tetrahedral compound: A compound in which four atoms or groups situated at the corners of a tetrahedron are linked by covalent bonds to an atom at the centre of the tetrahedron. For example, silicon is covalently linked to four oxygen atoms in a tetrahedral silicon.

tetrahedral sheet: Sheet of horizontally linked, tetrahedral-shaped units that serve as one of the basic structural components of silicate clay minerals. Each unit consists of a central four-coordinated atom (e.g. Si, Al, or Fe) surrounded by four oxygen atoms that, in turn, are linked with other nearby atoms (e.g. Si, Al, or Fe), thereby serving as inter-unit linkages to hold the sheet together.

octahedron: A polyhedron with eight triangular faces. In a regular octahedron all eight triangles are congruent equilateral triangles.

octahedral compound: A compound in which six atoms or groups situated at the corners of a octahedron are linked by covalent bonds to an atom at the centre of the tetrahedron. For example, aluminum is covalently linked to six oxygen or hydroxyl ions in octahedral aluminum.

octahedral sheet: Sheet of horizontally linked, octahedral-shaped units that serve as one of the basic structural components of silicate clay minerals. Each unit consists of a central six-coordinated metallic atom (e.g. Al, Mg, or Fe) surrounded by six hydroxyl groups that, in turn, are linked with other nearby metal atoms (e.g. Al, Mg, or Fe), thereby serving as inter-unit linkages to hold the sheet together.

phyllosilicates: Phyllosilicate minerals have layer structures composed of shared octahedral and tetrahedral sheets.

plane (of atoms): A flat (planar) array of atoms of one atomic thickness. Example: plane of basal oxygen atoms within a tetrahedral sheet.

sheet (of polyhedra): Flat array of more than one atomic thickness and composed of one level of linked coordination polyhedra. A sheet is thicker than a plane and thinner than a layer. Example: tetrahedral sheet, octahedral sheet.

layer: A combination of sheets in a 1:1 or 2:1 assemblage.

interlayer: Materials between structural layers of minerals, including cations, hydrated cations, organic molecules, and hydroxide octahedral groups and sheets.

unit structure: The total assembly of a layer plus interlayer material.

basal spacing: Distance between similar faces of adjacent layers.

Concepts

Fig. 6.3. Shape of silicon tetrahedron and aluminum octahedron (Kohnke, 1968).

The tetrahedral sheet is composed of silicon bounded to four oxygen atoms. Each unit consists of a central four-coordinated atom (e.g. Si) surrounded by four oxygen atoms that, in turn, are linked with other nearby atoms (e.g. Si), thereby serving as inter-unit linkages to hold the sheet together. The aluminum octahedral sheet is composed of aluminum bounded to six oxygen or hydroxyl ions. Each unit consists of a central six-coordinated metallic atom (e.g. Al) surrounded by six hydroxyl groups that, in turn, are linked with other nearby metal atoms (e.g. Al), thereby serving as inter-unit linkages to hold the sheet together.

Fig. 6.4. The structure of kaolinite: sheets of silicon tetrahedra and aluminum octahedra are linked by shared oxygen atoms (after Singer and Munns, 1996)

Phyllosilicate minerals have layer structures composed of shared octahedral and tetrahedral sheets. The silicon and aluminum layers are held together by share chemical bonds. Kaolinite has one layer of silicon atoms and one layer of aluminum layers in 1:1 structure (Fig. 6.4). It is non-expanding clay mineral and different layer are held together by hydrogen bonding which occurs between the plane of basal oxygen atoms within a tetrahedral sheet and the plane of hydroxyl group within the octahedral layer. The formula for kaolinite is (Si₄)^IV(Al₄)^VIO₁₀(OH)₈ which indicates that there is no substitution of Si⁴⁺ with Al³⁺ in the tetrahedral layer and no substitution of Al³⁺ with Mg²⁺, Zn²⁺, Fe²⁺, Ca²⁺, Na⁺, or K⁺ in the octahedral layer. In order to calculate the net charge of kaolinite one has to remember that the charge of Si is +4, Al is +3, oxygen is -2 and OH is -1. Thus net layer charge of kaolinite is:

= [4(+4)] + [4(+3)] + [10(-2)] + [8(-1)]
= 28 - 28
= 0

In nature, kaolinite has a small net negative change because the clay crystals have broken edges (Fig. 6.5).

Fig 6.5. Broken edge of a kaolinite crystal showing oxygens as the source of negative charge (Adapted N.C. Brady and R.R. Weil, 1996.)

Smectite minerals have three layers with the aluminum atoms lying between two layers of silicon atoms in a 2:1 structure, sharing the valencies of their oxygen atoms (Fig. 6.6). Montmorillonite, a type of smectite, has two layers of silicon atoms and one layer of aluminum layers in 2:1 structure (Fig. 6.6). It is expanding clay mineral and different layers are held together by bonding between divalent cations and water with basal oxygen atoms of the tetrahedral sheets. The formula for montmorillonite is (Si_7.8Al_0.2)^IV(Al_3.4Mg_0.6)^VIO₂₀(OH)₄. The formula indicates that there is substitution for Si⁴⁺ by Al³⁺ in the tetrahedral layer and for Al³⁺ by Mg²⁺ in octahedral layer. In order to calculate the net charge of montmorillonite, one has to remember that the charge of Si is +4, Al is +3, Mg is +2, oxygen is -2 and OH is -1. Thus net layer charge of montmorillonite per unit cell is:

= [7.8(+4)] + [0.2(+3)] + [3.4(+3)]+[0.6(+2)] + [ [20(-2)] + [4(-1)]
= 43.2 - 44.0
= 0.8 charge/unit cell

Fig. 6.6. Structure of montmorillonite (smectite): it is built of two sheets of silicon tetrahedra and one sheet of aluminum octahedra, linked by shared oxygen atom. Smectite is a 2:1 mineral (after Singer and Munns, 1996).

In Fig. 6.6, there are three isomorphous substitutions: two in the tetrahedral layers and one in the octahedral layer. The formula for smectite shows that there is greater isomorphous substitution in the octahedral layer than in the tetrahedral layer. Also, the amount of water present in the interlayer of montmorillonite results in swelling under wet conditions and shrinking in dry conditions. The net negative charge of montmorillonite has to be satisfied by cations, which swarm around negatively charged mineral.

In contrast to montmorillonite, illite is a 2:1 clay mineral with potassium (K) in the interlayer that restricts shrinking and swelling (Fig. 6.7). Potassium ions bind to the oxygen plane of the basal tetrahedral layers of adjacent units.

Fig. 6.7. Structure of fine mica, illite (after Singer and Munns, 1996).

The formula for illite is (Si_6.4Al_1.6)^IV(Al₄)^VIO₂₀(OH)₄K_1.4M_0.2⁺. There is no substitution for Al in the octahedral layer but there is significant substitution in the tetrahedral layer. The potassium (K) and metal (M) ions are present to satisfy the negative charge and will not be used in the calculation of negative charge. Thus net layer charge of illite per unit cell is:

= [6.4(+4)] + [1.6(+3)] + [4(+3)] + [ [20(-2)] + [4(-1)]
= 42.2 - 44.0
= 1.6 charge/cell

The charge is satisfied by K ions in the interlayer space and other cations on the exchange sites of this mineral.

In nature, 2:1 type of clay minerals without isomorphous substitutions are found. In case of pyrophyllite, there is no substitution of Si⁴⁺ with Al³⁺ in the tetrahedral layer and no substitution of Al³⁺ with Mg²⁺, Zn²⁺, Fe²⁺, Ca²⁺, Na⁺, or K⁺ in the octahedral layer. The formula for pyrophyllite is (Si₈)^IV(Al₄)^VIO₂₀(OH)₄. The net layer charge is:

= [8 (+4)] + [4(+3)] + [20(-2)] + [4(-1)]
= 44 - 44
= 0

Similarly, the formula for talc is (Si₈)^IV(Mg₆)^VIO₂₀(OH)₄. The net layer charge is:

= [8(+4)] + [6(+2)] + [20(-2)] + [4(-1)]
= 44 - 44
= 0

Isomorphous substitution is the replacement of one atom by another of similar size in a crystal lattice without disrupting or changing crystal structure of the mineral (Table 6.1).

Table 6.1. Ionic radii of elements common in silicate clays and their location in the crystal lattice (adapted from Brady and Weil, 1996).

Table 6.2. Comparative properties of common silicate clay minerals (Adapted from Brady and Weil, 1996).