Soil properties - Fundamentals of the Physical Environment - page 580

of the silica sheet. Montmorillonite has a similar structure to illite, except that there are
no potassium ions to bond the layers together, and water enters easily between the layers.
Thus the wet clay can expand to several times its dry volume. Vermiculite resembles
montmorillonite except that absorption of water between layers is limited to two
thicknesses of water molecules. Chlorite is made of mica layers held together by alumina
sheets. Figure 18.8 illustrates how the alumina and silica sheets condense together to give
the structures of kaolinite and montmorillonite.
The volume change caused by wetting is an important physical property of clays. Dry
sand and silts can take up water when the air in pore spaces is replaced, but that gives no
increase in volume. With clays, water can give forces of repulsion between particles, so
that the volume increases as water content increases. Swelling increases with increasing
surface area of the clay particles. In turn, surface area depends on the thickness of the
crystalline particles. It increases from the thicker kaolinite particles to the thin particles of
montmorillonite (Table 18.5, Plate 18.4).

CATION EXCHANGE

As mentioned in the previous section, the replacement of aluminium or silicon by an ion
of similar size in the octahedral or tetrahedral sheets is known as isomorphous
substitution. It is possible for aluminium (Al 3+ ) to replace some of the silicon (Si 4+ ) in the
tetrahedral sheets. Similarly magnesium (Mg 2+ ), iron (Fe 2+ or Fe 3+ ) and calcium (Ca 2+ )
may replace Al 3+ in octahedral sheets. When the replacing ion has a lower positive charge
than the ion it replaces, the clay mineral has a net negative charge. These substitutions
account for most of the negative charge in the 2:1 and 2:1 : 1 minerals, but only a minor
part in the 1:1 kaolinites. A second source of 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.
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 is usually given as milliequivalents per 100 g
soil. 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 is the
same as the atomic weight; for divalent cations it is half the atomic weight
Table 18.5 Size and swelling of clays

Surface area nanometres (m 2 g −1 )

Mineral Thickness Volume change

Montmorillonite 2 800 High

Illite 20 80 Medium

Chlorite 20 80 Medium