Abstract: In 1847 the name “montmorillonite” was given to a rose-red, clay-like mineral forming nests in a brown clay at Montmorillon, France. The first analysis, reported by Salvetat, gave 49.4 percent to SiO2, 19.7 percent to Al2O3, 0.8 percent to Fe2O3, 0.27 percent to MgO, 1.5 percent to CaO, 1.5 percent to alkalies, and 25.67 percent to H2O. Much later a substance which was originally called “taylorite” after William Taylor, who was the first to draw attention to it, was finally designated as “bentonite” because it was first found in the Fort Benton series of rocks in Montana. The most characteristic mineral component of bentonite is crystalline and definitely montmorillonite.
The chemical analyses of Wyoming bentonite and hectorite are compared with those of three different calcium bentonite deposits. The importance of more up-to-date crystal chemical considerations is discussed. The electrophoretic properties of bentonites are explained on the basis of the available counter ions. The formation of thixotropic bentonite gels is discussed in detail and proof is offered to show why calcium bentonites do not give thixotropic systems.
The base-exchange capacities of sodium-bentonites and of calcium-bentonites and their differences are explained on the basis of two different methods of determining this phenomenon.
Ultra- and electron photomicrographic studies reveal the difference between montmorillonites and the various types of bentonites now known. Most striking is the fact that most bentonites, specifically calcium bentonites, contain several different types of clayey minerals.
The most probable cause of the formation of bentonitic clays as well as of true montmorillonite is referred to and discussed.