An Introduction to Environmental Chemistry

maintain electrical neutrality and the sheet is classified as dioctahedral (Table 4.5).

Where divalent cations fill octahedral sites, all available sites are filled and the

sheet is classified as trioctahedral (Table 4.5).

Combining these sheets gives the basic clay mineral structure. The combina-

tion allows the apical oxygen of the tetrahedral sheet and the OH groups lodged

in the centre of the hexagonal holes of the basal tetrahedral sheet to be shared

with the octahedral sheet (Fig. 4.8). The various clay mineral groups (Table 4.5)

result from different styles of arrangement and mutual sharing of ions in the tetra-

hedral and octahedral sheets.

4.5.1 One to one clay mineral structure

The simplest arrangement of tetrahedral and octahedral sheets is a 1 : 1 layering,

shown in Fig. 4.8 and developed more fully in Fig. 4.9. These 1 : 1 minerals com-

prise the serpentine-kaolin group of clay minerals, of which the mineral kaolin-

ite is probably the best known (Fig. 4.10). In kaolinite, the 1 : 1 packages are held

together by hydrogen bonds (Box 4.1), which bridge between OH groups of

the upper layer of the octahedral sheet and the basal oxygens of the overlying

tetrahedral sheet. The hydrogen bonds are strong enough to hold the 1 : 1 units

together, preventing cations getting between layers (interlayer sites). Isomor-

phous substitution (Box 4.6) in kaolinite is negligible.

4.5.2 Two to one clay mineral structure

The other important structural arrangement is a 2 : 1 structure, comprising an

octahedral layer, sandwiched between two tetrahedral sheets with apical oxygens

pointing inward on each side of the octahedral sheet (Fig. 4.11). The mutual

sharing of two layers of apical oxygens in the octahedral sheets implies a higher

oxygen : OH ratio in the structure of the 2 : 1 vs. the 1 : 1 octahedral sheets. All of

the other clay mineral groups share this structure, the most important being the

88 Chapter Four

Table 4.5Simplified classification of clay minerals. After Martin et al. (1991) with kind permission from the
Clay Minerals Society.

Layer
type Group Common minerals Octahedral character Interlayer material

1 : 1 Serpentine–kaolin Kaolinite Dioctahedral None

2 : 1 Smectite Montmorillonite Dioctahedral Hydrated
exchangeable
cations
True (flexible) mica Biotite Trioctahedral Non-hydrated
monovalent
cations
Muscovite, illite Dioctahedral
Chlorite Chamosite Trioctahedral Hydroxide sheet