THE MICROSTRUCTURE OF 3 NA- THE IMPORTANCE OF PARTICLE GEOMETRY ON DEHYDRATION AND REHYDRATION( SMECTITES )

Recent pedological evidence of the widespread distribution of beidelli tes in soils indicates the need for a greater knowledge of the effect of charge location on the microstructural organization of Na+ smectite in gels. After equilibration at a suction pressure of 3.2 kPa before and after desiccation, TEM observations showed large differences betwe en a beidellite and both a low and high charge montmorillonite. Monola yers were rare; individual layers were instead organized in particles with larger interparticle distances. This has implication for theories relating swelling pressures to interlayer distances or surface areas and implies the need for a geometrical approach to the study of swelli ng in smectites. Location of isomorphous substitution in the tetrahedr al sheet of smectites results in an increased lateral extension of ove rlapping layers. This was reflected in a greater capacity to rehydrate after desiccation. Increased number of layers in particles were found with increasing surface charge density. The geometric organization of the particles is critical to the understanding of the ability of Nasmectite to hold water against an applied suction.