Agriculture Reference
In-Depth Information
O
O
Si
O
O
d
-
O
O
HO
OH
Si
O
Al
O
O
O
O
OH
HO
Al
O
O
H
d
+
Si
O
O
O
O
d
+
d
-
H
O
d
-
Si
O
O
H
d
+
O
d
-
O
O
O
O
Si
HO
OH
O
Al
O
O
HO
OH
Al
O
O
H
d
+
O
n
O
Si
O
O
O
n
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.
2.1.3.1
1 : 1 Clay—Kaolinite
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 hydro-
gen 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 sta-
bility of kaolinite and the formation of relatively stable structures. Some kaoli-
nite particles can even be larger than the 0.002 mm upper limit for clay! Both
surfaces also attract and hold water through these partial charges. The adsorp-
tive activity of kaolinite is associated with its surface electrons and partially
