**2. Bentonite**

Bentonite is an important rock of clay found in nature. It is an important source of montmorillonite in nature. It is a rock formed of highly colloidal and plastic clays mainly composed of montmorillonite [2]. In addition to montmorillonite, bentonite may contain some amount of crystalline quartz, cristobalite, and feldspar.

The variety of bentonite applications are the result of its useful and interesting chemical and physical properties. This range of properties include rheology, sorbent effects, plasticity and lubricity, high dry bonding strength, high shear and compressive strength, impermeability, and low compressibility [11].

Natural bentonite particles are indistinguishable from kaolin clay minerals viewed under scanning electron microscope; however, the main difference indicated was thickness. Sodium or potassium salts of bentonite exfoliate into thin plates that could be of 1 nm in thickness theoretically [12].

In general, the clay minerals based on bentonite may exhibit the properties of thixotropic gel formation with water, high water absorption, and high cation exchange capacity (CEC). These properties could be varied in clay minerals depending upon the nature of interstitial water and exchangeable cations in the interlayer space.

Bentonite, which mainly contained montmorillonite, was discovered in about 1890 in USA, and the name "bentonite" is associated with the name of an American geologist for the one time Fort Benton in the eastern Wyoming Rock Creek area, USA [2].

Montmorillonite derived from bentonite may contain sodium or calcium. Sodium montmorillonite is the main fraction in bentonite found in combination with 10–20% of various minerals including feldspar, calcite, silica, and gypsum [10].
