Geoscience Reference
In-Depth Information
mutually repulsed. The same repulsion happens with all such triple layers, i.e.,
between T-O-T confi gurations. Provided that there is enough water available, water
molecules can simply penetrate into the space between the repulsed triple layers and
eventually form a fi lm with a thickness of several water molecules. However, there
are still outer edge to outer edge bonds of suffi cient force to hold bunches of about
10-30 triple layers together. At this point, we have explained that all volume changes
of smectites during their wetting (swelling) and during their drying (shrinkage) are
the direct result of changes of water fi lm thickness between T-O-T confi gurations.
Nevertheless, an interesting effect arises during drying after crossing a certain
boundary of water content reduction. At that diminishing threshold of water content,
the cohesion between triple layers is broken to such an extent that we are fi nally able
to observe with naked eye the appearance of fi ssures in a clay predominated by
smectites.
As water enters into the spaces between triple layers T-O-T, dissolved inorganic
compounds are introduced into this negatively charged domain. Cations of the inor-
ganic compounds, attracted to the fi eld of negative charges, behave as if they are
weakly bonded to the clay mineral. However, when the solution outside the mineral
is replaced by another solution containing another type of cation, the cations from
the former solution are replaced by those in the latter solution. Since this replace-
ment or exchange of cations occurs in the space between triple layers, they are
called exchangeable cations. Inasmuch as each clay mineral has a certain capacity
to retain exchangeable cations, this feature is denoted as cation exchangeable capac-
ity with the frequent abbreviation CEC. This feature or property is used as a simpli-
fi ed measure of soil fertility because it offers a numerical value of the retention of
possible plant nutrients commonly expressed as milliequivalent of hydrogen per
100 g of dry soil (meq/100 g). The term milliequivalent describes the weight of a
substance in milligrams divided by its valence. CEC is closely correlated to the
specifi c surface (m
2
/g). In montmorillonite the external specifi c surface makes up
about 20 % of the total surface, and 80 % of it belongs to the internal surface, i.e.,
the surface of T-O-T layers inside of the crystal of clay mineral. The total specifi c
surface accessible to water molecules is about 300-400 m
2
/g and CEC is in ranges
80-140 meq/100 g. Just for a rough imagination, if you have a handful of clay soil
that is 50 % montmorillonite, you have in your hand soil particles that have an inter-
nal surface area ranging between about 1,000 and 3,000 m
2
or approximately the
size of a family house garden! Kaolinite belongs to clay minerals with the smallest
value of the specifi c surface between about 5 and 15 m
2
/g, and its CEC is
3-15 meq/100 g. The values are again very approximate - they depend on the degree
of perfectness of crystal lattice and on the environmental conditions when the clay
mineral was formed.
Illite
is a clay mineral with a crystal lattice also confi gured by three sheets T-O-
T. When compared to smectites, illite's main difference is in the bonding of many
of individual triple layers together by the K
+
cation that fi ts perfectly between the
layers to balance the negative charge of the planar surfaces of triple layers that we
mentioned when we described the crystal lattice of montmorillonite. We consider
the K
+
in this position as fi xed and an integral part of its crystal lattice. If all such
