Introduction to Soil Chemistry

d -
Si
O
O
O Al
O
O
H d+
O O
HO OH
O
HO
O
d -
Si
O
Al
O
O
O
O
OH
H d+
n
soil solids 27
2.1.3.1 1:1 Clay—Kaolinite
O
Si O
The 1:1 kaolinite clay is depicted in Figure 2.2. One surface is composed of
oxygens with lone pairs of electrons. These electrons are in p orbitals and thus
extend away from the oxygens into space. The oxygens are partially negative
as indicated by the symbol d - because they are more electronegative than the
surrounding atoms, namely, Si. The other surface is composed of —OH groups
where the hydrogens are partially positive as indicated by the symbol d + .The
electrons in the sigma bond between the oxygen and hydrogen are drawn
closer to the oxygen because it is more electronegative, thus leaving the hydrogen
slightly positive.
The consequence of these partial charges it that one surface of kaolinite is
compatible and attractive to the other surface. This results in increased stability
of kaolinite and the formation of relatively stable structures. Some kaolinite
particles can even be larger than the 0.002mm upper limit for clay! Both
surfaces also attract and hold water through these partial charges. The adsorptive
activity of kaolinite is associated with its surface electrons and partially
O
O
O
HO
O
HO
O
O
Si
O
Al
O
Al
O
O
O
O
O
O Si O
d
H
O
O
H +
d +
Si O
O
Figure 2.2. The left structure represents kaolinite, a 1:1; and the right, a 2:1 clay mineral. These
representations are intended to show surface groups, surface pairs of electrons, unsatisfied bonds,
and associations between clay particles. Note that clay structures are three-dimensional and these
representations are not intended to accurately represent either the three-dimensional nature or
the actual bond lengths; also, the brackets are not intended to represent crystal unit cells.
OH
d -
OH
d -
n