This is what justifies
the term
neutral hydrolysis
, which is often used.
However, as weathering proceeds, the pH falls, according to the
consumption of basic cations. When it drops below 4.5,
H
2
CO
3
is not
dissociated and no longer acts as a buffer (Grimaldi 2004). The same is
true of organic acids. In these conditions of
acid hydrolysis
, kaolinite may
be dissolved. Quartz may or may not be concentrated. We shall return
to this in Chapter 5 and Chapter 11.
Hydrolytic reactions are not very rapid considering the low solubility
of aluminium that is always involved. Thus the later stages of evolution
can only be reached if the duration is long. This is one of the reasons
why the products of total hydrolysis are mostly abundant on ancient
crystalline shields of the world (Africa, South America…).
Time
3.4
ATTEMPT AT MODELLING
We intend to see here how the particle-size distribution in soil profiles
is transformed from the bottom up during weathering. For this, we shall
develop a few theoretical schemes and show that they correspond very
well to what is observed in nature. They are fully supported by earlier
studies of the author achieved through computer simulation (Legros
1982).
We should first keep in mind the particle-size classes. Coarse sands
are the particles between 2000
µ
m and 200
µ
m, fine sands go from 200
to 50
µ
m, coarse silts from 50 to 20
µ
m, fine silts from 20 to 2
µ
m and
clays, in the particle-size sense, are the particles smaller than 2
µ
m.
Skeleton grains are the assemblage of sands and silts.
3.4.1 Fragmentation of the Skeleton Grains
The fragmentation is simulated by assuming that all the soil particles
are of regular shape (rectangular parallelepipeds). These particles are
divided by varying the probability, the regularity and the intensity of
fragmentation (Fig. 3.14). In the computer program, the particles are
arranged in a table of 2000 boxes, the number of each box corresponding
to the length of the particles it contains. For example, particles of size
1100
µ
m (these are coarse sands) are put in box no. 1100. In these
conditions, the fragmentation is simulated by extracting the particles
