Geoscience Reference
In-Depth Information
are stacked into layers, the bonding between the layers, and
the substitution of other ions for Si and Al.
Properties of clay minerals
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 19.5
,
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
Types of clay minerals
Figure 19.5
gives a schematic representation of the
structure of five common clay minerals.
Kaolinite
is made
of a silica sheet and an alumina sheet sharing a layer of
oxygen atoms. The layers are held together by strong
hydrogen bonding and the structure is non-expanding.
Illite or clay mica has repeating layers consisting of one
alumina sheet sandwiched between two silica sheets. The
layers are firmly bonded together by potassium (K) ions,
which are just the right size to fit into the hexagonal holes
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 19.6
illustrates how the alumina and silica sheets condense
together to give the structures of kaolinite and montmo-
rillonite.
Kaolinite
1:1
non-expanding
Silica
Alumina
Silica
Alumina
Silica
Weak
bonds
(water
molecules)
Vermiculite
2:1
expanding
Silica
Silica
Alumina
Alumina
Firm
bonds
(Potassium
ions)
Silica
Silica
Illite (Mica)
2:1
non-expanding
K
K
K
K
Silica
Alumina
Silica
Silica
Silica
Alumina
Alumina
Silica
Silica
Chlorite
2:1:1
non-expanding
Montmorillonite
2:1
expanding
Firm
bonds
Weak
bonds
Brucite
Silica
Silica
Alumina
Alumina
Silica
Silica
