The Microstructure of Three NA+ Smectites: The Importance of Particle Geometry on Dehydration and Rehydration

Frederic Hetzel1, Daniel Tessier2, Anne-Marie Jaunet2 and Harvey Doner1
1 Department of Soil Science, University of California at Berkeley Berkeley, California 94720
2 Institut National de la Recherche Agronomique, Versailles, France

Abstract: Recent pedological evidence of the widespread distribution of beidellites 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 between a beidellite and both a low and high charge montmorillonite. Monolayers 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 swelling in smectites. Location of isomorphous substitution in the tetrahedral sheet of smectites results in an increased lateral extension of overlapping 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 Na+ smectite to hold water against an applied suction.

Key Words: Beidellite • Layer charge • Microstructure • Montmorillonite • Particle • Smectite • TEM

Clays and Clay Minerals; June 1994 v. 42; no. 3; p. 242-248; DOI: 10.1346/CCMN.1994.0420302
© 1994, The Clay Minerals Society
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