Environmental Engineering Reference
In-Depth Information
TABLE 5.30
Characteristics of Common Clay Minerals
Mineral
Origin (see
Section 6.7.3)
Activity
Particles
Kaolinite
Chemical weathering of feldspars
Low. Relatively stable
Platy but lumpy
material in the presence
of waters
Final decomposition of micas and
pyroxenes in humid climates or well-
drained conditions
Main constituents of clay soils in
humid-temperate and humid-tropical
regions
Halloysite
Similar to kaolinite, but from feldspars
Low, except properties
Elongated rod-
and mica (primarily sialic rocks)
are radically altered by
like units, or
intense drying.
a
hollow cylinders
Process not reversible
a
Illite
Main constituent of many clay shales,
Intermediate between
Thin plates
often with montmorillonite
kaolinite and
montmorillonite
Montmorillonite
Chemical decomposition of olivine
Highly expansive and
Under electron
●
●
(smectite)
(mafic rocks)
the mast troublesome
microscope,
of the clay minerals in
appears as a mass
slopes and beneath
of finely chopped
foundations
lettuce leaves
Partial decomposition of micas and
pyroxene in low rainfall or poor
drainage environment
●
Constituent of marine and clay shales
Used as an
impermeabilizing agent
●
●
●
Alteration of rock during shearing by
faulting
●
Volcanic dust
a
In compaction tests on halloysites, it was found that higher densities were obtained on material air-dried and
then brought back to the desired moisture content, than with material at natural moisture content that was
either wet or dried to the desired moisture (Gibbs et al., 1960). Therefore, when halloysites are used as
embankment material, testing procedures should duplicate field placement procedures.
Calcium clays are formed essentially in freshwater deposition. Hydrogen clays are the
result of prolonged leaching by pure or acid water, with the resulting removal of all other
exchangeable bases.
Base exchange refers to the capacity of colloidal particles to change the cations adsorbed
on the surfaces. Thus, a hydrogen clay can be changed to a sodium clay by the constant
percolation of water containing dissolved sodium salts. The permeability of a clay can be
decreased by such changes and the sensitivity increased. Base exchange may explain the
susceptibility of some soils to the phenomenon termed ”dispersion” or erosion by piping.
Soils with a high percentage of sodium cation relative to calcium and magnesium cations
appear to have a high susceptibility to dispersion (see
Section 10.5.5).
Exchange capacity refers to the quantity of exchangeable cations in a soil; not all cations
are exchangeable. They increase with the acidity of the soil crystals.
Acidity of a clay is expressed by lower values of pH, or higher values of the silica-
sequioxide ratio SiO
2
/R
2
O
3
, where
R
2
O
3
Fe
2
O
3
Al
2
O
3
(5.6)
