Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 56

2.5.3 Soil Composition
Figures 2.9 and 2.10 show that the solid phase of soils contain both inorganic and organic
constituents, and that the inorganic components can be minerals as well as other quasi-
crystalline and noncrystalline materials.
2.5.3.1 Primary Minerals
We deine primary minerals as those minerals derived in unaltered form from parent rock
material, generally through physical weathering processes. The more common ones found
in soils are quartz, feldspar, micas, amphiboles, and pyroxenes. By and large, primary
minerals are generally found as sands and silts, with a small portion of clay-sized fractions
qualifying as primary minerals. We classify particles less than 2 μm in effective diameter
as clay-sized. This classiication is made because it is necessary to distinguish between
clay-sized particles and clay minerals .
2.5.3.2 Secondary Minerals
Secondary minerals are derived as altered products of physical, chemical, and/or biologi-
cal weathering processes. These minerals are layer silicates, commonly identiied as phyl-
losilicates, and they constitute the major portion of the clay-sized fraction of soil materials
in clays. Because of the possibility for confusion in usage of terms and names, it is impor-
tant to distinguish among the terms clays , clay soils , clay-sized , and clay minerals .
Clays and clay soils refer to soils that have particle sizes <2 μm effective diameter. Clay-
sized refers to soil particles with effective diameters less than 2 μm. No speciic reference
to the kind or species of particles is required, since attention is directed toward the size
of the particles. Clay minerals refer speciically to the layer silicates. These are secondary
minerals. They consist of oxides of aluminum and silicon with small amounts of metal
ions substituted within the crystal structure of the minerals. Because of their size and
their structure, secondary minerals have large speciic surface areas (SSAs) and signiicant
surface charges. The major groups of clay minerals include kaolinites, smectites (montmo-
rillonites, beidellites, and nontronites), illites, chlorites, and vermiculites.
2.5.3.3 Soil Organic Matter
Soil organic matter (SOM) can exist in soils in proportions as low as 0.5% to 5%. Although
their proportions may be small, their inluence on the bonding of soil particles and aggre-
gate groups, together with their ability to attenuate contaminants, cannot be overstated. SOM
originates from vegetation and animal sources and is generally categorized in accord with its
state of degradation into humic and non-humic material. Humic materials or substances are
those organics that result from the chemical and biological degradation of non-humic mate-
rial. Non-humic material or compounds, meanwhile, are organics that remain un-decom-
posed or are partly degraded. Humic substances are classiied into humic acids, fulvic acids,
and humins, with the distinction being made of the basis of their solubility to acid and base.
2.5.3.4 Oxides and Hydrous Oxides
The general list of oxide and hydrous oxide minerals includes the oxides, hydroxides,
and oxyhydroxides of iron, aluminum, manganese, titanium, and silicon. The common