At the present time two hypotheses have been proposed to explain the formation of clays. One of them supposes that these minerals arise from the crystallisation of a gel due to the breaking down of the original mineral; one would thus have schematically:

Original mineral → Gel. → Clay.

The application of the de Saladin-Le Chatelier differential thermal analysis method to the identification of soil clays has given rise to a certain number of difficulties. In fact, it is possible for these clays to be altered either in situ by the action of atmospheric and biochemical agents or in the laboratory during their extraction. Mr Henin and I have been investigating what influence chemical agents can exert on the thermal diagrams of different types of clays. The treatments have been effected by normal solutions of cations in the form of chlorides, carbonates and nitrates, or by N/10 acid or basic solutions.

These authors are studying the influence of pH and particle size on sedimentation volume, and on the velocity with which it is reached. They are using fractions from > 1.2 μ to < 0.3 μ, obtained by sedimentation, and buffer solutions of pH 1, 3, 5, 7, 8, 9, 10, as well as distilled water. For all pH's the volume of sedimentation and the time taken to attain it rise with decreasing particle size. For a given fraction the volume of sedimentation rises in the pH's near neutrality (7 and 8) and in distilled water, depending partly on the ions present, their nature and concentration, in the buffering solutions employed. The thixotropy and the base exchange capacity rise parallel with the sedimentation volume.

This is normally measured in terms of resistance to shear, which is a function of the internal friction and cohesion of the soil. The internal friction is primarily a characteristic of the coarser soil fractions and results from the interlocking of the particles; its value is a function of the normal stress on the soil. The cohesion of a soil is produced by forces binding the particles together, and is independent of the normal stress. These binding forces are accounted for partly by the, surface tension at the air-water interface of the water films surrounding the particles and partly by the intermolecular forces acting between films of water adsorbed on the surfaces of neighbouring particles.

One of the most interesting and practically most important questions regarding the behaviour of clays, and particularly montmorillonites, is concerned with the water binding mechanism in relation to the mechanical properties, elasticity and plasticity.