THE BEIDELLITE-SAPONITE SERIES - AN EXPERIMENTAL APPROACH

The study of the octahedral Al-Mg series in smectites was conducted us ing synthesis methods at 200-250-degrees-C. All synthesized clays were tetrahedrally charged thus leading to a solid-solution between beidel lite and saponite end-members. The TEM chemical microanalyses indicate d that the Al-Mg series was continuous between these two end-members w ithout chemical gap, at the clay particle scale (mean particle dimensi on about 1 mum and stacking of almost ten layers). The investigations by X-ray diffraction (at the scale of lattice coherences between 50 an d 150 angstrom) indicated three different kinds of interferential cohe rence domains : (1) a dioctahedral essentially aluminous one, (2) a tr ioctahedral essentially magnesian one and (3) an intermediate one. In most samples, two kinds of domains are associated. A sample with an in termediate octahedral chemistry was made up of particles with stacked layers having a constant chemistry and, thus, showing an apparent ditr ioctahedral lattice. However, time studies show this special lattice t o be metastable. Along the series and according to IR data, Al3+ and M g2+ cations present a strong segregation tendency. The beidellite-sapo nite octahedral series which appears continuous at the clay particle s cale 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.