Etude en microscopie et microdiffraction électroniques de l'hectorite, de la montmorillonite et de la nontronite

Abstract

From among the minerals of the montmorillonite group, hectorite, montorillonite and nontronite have been studied by means of electron microscopy and selected area diffraction methods. It was possible to demonstrate that hectorite has a strong resemblance to the ideal structure attributed to the layer and to determine the symmetry of the projection of the basic layer on to its plane (bidimensional space group) in the case of montmorillonite and nontronite. The symmetry c 1 m 1 (equivalent to the tridimensional group C 2) was assigned to montmorillonite (WYOMING mineral). Nontronite corresponds to the symmetry c 2 m m (equivalent to the tridimensional group C 2/m). In the latter case it was shown that the structure was obtained from the ideal structure by a slight rotation of the tetrahedrons of the SiO2 layer and by a certain distorsion of the octahedral layer . The investigations by means of selected area diffraction also made it possible to establish that the particles observed under the electron microscope were in certain cases aggregates of basic particles in reciprocal orientation. Hence, certain varieties of montmorillonite (e. g. CAMP BERTEAUX mineral) show the presence of particles consisting of a mosaic of small elements in definite reciprocal orientation at a multiple of 60°. Whereas hectorite appears to be formed by elongated basic particles along the axis a, associated in a parallel direction with each other with a fluctuation of orientation around the axis a.