Extract from this site:
Bentonite is naturally occuring hydrated Aluminium Silicate. The most important use of Bentonite is based on its natural swelling property.one can surely say that Bentonite is most economic natural thickner available to industries.
It has ability to absorb water several times its wieght and give thickstropic fluid.
Chemically Bentonite is HYDRATED ALUMINIUM SILICATE.