Dear Mahmoud, 

as indicated by the two previous answers, bentonite is generally used to describe clay in the context of sedimentary geology. Generally bentonite contains a significant fraction of smectite. Smectite is defined in clay mineralogy as a  2:1 clay - consisting of an octahedral sheet sandwiched between two tetrahedral sheets. Smectites may swell by the uptake of water in the clay interlayer spacing. As an example, during oil and gas exploration, serious problems may occur in shale formations, which contain a high fraction of smectites. These problems include borehole instability, associated with the uptake of water by smectites from the (water-based) drilling fluid. Smectites are comprised of layers of negatively charged aluminosilicate sheets held together by charge-balancing counter-ions such as Na+ and Ca2+. In the presence of water these cations tend to hydrate, thereby forcing the clay layers apart in a series of discrete steps. This causes the smectite to swell. If you are interested in more details regarding the molecular mechanism of smectite swelling, I am attaching a link for your information.

Article Monte Carlo Molecular Modeling Studies of Hydrated Li-, Na-,...

Dear Mahmoud,

Bentonite is a term mainly used to describe clay in its sedimentary environment and as a term for smectitic clay mined and sold in bulk and industrial applications such as in drilling muds. It was named after the Benton Shale near Rock River, Wyoming in the USA.

Bentonite is considered by clay mineralogists to be a impure clay that consists mainly of the smectite montmorillonite in terms of its mineralogy. However there are bentonites that have considerable amounts of illite and/or kaolinite. Often the dominent exchangeable cation (K, Na, Ca) is added to the name.

Cheers, Theo

True bentonite is a clay rock made from volcanic ashes. Bentonite is mainly made of smectite, but you may also find carbonates, pyrite, quartz in it. Sometimes, the term bentonite refers to a clay rich material whatever its origin.

Smectite is one of the many kinds of clays which has the property to swell in contact with water. Smectite is also a big family.

Bentonite is one of the mineral components of drilling muds, because of its high swelling clay fraction. It gives specific properties (rheology, lubricant) to the mud.

Dear Mahmoud, 

as indicated by the two previous answers, bentonite is generally used to describe clay in the context of sedimentary geology. Generally bentonite contains a significant fraction of smectite. Smectite is defined in clay mineralogy as a  2:1 clay - consisting of an octahedral sheet sandwiched between two tetrahedral sheets. Smectites may swell by the uptake of water in the clay interlayer spacing. As an example, during oil and gas exploration, serious problems may occur in shale formations, which contain a high fraction of smectites. These problems include borehole instability, associated with the uptake of water by smectites from the (water-based) drilling fluid. Smectites are comprised of layers of negatively charged aluminosilicate sheets held together by charge-balancing counter-ions such as Na+ and Ca2+. In the presence of water these cations tend to hydrate, thereby forcing the clay layers apart in a series of discrete steps. This causes the smectite to swell. If you are interested in more details regarding the molecular mechanism of smectite swelling, I am attaching a link for your information.

Article Monte Carlo Molecular Modeling Studies of Hydrated Li-, Na-,...

Hi! Just to complete the previous (all correct) answers: in Slovakia, a clay is classified as bentonite if the smectite content is 70% or more.

Renata Adamcova , how the clay is classified if the smectite is less than 70% ?

Bentonite is a naturally occurring material composed predominantly of the clay mineral smectite.  Most bentonites are formed by the alteration of volcanic ash in marine environments and occur as layers sandwiched between other types of rocks.  The smectite in most bentonites is the mineral montmorillonite.

Bentonite is mainly made of smectite also formed by the alteration of volcanic ash, while smectite has a high expansion (swelling) capability in the presence of H2O.