The beidellite-saponite series: an experimental approach

Abstract

The study of the octahedral Al-Mg series in smectites was conducted using synthesis methods at 200-250°C. All synthesized clays were tetrahedrally charged thus leading to a solid-solution between beidellite and saponite end-members. The TEM chemical microanalyses indicated that the Al-Mg series was continuous between these two end-members without chemical gap, at the clay particle scale (mean particle dimension about 1 μm and stacking of almost ten layers). The investigations by X-ray diffraction (at the scale of lattice coherences between 50 and 150 A) indicated three different kinds of interferential coherence domains: (1) a dioctahedral essentially aluminous one, (2) a trioctahedral essentially magnesian one and (3) an intermediate one. In most samples, two kinds of domains are associated. A sample with an intermediate octahedral chemistry was made up of particles with stacked layers having a constant chemistry and, thus, showing an apparent ditrioctahedral lattice. However, time studies show this special lattice to be metastable. Along the series and according to IR data, Al 3+ and Mg 2+ cations present a strong segregation tendency. The beidellite-saponite octahedral series which appears continuous at the clay particle scale is, in fact, made up of associated different lattices or clusters, as shown by the study at a finer scale. So, the true solid-solution presents a wide miscibility gap