The main groups of crystalline materials that make up clays are the minerals kaolinite, illite, and
montmorillonite. Kaolinite has a structure that consists of one silica sheet and one alumina sheet bonded
together into a layer about 0.72 nm thick and stacked repeatedly (Figure 2.7a). The layers are held
together by hydrogen bonds. Tightly stacked layers result from numerous hydrogen bonds. Kaolinite is
common in clays in humid tropical regions. Illite consists of repeated layers of one alumina sheet sandwiched
by two silicate sheets (Figure 2.7b). The layers, each of thickness 0.96 nm, are held together by
potassium ions.
Montmorillonite has a structure similar to illite, but the layers are held together by weak van der
Waals forces. Montmorillonite belongs to the smectite clay family. It is an aluminum smectite with a