Geoscience Reference
In-Depth Information
transformation of smectite to kaolinite, in the presence of iron oxide, may occur
under enhancement of drainage conditions during landscape evolution. In the case
of sedimentary environments (Morgan et al.
1979
) and well-drained red-black
soils (Herbillon et al.
1981
), the transformation may proceed through an inter-
mediate step consisting of interstratified kaolinite-smectite.
1.1.3 Minerals Other than Silica and Clay
In addition to silica and clay minerals, the subsurface contains a variety of min-
erals (e.g., oxides, carbonates), which may react with organic and inorganic
contaminants. Gilkes (
1990
), summarizing the properties of the metal oxides in
earth materials, states that iron oxides (e.g., hematite a-Fe
2
O
3
, magnetite b-Fe
2
O
3
,
goethite a-FeOOH, and lepidocrocite b-FeOOH) are common constituents with
crystals that vary greatly in size, shape, and surface morphology. The surface of
iron oxides in the subsurface environment often is hydroxylated either structurally
or through hydration of the Fe atoms. Crystals of the aluminum oxides that
commonly occur (e.g., gibbsite, boehmite) are small but often larger than the
associated iron oxides. Other oxide minerals are less abundant than Fe and Al
oxides, but because of their very small crystal sizes and large surface areas, they
may affect very significantly the geochemical properties of the subsurface and their
interaction with chemicals. For example, the various Mn oxides that can be found
in the subsurface can occur as very small (about 10 nm), structurally disordered
crystals. Similarly titanium oxides (rutile, anatase, TiO
2
) and even corundum, a
rare pyrogenic mineral, occur within the clay fraction as approximately 30 nm
crystals. The ability of Fe and certain other metal ions to undergo redox reactions
further increases the role of metal oxides in the activity of the solid phase.
Other major components found in the subsurface include significant quantities
of relatively high surface area, soluble calcium carbonate (CaCO
3
), and calcium
sulfate (CaSO
4
). It is difficult to estimate the contribution of amorphous materials
(e.g., allophone or imogolite) to the surface activity of earth materials. Amorphous
materials often coat crystal minerals, which may further affect interaction of these
minerals with contaminants.
1.1.4 Organic Matter
Soil organic matter is a general term for the nonliving portion of the organic
fraction in the soil layer. It is a mixture of products resulting from microbial
transformation of organic residues and includes organic compounds originating
from undecayed plant and animal tissues, their partial decomposition products, and
the near-surface biomass. Major components of the soil organic matter and their
definition are presented in Table
1.1
.
