Geoscience Reference
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environment, in which not one but several minerals are decomposed
together to form many others simultaneously. It would be more
appropriate to write the reaction as proposed in Figure 3.13.
Kaolinite
Nontronite
Gibbsite
Haematite
Progress of the reaction over time
Fig. 3.12
Simulation of the weathering of a granite by percolation of rainwater at 25° C for
the present-day content of CO
2
in the atmosphere (Schmitt 1999 in Thiry
et al
. 1999).
Granite
Regolith
Tropical soil
Biotite
+ Feldspar
+ Muscovite
+ Quartz
Kaolinite
+ Nontronite
+ Quartz
Kaolinite
+ Gibbsite
+ Haematite
Fig. 3.13
Summary of the simulated hydrolysis of a granite in tropical environment.
3.3.5
Guiding Parameters of Hydrolysis
We have already indicated (Chap. 2, § 2.2.3, Table 2.2) the greater or
smaller fragility of minerals. They can also be compared by determining
the rate at which silica is released into the medium. The following scale
of decreasing resistance to hydrolysis is obtained, which is the Goldich
sequence established in 1938:
Nature of the mineral attacked
Quartz > muscovite > orthoclase > plagioclase > olivine
When the products obtained are insoluble, they disappear from the
aqueous solution, which allows the reactions to continue.
Nature of the products obtained
