Bentonite is a type of clay that is often formed from the alteration of volcanic ash and is composed primarily of smectite minerals, usually montmorillonite.They are composed of single crystals, most of which have the largest dimensions of less than 2 µm. Smectite crystals themselves are three layered clay minerals. They consist of two tetrahedral layers and one octahedral layer.In tetrahedral layers of montmorillonite composed of [SiO4] – the tetrahedra enclose the octahedral [M(O5,OH)] layer.Mainly Al, Mg, but Fe is also often found.
Silicate layers have a small negative charge that is compensated by exchangeable ions in the intercrystalline region. The charge is so weak that cations (in their natural form, mainly Ca2+, Mg2+ or Na+ ions) can be adsorbed by their hydrate shell in this regio

The degree of hydration causes intercrystalline swelling.

Bentonites, depending on the nature of their origin, contain a variety of secondary minerals in addition to montmorillonite. These minerals may include quartz, feldspar, calcite and gypsum. The presence of these minerals can affect the industrial value of a deposit and increase or decrease its value depending on the application.

Physical properties of bentonite:

1- Bentonite has strong colloidal properties and its volume increases several times when in contact with water and creates a gelatinous and viscous liquid. Its properties are used in the paint industry by producing Benton Gel.
  2-Water absorption properties
  3- High viscosity
  4- High plasticity
  5- Thixotropy
Excavated bentonite is quite solid, even at approximately 30% moisture content.
This material is first crushed and activated by adding soda ash (Na2CO3) if necessary. The bentonite is subsequently dried (air and/or forced drying) to a moisture content of approximately 15%.
Depending on the final application, bentonite is either sieved (in the form of granular bentonite) or ground (in the form of powdered bentonite and in the form of ultrafine powder).
For certain applications, bentonite is treated by removing associated gangue minerals, or with acids to produce acid-activated bentonite (bleaching earths), or with organic matter to produce organic clays.
Two types of bentonite are known and the use of each depends on specific physical properties.
  1- Sodium bentonite
  2-Calcium bentonite
1-Sodium bentonites absorb large amounts of water and swell up to several times their initial volume, creating a permanent suspension of gel-like masses. These have been used to seal dams. In the bonding of foundry sands, asbestos and mineral wool. as drilling mud; In Portland cements and concretes, ceramics, emulsions, insecticides, soaps, pharmaceuticals and paints. in paper production; to clarify water, fruit juices and alcoholic beverages; and as a water softener to remove calcium from hard water.
2-Calcium bentonites are non-swelling and break down into a fine grain that is widely used as absorbent clay, sometimes called fuller’s earth.