THE INFLUENCE OF IONIC SUBST ITUT ION ON THE HYDROTHERMAL STABIL ITY OF MONTMORILLONOIDS 1 By

Both from the point of view of industrial use and in connection with theories concerning the origin of the montmorillonite minerals it is desirable to study the formation and stability of these minerals under high water pressures and elevated temperatures. Close control of the composition is possible by studying synthesized phases having the desired composition. Using synthetic mixtures one can also study the variation in properties as a function of the ions substitu-ting in the different positions in the lnontmorillonite lattice. The experimental techniques used have been described in some detail by Roy and Osborn (1952) and Roy and Roy (1955). Hydrothermal runs were at least two weeks long and all products were characterized by x-ray diffraction patterns and, where significant, by electron microscopy. The "crystallinity " of the syn-thetic phases was always as good as or better than most natural montmoril. lonoids. The results are based on data from about 550 runs. One of the main purposes was to supplement the lower-temperature phase equilibria of the system MgO-A120~-SiOz-H20, studied by Roy and Roy (1952