Connecting a phyllosilicate sheet onto the front OH balls produces a dioctahedral
aluminosilicate sheet, as shown in Fig. 3.2c, where Si ions are placed (small black
balls) in the center of SiO 4 tetrahedrons (Fig. 3.2d). The reaction of the gibbsite sheet
and the phyllosilicate sheet can be formulated simply as
(OH) 2 Al-Front OH (gibbsite, Fig. 3.2(b))
þ SiO 1.5 O
À H
þ (one fourth of phyllosilicate unit cell coordinated with H
þ , Fig. 2.3)
¼ (OH) 2 AlÀOÀSiO 1.5 þ H 2 O
where an Al–O–Si bond is formed with the release of a molecule of H 2 O (Fig. 3.2e).
A front OH group remains at the center of the Si tetrahedron ring (Fig. 3.2c). Further
schematic simplification of the tetrahedral and octahedral sheets in Fig. 3.2e gives
Fig. 3.2f.
3.2.3 Major Layer Aluminosilicates in Soil
Using the simplification of the tetrahedral sheet and octahedral sheet shown as
Fig. 3.2f, the major layer aluminosilicates in soil are represented by six different
aluminosilicates in Fig. 3.3. These six layer aluminosilicates can be grouped as 1:1
and 2:1 types. The 1:1 type aluminosilicates have a stacking of one tetrahedral layer
and one octahedral layer, whereas the 2:1 type aluminosilicates have an octahedral
layer sandwiched by two tetrahedral layers. The major 1:1 type aluminosilicates in
soil are kaolinite and halloysite. Halloysite (1.0 nm) has an interlayer of water,
Tetrahedral sheet
Octahedral sheet
0.7
nm
1.0
1.4
1.0
1.4
1.4
+
H 2 O
K +
1:1 type
2:1 type
Kaolinite
Halloysite
Micaceous
Vermiculite
Smectite
Chlorite
: Hydrated cations
+
+
minerals
Fig. 3.3 Schematic of phyllosilicate clay minerals. Arrows show basal spacings
3.2 Construction of Layer Aluminosilicate Models
41