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Quasi-Crystals in Na-Ca Systems¹

ABSTRACT

Drying of Ca-clays pushes platelets into closely oriented bundles, or quasi-crystals, which maintain their cohesiveness when rewetted.

Exchange capacity measurements on previously dried Casystems using recommended techniques were 20% lower in bentonites and 40% lower in soils than the actual exchange capacities of these materials. These underestimates were attributed to inadequate equilibration time and the slowness with which Ca ions diffuse out of the interplatelet exchange positions.

Dried Ca-bentonites maintain a 10% higher level of adsorbed Ca than nondried Ca-bentonites when both are equilibrated with the same mixed Na-Ca solution if the total salt concentration is 0.1N. If the total salt concentration is lowered to 0.045N, swelling pressures between platelets force quasi-crystals to disintegrate, and the quasi-crystal effect on Ca adsorption is greatly diminished. Adsorption energies of calcium ions on internal surfaces of quasi-crystals are apparently higher than on external surfaces and consequently the ratio of Ca/Na is greater on internal than on external surfaces.

Hysteresis loops in swelling pressures curves recorded at increasing and decreasing Na-saturation are apparently indicative of quasi-crystals. Swelling pressure curves for dried Ca-clays exhibit hysteresis loops which eventually disappear as crystals are destroyed by repeated Na-saturations. Nondried Ca-clays exhibit no hysteresis loops when confining pressures are low.

¹ Contribution from Colorado State University and Northern Plains Branch, SWC, ARS, USDA. Scientific Series no. 1305.

² Research Assistant, Colorado State University; and Professor of Soils, Colorado State University, and Soil Scientist, USDA, Fort Collins, Colo.

Received for publication June 21, 1968. Accepted for publication December 30, 1968.