Diffusion of Exchangeable Cations in Vermiculite

Abstract

VERMICULITE consists essentially of mica-like silicate layers separated by double sheets of water molecules carrying exchangeable cations, the cations being located midway between the water sheets in octahedral co-ordination1. In certain instances, replacement of one type of interlayer cation by another causes swelling or shrinking of the crystal lattice perpendicular to the plane of the silicate layers. These changes in the c-dimension of the unit cell are readily observable by X-ray diffraction procedures; replacement of magnesium by strontium for example, involves an increase in interlayer distance of about 0.6A., corresponding to an increase in basal spacing from 14.4A. to 15.0A. When a flake of vermiculite in which interlayer magnesium has been partly replaced by strontium is examined by X-rays, 14.4A. and 15.0A. reflexions are observed together, with the latter growing in intensity at the expense of the former as the replacement proceeds (Fig. 1).