Structure Of Illite

Alumina has an octahedral structure consisting of an aluminum atom with six oxygen atoms arranged in an octahedron around it. These alumina octahedra are then joined in a structure which is replicated to form a sheet or layer. The structure is the same as the mineral gibbsite [AL2(OH)6] (see Figure 4). These sheets of alumina and silica alternate to form the various clays. The clays we are most concerned with are either two-layer or three-layer clays.

Figure 4

Structural Diagram of a Single Alumina Octahedron and Gibbsite

Figure 4

Structural Diagram of a Single Alumina Octahedron and Gibbsite

Gibbsite Structure

Most of the clays we encounter either as drill solids or as commercial clays have a platelike shape. The particles may be several microns wide, but only a few Angstroms thick. Since a micron (micrometer) is 1 x 10-6 m and an Angstrom (A) is 1 x 10-10 m, a clay particle will be roughly 10,000 times wider than it is thick. It is this form, in the case of bentonite, which makes it beneficial in reducing filtrate loss.

The principal clays are:

  • Kaolinites
  • Illites
  • Chlorites
  • Smectites
  • Attapulgite and Sepiolite Kaolinites

Kaolinites are a two-layer clay composed of a tetrahedral silica sheet and an octahedral alumina sheet (see Figure 5). The silica sheet is oriented so that the tips of the tetrahedra are in the same plane as the oxygen or hydroxyl groups on the alumina sheet. The kaolinite particles are held together by hydrogen bonding and the spacing between layers is about 2.76 A. The hydrogen bonding is strong enough to exclude water from the clay surface; consequently, kaolinites are considered non-swelling clays. The cation exchange capacity of Kaolinites is typically 3-15 milliequivalents (meq)/100 g.

Figure 5 Lattice Structure of Kaolinite

Figure 5 Lattice Structure of Kaolinite

Bonding Gibbsite Layers

Illites

Illites are hydrous micas three layer clays which structurally resemble montmorillonites. Illites have no expanding lattice; therefore, no water can penetrate between the layers. They are composed of an alumina octahedral layer sandwiched by two silica tetrahedral layers (see Figure 6). Some of the silicon atoms in the illite structure are replaced by aluminum atoms. The resulting charge discrepancy is balanced by the association of potassium ions between layers. In some Illites the substitution of silicon by aluminum may be lower and the potassium may be replaced by divalent cations such as calcium or magnesium. In these cases the Illites may exhibit swelling tendencies similar to montmorillonites. The cation exchange capacity of Illites is 10-40 meq/100 g.

Figure 6 Structure of Illite

Figure 6 Structure of Illite

Chlorite Structure

Chlorites

Chlorites are three-layer clays separated by a layer of brucite (see Figure 7). There is a strong bonding between layers and for this reason chlorite is a non-swelling clay. The cation exchange capacity for chlorites is 10-40 meq/100 g.

Illite Structure
Rev. 6/94 4-14

Smectites (Montmorillonites)

Smectites are a family of three-layer clays of which montmorillonites are members. They consist of an alumina octahedral layer sandwiched between two silica tetrahedral layers (see Figure 8).

Figure 8 Structure of Smectites

Figure 8 Structure of Smectites

Structure Illite

The aluminum atoms in the central layer may be replaced by magnesium or iron atoms causing a charge imbalance. This imbalance is countered by the association of positive cations at the particle surface. These cations may be monovalent, sodium for example (see Figure 9), or divalent such as calcium.

Figure 9

Structure of Sodium Montmorillonite

Figure 9

Structure of Sodium Montmorillonite

Illite Bond

The character of the exchangeable cation influences the extent to which the montmorillonites will swell. The divalent cations, because of the extra charge, tend to associate with adjacent particles and consequently, restrict swelling of the clay. For this reason calcium montmorillonite is a poorer viscosi-fier than sodium montmorillonite. Due to their structure, the bonds between particles are weaker than other clays which adds to the ability of the montmorillonite to hydrate. This is the principal reason sodium montmorillonite is the most common commercial clay. The cation exchange capacity for smectites is 60-150 meq/100g.

Attapulgite and Sepiolite

Two other commercial clays, attapulgite and sepiolite, are used in special situations in which montmorillonite will not perform. These clays differ in structure from the more common clays in that they are elongated rod-shaped particles. Although there is water associated with these clays, they do not hydrate. These clays viscosity by shearing which causes fracturing along the axis of the rods and exposes charges which cause the rods to attract each other. Since these clays are shear dependent, they are as effective in saltwater as in freshwater. Sepiolite has the added advantage of being very temperature stable. Because they yield through shearing, little or no viscosity increase will be seen in the pits since the shearing of the hopper and mixers is usually insufficient to cause the clay to yield. It may take several trips through the bit before maximum benefit is obtained from the clay. For this reason, it is easy to overtreat with these clays in an effort to raise the viscosity of the mud. These clays provide little filtration control because of their shape. It is usually necessary to add filtration control agents when using these clays.

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