Geoscience Reference
In-Depth Information
Fig. 5.6
The three-
dimensional arrangement of
Si tetrahedrons in the crystal
lattice of quartz
It is a typical feature of nature that total regularity without any substitutions
exists only in our models. Under natural conditions of reality, substitutions like
those illustrated above weaken the bonds at specifi c locations within the crystal and
satisfy the aims of weathering. The higher the number of those substitutions as well
as other irregularities, the easier the mineral is weathered. And the more of those
“weakened” minerals appearing in a rock, the easier is the rock's weathering. These
substitutions in a crystal lattice, possible only when the exchanging cations are
nearly the same size, are called isomorphic substitutions. Such substitutions are
responsible for changing the electrical charge on secondary minerals. For example,
when the smaller Al
3+
replaces Si
4+
in a tetrahedron, the excess of the negative
charge appears on the surface of a mineral. And the smaller the mineral, the stronger
is the fi nal effect with the net negative charge on the surface of the mineral being
more distinct. Similarly, the isomorphic substitution of Al
3+
by Mg
+2
, Fe
2+
, or Ca
2+
each creates a distinct negative surface charge density.
The role of aluminum in isomorphic substitution is worth of a detailed study in
origin and evolution of soils. Aluminum can have two roles in a certain mineral:
once it is substituting, and at a just slight distance it could be substituted. These two
different roles could be a factor, which makes the processes less uniform and leads
to the formation of intrigue plots of the play of mineral transformations in the soil
evolution.
Weathering is frequently described as the combination of four basic chemical
processes: hydration, hydrolysis, oxidation, and reduction. Although we could add
two more, dissolution and precipitation, both of them have to precede together
obeying explicit chemical reactions that lead to new simple solutions. Hence, we
eventually include them into one of the basic processes - the transformation of the
products of weathering.
