Kaolinite Clay

The basic unit of this type of clay is formed by atomic bond of the unsatisfied face of silica sheet and either face of aluminum sheet as seen in Figure 1.

Figure 1. Kaolinite clay formation

The bond between two sheets is strong and, also, it is the primary bond. However, the stack of two sheets (with thickness 7.2 Å [Angstrom]) is not a form of clay yet. Many layers of this basic kaolinite unit make a kaolinite clay particle. Figure 3.3 shows an electron photomicrograph of well-crystallized kaolinite clay particles.

Figure 2. Electron photomicrograph of kaolinite clay

From the picture, it can be estimated that the diameter of a particle is about 5 μm, and the thickness of the particle is about one-tenth of that (i.e., 0.5 μm). Thus, it is required to have about 700 layers of the basic unit to make a kaolinite clay particle in the picture. The bond between each basic silica and aluminum sheet unit is the one between exposed OH^- and satisfied O^2- and is called a hydrogen bond. This bond is not as strong as the previous atomic bond (primary bond) but much stronger than the bond between exposed O^2- and O^2- in case of montmorillonite clay, which will be discussed later. Hydrogen bond is categorized as a primary bond in many literatures, but it shall be noted that this is a marginally strong bond. Because of its nature of bonds within the kaolinite particle, this clay is rather stable, has less swelling and shrinking characteristics, and is less problematic.