Fig. 2. The crystalline structure of clay. (A) The simplest clay mineral is called kaolinite. The micelles of kaolinite consist of two different layers, one of silica and the other of alumina. Kaolinitic clay micelles are relatively large and are bound together tightly. The distance between the alumina and silica layers is rigidly fixed and does not increase when water is adsorbed on the surfaces of the individual micelles. The internal space is not available for surface reactions. Such clays do not shrink greatly when dry or expand much when hydrated. (13) Montmorillonite is more complex, and each micelle consists of an alumina layer sandwiched between two layers of silica. These clays are not bound together tightly, and they swell when wet because the hygroscopic surfaces between layers adsorb water and force the layers apart. All surfaces can adsorb water and minerals. (C) The micelles of illite consist of an alumina layer between silica layers, and adjacent layers are held together by potassium atoms. Surface adsorption can take place on the surfaces between crystals to a limited degree. Illite is less abundant than kaolinite or montmorillonite.