Geoscience Reference
In-Depth Information
Alumina
sheet
Silica
sheet
Alumina
sheet
Silica
sheet
Silica
sheet
Kaolinite structure
Montmorillonite structure
Figure 19.6 Structures of kaolinite and montmorillonite clays.
Table 19.5 Size and swelling of clays
Surface area
nanometres
Volume
Mineral
Thickness
(m 2 g -1 )
change
Montmorillonite
2
800
High
Illite
20
80
Medium
Chlorite
20
80
Medium
Kaolinite
100
15
Low
electric charge is unsatisfied charges at the edges of the
particles, the broken bonds. The hydroxyl (OH - ) groups
at the edges become ionized at high pH values and give
an increasing negative-charge capacity as pH rises. This
charge is thus pH-dependent . The ease with which the
hydrogen ion (H + ) can be exchanged also increases as the
pH increases and thus the total charge due to 'broken
bonds' increases as pH increases. Conversely at low pH
values many positively charged sites are found on the clay
colloids, though the net charge of the colloid is overall
negative.
Cation exchange properties
The overall net negative charge of clay minerals is the
cation exchange capacity (CEC), the capacity of the
negatively charged colloid surface to attract positively
charged ions (cations). Cation exchange capacity was
traditionally given as milliequivalents per 100 g soil (me
100g -1 ). The equivalent weight is the weight, in grams, of
that element needed to displace one gram of hydrogen.
For monovalent cations (Na + ,K + ) the equivalent weight
Plate 19.4 Dark Brown Subtropical Clay (FAO: Pellic
Vertisol) in Syria with clay minerals of 2 : 1 montmorillonitic
type. Continual expansion and shrinkage of the clay minerals,
with wetting and drying, causes the shiny 'slickenside' faces
in the subsoil.
Photo: Ken Atkinson
 
 
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