Geoscience Reference
In-Depth Information
broken and bent tubes. In our weathering model they are the forces keeping the
atoms in the regular form of the crystal lattice. Their absence means that the atoms
are liberated from their fate to stay at required positions within the lattice. There are
also separated scaffolding connections with embedded screws and even many
individual screws scattered in and about the pile. They represent freed atoms no
longer attached to the lattice. In addition to these separated parts, there are large,
intact remnants of the recently plastered scaffolding in the pile. They demonstrate
portions of the mineral not affected by weathering. All of the materials in the pile
are mixed with the powder of plaster that originated from the dusty products of
earlier weathering.
A more complicated explanation of the silicate crystal structure in soils follows
even though we tried to simplify the subject as far as possible. Although less patient
readers may jump over these paragraphs and continue to read near the end of this
subchapter, they should keep in mind that chemical weathering causes small com-
plete parts of an original crystal to be torn out of the lattice by breaking the weakest
bond. At the same time other parts of the crystal lattice are completely disturbed
with individual ions being released to form simple solutions in water that are fre-
quently washed away. On the other hand, they could be involved in the creation of a
new mineral. What are left in place are amorphous forms, materials without defi nite
confi gurations that are frequently transformed into new minerals of submicroscopic
size, the clay minerals. The great majority of all such fragments and mineral trans-
forms obey the same basic principles - those of the crystal lattices of silicates.
The most frequent minerals in the Earth's crust are silicates and quartz. Because
quartz is very resistant to weathering, we have the tendency to say that it does not
weather at all. Here, we shall describe weathering of silicates without concealing
that other minerals are either more or less easily weathered. But silicates are the
majority of all minerals, and when their weathering produces new minerals different
from those in rock, we call them secondary minerals. We shall show that they have
extreme infl uence on the physical and chemical properties of soils.
The structure of crystal lattice of minerals is determined by the principle of the
most dense arrangement of ions composing the mineral. In order to make the rather
complicated structure more transparent and applicable in our popular writing on
soils, we shall use a most simplistic approach even though it brings along a danger
of pitfalls commonly related to oversimplifi cation. By assuming that the volume of
space occupied by ions is spherical and formed by a force fi eld impervious to neigh-
boring ions, we can apply and use a model of solid spheres. The ions - spheres of
silicates - are arranged into simple geometric confi gurations. First, we demonstrate
the confi gurations of centers of ions forming the lattice.
There are pyramidal forms composed of four triangles - one is considered the
base and the other three triangles are the walls or faces that share a common vertex.
Any of the four faces could be considered the base. Although this form is easily
recognized as a triangular pyramid, its scientifi c name is tetrahedron, derived from
the Greek
tetra
meaning four,
hedra
meaning face or seat, or
hedron
meaning having
bases or sides (Fig.
5.1
).
