Clay minerals

The reactions to illustrate weathering of complex silicates during acid hydrolysis (eqns. 4.13 & 4.14) predict that clay minerals will be an important solid product and this is confirmed by looking at soils. Clay minerals are important constituents in most soils. These sheet silicates that are less than 2 mm (Section 4.2.3) are constructed of layers of atoms in tetrahedral and octahedral coordination, known as tetrahedral and octahedral sheets.

The tetrahedral sheets are layers of SiO4 tetrahedra which share three oxygens with neighbouring tetrahedra. These basal oxygens form a hexagonal pattern (Section 4.2.3). The fourth tetrahedral (apical) oxygen of each tetrahedron is arranged perpendicular to the basal sheet (Fig. 4.7a). The sheet carries a net negative charge.

The octahedral sheet is composed of cations, usually aluminium, iron or magnesium, arranged equidistant from six oxygen (or OH) anions (Fig. 4.7b). Aluminium is the common cation and the ideal octahedral sheet has the composition of the aluminium hydroxide mineral, gibbsite (Al(OH)3). Where octahedral sites are filled by trivalent aluminium, only two of every three sites are occupied to

Fig. 4.7 (a) A sheet of SiO4 tetrahedra linked via basal oxygens, with apical oxygens pointing upward. (b) Octahedra and the octahedral sheet: (i) the atoms packed together; (ii) the octahedron expanded; (iii) conventional representation of an octahedron; (iv) conventional representation of an octahedral sheet, showing aluminium equidistant between six hydroxyls — forming the mineral gibbsite.

Fig. 4.7 (a) A sheet of SiO4 tetrahedra linked via basal oxygens, with apical oxygens pointing upward. (b) Octahedra and the octahedral sheet: (i) the atoms packed together; (ii) the octahedron expanded; (iii) conventional representation of an octahedron; (iv) conventional representation of an octahedral sheet, showing aluminium equidistant between six hydroxyls — forming the mineral gibbsite.

Table 4.5 Simplified 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

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 combination 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.

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