Environmental Engineering Reference
In-Depth Information
of cations (Na
) more readily overcomes the negative charge on the clay surface).
Hence the repulsive forces are lower;
(b)
Cation valence
: Exchange of Na
cations by Ca
cations leads to a smaller, higher
charge density diffuse double layer and hence lower repulsive forces.
Other factors which affect the diffuse double layer include:
-dielectric constant of the electrolyte;
-
temperature.
More details are given in Mitchell (1976, 1993).
7.4
IDENTIFICATION OF CLAY MINERALS
When required, the identification of clay mineral(s) present in a soil is usually carried out
using at least two of the following techniques. It should be noted that most soils have clay
sized particles mixed with silt and sand sized particles, the latter usually being quartz or
other rock mineral. The results of the clay mineralogy must therefore be considered allow-
ing for that proportion of the soil they represent in total.
7.4.1
X-ray diffraction
A sample of the powdered soil is subject to x-rays, which are diffracted depending on the
mineral crystals present. Each clay mineral has a characteristic pattern which is known.
Mitchell (1976, 1993) discusses the technique.
7.4.2
Differential thermal analysis (DTA)
A sample of the soil, and an inert substance are heated at a controlled rate (10°C/minute)
up to 1000°C, and the differences in temperature in the soil and the inert substance are
recorded.
Endothermic reactions (ie. heat being absorbed) are usually related to the driving
off of:
- Adsorbed water;
-Water of hydration.
Exothermic reactions are usually related to recrystallisation and oxidation. Each clay min-
eral has a characteristic thermogram allowing its identification. Mitchell (1976, 1993) indi-
cates that the method can be calibrated for quantitative analysis, accurate to the determined
value plus or minus 5%.
7.4.3
Electron microscopy
Scanning electron microscopy can be used to assist in identification of clay minerals.
Magnifications from X20 to X150,000 can be used and the micrographs compared
with those for known clay minerals. The main clay minerals have distinctly different
micrographs.
In practice the presence of several clay minerals, and silt and sand complicates identifi-
cation but, coupled with x-ray diffraction or differential thermal analysis, the method can
be useful.
