Geoscience Reference
In-Depth Information
Traces of the hyperplanes
(these are lines) corresponding
to the minerals considered
Gibbsite
Kaolinite
Water
log H SiO
4
4
Fig. 3.11
Principle of constructing a geochemical diagram (Aurousseau and Pagès 1985).
Thus the projection allows us to visually examine if the water is
undersaturated or supersaturated with respect to certain minerals, here
gibbsite and kaolinite. But this method, widely used and apparently
highly scientific, is not lacking in drawbacks:
Closely projected points might pertain to waters very different in
reality; observation of the points and lines in the drawings done
in two dimensions has to be supplemented by measurement of
distances in
n
-dimensional space.
Widely separated points may actually pertain to waters of the same
family in equilibrium with a single mineral not represented.
The choice of the projection system (in 2D) retained is not always
justified clearly so that it hardly makes understanding of the
phenomena easier!
3.3.4 Computer Simulation of Weathering
Simulation of hydrolysis has been done by means of the computer
(Schmitt 1999 in Thiry
et al
. 1999). A rock itself was considered and
not an isolated mineral. Starting from a granite (Fig. 3.12) the gradual
disappearance of each of the primary minerals was studied, from the
most soluble to the most resistant: biotite, feldspar, muscovite and,
lastly, quartz. In the example chosen, these minerals were replaced by
an assemblage of kaolinite, gibbsite and haematite. During the reaction,
though, a 2/1 clay mineral appeared ephemerally. It was nontronite, which
is ferriferous and thus hides iron in its structure. When the nontronite
disappeared, the iron is expressed in the form of haematite.
Finally, kaolinite starts to get decomposed and it disappears almost
completely in the simulation conducted at 35 ºC, which represents the
tropical environment better. Thus the reaction sequence in Figure 3.9:
'feldspar Æ 2/1 clay mineral Æ 1/1 clay mineral Æ gibbsite'
now appears
validated. But it simplifies to excess what occurs in the natural
