Various workers in recent years have found that the Gouy‐Chapman theory of the diffuse electrical double layer provides useful predictions of swelling behavior of certain 2:1 clays in dilute solutions. Calcium‐saturated montmorillonite was apparently an exception. It swelled appreciably in dilute CaCl2 solutions, but much less than predicted by theory, and to an extent depending on previous compression history. Earlier swelling studies with calcium montmorillonite were repeated with similar (but not identical) results. In addition, specimens with various histories of compression were examined by X‐ray diffraction methods. Every specimen showed [001] spacings of 18.8Å., regardless of degree of compression or reswelling. Analysis of the 18.8Å. diffraction peaks by literal use of the Scherrer equation indicated that in uncompressed material (prepared by resin exchange from the sodium form) packets, or “tactoids” averaging about 4.5 montmorillonite particles each, produced the diffracted beam. The indicated number of particles increased irreversibly to about 8 as the maximum pressure experienced by a specimen increased to 100 atm. These results suggest that volume changes of compressed specimens were not associated with elastic bending of montmorillonite particles. Literal use of the Scherrer equation permits estimation of average spacing between tactoids from water content data. The slope of curves relating this spacing to pressure were surprisingly close to that predicted by application of double‐layer equations, but large “dead volume” corrections must be applied if this is to be taken as evidence that swelling in calcium montmorillonite is in reasonable agreement with double‐layer theory. It is concluded that the lowest energy state for calcium montmorillonite in dilute CaCl2 is represented by a single “crystal” with 18.8Å. [001] spacing. Where attainment of this ordered state is prevented by imperfect orientation, swelling can occur, and is not incompatible with osmotic repulsion inherent in the diffuse double‐layer model.
An attempt has been made to discover the mechanism of iron cementation in clays by comparing the aggregation effected in dispersed pure mineral clay by its exposure to different stages in the active hydrolysis of a ferric chloride solution. Contact of the clay with the early products of the hydrolysis is apparently necessary for the achievement of stable bonding; clay mixed with the gelatinous precipitate formed at the end of the hydrolysis is not stably aggregated, but a combination of these two phases produces aggregates which are particularly resistant to normal dispersion treatment. The results are discussed in terms of the kinetic steps in the hydrolysis of iron(III) solutions, and their implications for structure formation in soils are outlined.
Factors involved in increasing salinity, which has followed clearing in the Western Australian wheat belt, have been investigated in a typical valley. Field and laboratory studies show that salinity is associated with a three-component hydrologic system involving surface, soil, and aquifer waters. The amounts of surface and soil water have increased after clearing. The third component, which is not thus affected, consists of a confined and continuous fine-textured aquifer which extends under most of the area and which has its intakes adjoining rock outcrops. This aquifer is the major repository for water and salt in the landscape, and salinity of overlying soils results where capillary contact with the surface occurs in the valley bottoms. The increase in volume of surface water does not lead directly to increased salinity, but rather, ultimately, to a removal of salts from the system via the main drainage lines. Increased soil water, however, serves to affect salinity in two ways. Firstly, it increases the area and duration of capillary contact between the confined aquifer and the soil surface in valley floors. Secondly, it leads to the development of seepage spots in the coarse-textured soils on valley sides, thus resulting in the formation of saline patches. The main part of the salt in the landscape has accumulated through atmospheric accession.
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