Stevensite, a magnesium end member of trioctahedral montmorillonites, contains a variable amount of interlayer water molecules. The elimination of these interlayer water molecules on heating accompanied with some kinds of structural changes was traced in the experiments with a high temperature X-ray diffractometer and a thermobalance. Dehydrated stevensite generally has an interstratified structure composed of two or three of the fundamental layers having thicknesses of 15.4 A, 12.5 A, 9.7 A and 9.5 A. The 15.4 A later has double sheets of hexagonally arranged water molecules in each interlayer position, and each water sheet is composed of about five water molecules per unit mesh of the ab plane (a=5.2 A and b=9.1 A). The 12.5 A layer has a single water sheet, equivalent to one of the components in the double sheets of the 15.4 A layer, in each interlayer position. The 9.7 A layer has a structure similar to that of mica, and has a sheet of two water molecules per above-mentioned mesh in each interlayer position. The 9.5 A layer has a talc-like structure and does not contain interlayer water. During dehydration several kinds of regular structures appear. They are as follows in the order of decreasing hydration degree: 15.4 A type, composed only of 15.4 A layers;
25.0 A type, having regular sequence of 15.4-9.7A layers;
22.0 A type, having regular sequence of 12.5 A-9.7 A layers;
84.0 A type, having perhaps regular sequence of 12.5 A-9.7 A-9.7 A-9.7 A-9.7 A-9.7 A-12.5 A-9.7 A layers;
9.7 A type, composed only of 9.7 A layers;
19.2 A type, having regular sequence of 9.7 A-9.5 A layers; and
9.5 A type, composed only of 9.5 A layers.
Random interstratifications occur between two adjacent regular structures.
Beyond the formation of the structure of the 9.5A type, the layer structure is unstable, and it finally breaks down into enstatite and silica.
From the knowledge of structures and modes of interstratifications, dehydration mechanism of stevensite, reversible and irreversible, is also discussed.
hand, the structures of the irreversible dehydration stage have no 5-H₂O sheet and thus potential bonds which are strong enough to attract other 5-H₂O sheets can not be expected.
Though many problems remain still unproved, it can be concluded that the dehydration of stevensite in the interlayer positions is quite continuous and that the phenomenon is explained by the continuous change of modes of interstratifications which occur among fundamental layers of several kinds. Such interstratifications take place with some regularities by eliminating the interlayer water sheets and by changing the types of the interlayer water sheets, and become regular in some cases and random in the other.