Clays and Their Minerals

Clay needs special attention because of its small particle size. As discussed in the grain size distribution section, soils with their particle diameters less than 5 μm (2 μm in some classification systems) are classified as clay or clay-size particles. In such a small size, electrical interactive forces become more significant as compared to the physical frictional interactive forces in the case of larger grain soils (sand and gravel).

To understand various unique engineering behaviors of clay, it is most beneficial to study microstructures of clay particles first. The microstructural observation greatly helps to understand macrobehavior.

Figure 1.  Silica and aluminum sheets

In nature, basically there are three types of clay minerals-namely, kaolinite clay, illite clay, and montmorillonite clay. These clays have different atomic structures and behave differently and are all made of two basic atomic sheets- namely, silica tetrahedral sheets and aluminum octahedron sheets, as seen in Figure 1. Naturally abundant atom silica (Si) and aluminum atom (Al) occupy the center positions of the sheets, and oxygen atom (O^2-) and hydroxyl (OH^-) are strongly bonded to those core atoms, respectively. These bonds are either ionic or covalent, and actual bonds in silica and aluminum sheets are combinations of these two types of bonds.

Note that the ionic bond is due to exchange of orbiting electrons of two atoms such as Na+ (sodium ion) and Cl^- (chlorine ion) to make NaCl (sodium chloride = salt), and the covalent bond is due to sharing electrons in their orbits such as two H⁺ (hydrogen ions) to form H₂ (hydrogen gas). These atomic bonds are very strong and can never be broken by ordinary physical forces. They are called the primary bonds.

A silica tetrahedral sheet is symbolized with a trapezoid, of which the shorter face holds electrically unsatisfied oxygen atoms and the longer face holds electrically satisfied oxygen atoms. An aluminum octahedron sheet is symbolized with a rectangle with top and bottom faces having the same characteristics of exposed hydroxyl (OH^-).

In most instances in nature, sheets are further bonded together, basically due to the unsatisfied face of a silica sheet to form various clay minerals.