Geoscience Reference
In-Depth Information
long time, a tetrahedral sheet is eventually broken and displaced without keeping its
original confi guration. Kaolinites are the result of such weathering. However, the
weathering process does not end. It could continue until bauxite is formed with an
aluminum cation remaining in the center of its octahedral. Having oversimplifi ed
the formation of clay minerals by the process of weathering, we omitted numerous
steps of their synthesis that produce a diversity of qualities and properties. These
features hinge on the chemical composition of water fi lling the pores, conditions
and frequencies of water drainage or water logging, prevailing acidity and tempera-
tures, and a multitude of other timely and impinging conditions.
Just one group of clay minerals deviates from slats models. They are
allophanes
.
They originate as products of weathering of volcanic materials, mainly of volcanic
glass. Their name was derived from Greek
allos
meaning other or different and
phanos
meaning to appear. They were earlier classifi ed as mainly amorphous
hydrous aluminosilicates, but now they are regarded as minerals formed by fi ne
spheres of 3.5-5 nm outside diameter. These nano-spheres remain empty and have
walls formed by tetrahedrals Si-O-OH bound to octahedrals Al-O-OH where the
Al octahedrals may prevail on some parts of their spherical structure. The chemical
compositions of various kinds of allophanes fall in a relatively narrow range and
have SiO
2
:Al
2
O
3
ratios mostly between 1.0 and 2.0. In addition to their dominant
spherical form, some of them also have ring-shaped forms. The specifi c character-
istics of allophanes can be attributed to their spherical shape. Their apparent specifi c
surface is high. They are characterized by high phosphate retention. Their low bulk
density is attributed to their empty spherical volume and to spheres having roughly
the same diameter. Soils with clay fractions dominated by allophanes have similar
physical properties such as a high cation exchange capacity, a high porosity, and a
large hydraulic conductivity.
5.2.3
Soil's First Name: Texture
It has been a well-known fact to
Homo sapiens
for at least the last 12,000 years that
different kinds of soils have various properties. Their knowledge on soil qualities is
linked to the start of the greatest revolution in mankind's history, i.e., to the start of
agriculture at the end of Paleolithic period. Actually, this was the time when the
Neolithic period began. Without knowledge of soils, establishing a life related to
regular agriculture was impossible. Although no documents were written during
this era because at that time script had not yet been invented, the style of life, the
selection of land for very simple settlements, and the regular planting of fi rst domes-
ticated barley and wheat were all inconceivable without sorting and optimizing land
parcels of a large area occupied by the families of settlers. That large area was the
region of the Middle East extending from the Nile valley to the Tigris and Euphrates
rivers including the slopes of the Zagros and Anti-Taurus mountains where the fi rst
agronomists found the best soils as deposits of mountain slopes and fertile alluvial
