Geology Reference
In-Depth Information
fixed negative charge, arising from isomorphous substitution in Si
tetrahedral sheets, and a variable, pH-dependent charge similar to that
on clay edges and hydrous oxide surfaces. The two main minerals in this
group, allophane and imogolite, are found in young soils and soils of
volcanic origin.
5.2.3.3 Hydrous Oxides. This term is generally taken to include the
oxides, hydroxides, and oxyhydroxides of aluminium, iron and manga-
nese, which form in soil when these elements are released from primary
minerals by weathering. They exist mainly as small particles in the clay-
sized fraction of a soil (
o
2 mm), and also as coatings on other soil
minerals or as components of larger aggregates.
The most common aluminium oxide found in soil is gibbsite (gAl(OH)
3
),
which may be the final product resulting from the precipitation of
amorphous aluminium hydroxide. A number of iron oxides are found in
soil, the commonest being ferrihydrite, goethite, and haematite. Iron
released from weathering of the ferromagnesian minerals precipitates out
of solution as either ferrihydrite or goethite depending on soil conditions
(low organic matter and a high rate of Fe release favour ferrihydrite
formation). Ferrihydrite is a poorly ordered oxide of small particle size,
whereas goethite is a well-structured mineral and is the commonest of the
iron oxides. Haematite is derived by structural changes to ferrihydrite, and
is commonly found in tropical soils. Manganese oxides occur in a large
number of forms, often with variable valency, for example, birnessite, and
in mixed oxides, especially with iron. They are the least well understood of
this group of minerals.
The hydrous oxides have a variable, pH-dependent charge and are
important in soils as aggregating agents and as adsorptive surfaces (see
Section 5.5) (Figure 7).
5.2.4 Weathering Processes (See Also Chapter 3)
The processes that cause changes to the rocks and minerals of the soil
parent material are collectively known as weathering. In the early stages,
physical weathering dominates and results in fragmentation of rock, a
decrease in particle size, and increase in surface area, but no chemical
change. It is brought about by a number of actions:
Abrasion of the rock caused by the action of ice, water, and wind
leads to physical disintegration.
Temperature change due to diurnal temperature variation and by
freeze-thaw cycles can create significant pressures that will cause
rock to split along lines of weakness.
