Agriculture Reference
In-Depth Information
positive surface hydrogens, and thus the two-face kaolinite can attract anions,
cations, water, and electrophilic and nucleophilic compounds.
When a particle of kaolinite is broken or at the edges of the particles, bonds
will be unsatisfied. This leaves negative charges, which attract cations. Thus
kaolinite has cation exchange capacity. Typically these charges are satisfied by
metal cations, particularly Ca
2+
,Mg
2+
,K
+
, and Na
+
. Other cations can be
attracted to these sites including positively charged organic molecules such as
quaternary amines and ammonium. Because the crystals are relatively large
and the charges develop only at the edges, the cation exchange of kaolinite is
small compared to that of other clays.
2.1.3.2
2 : 1 Clays—Fine-Grained Micas and Smectites
The 2 : 1 clays are smaller and much more complex than 1 : 1 clays. Because
they are smaller, they have a larger surface area and more edges. This results
in both increased cation exchange and adsorption. However, the adsorption
will be different from that occurring in kaolinite because the surfaces of the
particles are the same. In this clay cation exchange is not limited by edge
effects because of a phenomenon called
isomorphous substitution
, which
results in increased cation exchange, changes in the shape of the particles, and
changes in the way they interact with water and cations.
A first approximation to the makeup of 2 : 1 clays can be seen in Figure 2.3.
The particles consist of a sandwich of aluminum octahedral between two
sheets of silicon tetrahedral. One result of this arrangement is that both sur-
faces are made up of only oxygen. This means that both surfaces are slightly
negative and thus two particles repel each other if there is no positive or par-
tially positive species, such as the hydrogens on water molecules, between
them. In Figure 2.3 water is shown between the clay particles, which allow them
to come closer together, and in reality water molecules are always present
between the clay layers. As with all soil solutions, this water contains cations,
further enabling the particles to come together.
Another characteristic of 2 : 1 clays is isomorphous substitution, where
iso
means same and
morphous
means shape. During the formation of clay, cations
other than aluminum and silicon become incorporated into the structure. In
order for this to work and still ensure stable clay, the cation must be of about
the same size as either aluminum or silicon; hence the term
isomorphous
.
There are a limited number of cations that satisfy this requirement. For silicon,
aluminum as Al
3+
and iron as Fe
3+
will fit without causing too much distortion
of the clay structure. For aluminum, iron as Fe
3+
, magnesium as Mg
2+
, zinc as
Zn
2+
, and iron as Fe
2+
will fit without causing too much structural distortion
(see Figure 2.3).
The two 2 : 1 clay types are distinguished by the type of substitution in the
layers. Nonexpanding fine grained mica-type clay has isomorphous substitu-
tion of Al
3+
in the silicon tetrahedral sheet. Thus, in a tetrahedral sheet of fine
mica, aluminum may be substituted for a silicon atom. The expanding clays
