Abstract Clay minerals have long been recognized as playing an important role in industry, such as foundry binding sands, paper production, ceramics and refractory materials, civil engineering, waste management and many more. Clay surfaces act as Lewis and Brensted acids and are naturally occurring source of inorganic catalysts that facilitate a number of organic reactions (e.g., Adams and Clapp, 1986 ; Brown, 1994 ; Breen et al., 1995a , 1997). Clays are important in areas of environmental concern, for example, in controlling migration of pesticides through soil (e.g., Sawhney, 1996 ; Johnson, 1996; Stucki, 1997 ; Moronta et al., 2002 ; Sheng et al., 2002 ). Various chemical and physical-mechanical properties, such as cation exchange capacity (CEC), porosity, and permeability, of the raw and/or modified materials are frequently studied to optimize the utilization of clay minerals. However, the occurrence of new deposits as well as new industrial applications of these minerals, mean that new information on both raw and modified clays will always be required. Bentonites are exceptional among naturally occurring clay ores due to properties such as high cation exchange capacity, high surface area, the ability to swell in water and their capacity to sorb large quantities of inorganic and organic chemicals. They are plastic, impermeable, and have a high viscosity when suspended in water. The largest volumes of bentonites for industrial uses are in foundry moldings, geological explorations as a component for drilling fluids, in civil and geological engineering as barriers or liners under the reservoirs and waste disposal sites and in chemical industry for the manufacture of materials with sorptive and catalytic properties ( Grim, 1968 ; Grim and Guven, 1978 ; Kendall, 1996 ). A substantial amount of bentonites is currently used as adsorbents for pet’s litters. The main components of bentonites are minerals from the smectite group, which substantially affect the properties (CEC, swellability and surface area) and uses of the clay ore. Smectites are formally defined as the group of phyllosilicates with layer charge between 0.4 and 1.2 e − per unit cell, arising from the non-equivalent substitution of central atoms in layers with lower valence cations. The structure of smectites consists of 2:1 layers formed by two tetrahedral sheets linked with an octahedral sheet. Tetrahedral sheets contain normally Si, Al or Fe as central atoms. Two types of octahedral sheets occur in smectites: the dioctahedral type (e.g., in montmorillonite or nontronite) where two-thirds of the octahedral sites are occupied mainly by trivalent cations, e.g., Al (III) or Fe (III), and the trioctahedral type (e.g., in hectorite or saponite), with most of the sites occupied by divalent cations, such as Mg (II). The negative charge of the layers is balanced by hydrated exchangeable cations (mostly Ca 2+ , Mg 2+ , Na + ) located in the interlayer space between adjacent sheets. The extent of hydration varies greatly and depends on many factors related to the composition of the layers and the nature of interlayer cations.