Montmorillonite is not uncommon in the china clay of Devon and Cornwall but occurs in such small proportions that few commercial samples will give X-ray diagrams showing a line at 15-18 Å, in the oriented aggregate technique of Nagelschmidt (1940) and MacEwan (1946). Particle size fractionation is an essential starting point for the study of these clays, montmorillonite concentrating at the fine end. The sensitivity of the X-ray method may be greatly increased by a preliminary concentration in this manner. However, there are many clays with properties which suggest that a trace of montmorillonite is present, but in which it cannot even be identified by X-rays. If the fractionated clays are converted to the H-clay and the cation exchange capacity of the fractions measured by titration with alkali the presence of montmorillonite is shown by the much higher exchange capacity of the finest fractions. In this way montmorillonite may be detected, and roughly estimated, when the proportion present is as low as 0.2 per cent. of the total clay.

In a series of test tubes, increasing amounts of pure cetyltrimethyl ammonium bromide (Lissolamine A) are added to a dilute clay suspension containing a known amount of clay. Visual examination of the contents of the tubes shows a stability minimum to exist at a certain concentration of the bromide. It may be assumed that in this stability minimum the clay micelles are neutralized by quantitative exchange of the exchangeable ions of the clay by an equivalent amount of strongly adsorbed organic cations of the bromide. Consequently the b.e.c, of the clay may be calculated from the concentration of the cation-active compound at the stability minimum. Since, however, by visual examination the stability minimum cannot be distinguished sharply enough, a better criterion had to be developed. As there are reasons to assume that the neutralized micelle will act as an emulsifier for oil-water systems, an emulsification test was expected to offer a way out. It was indeed found that on shaking the contents of the tubes with gas-oil, a rather stable emulsion was formed at a certain critical concentration of the cation active compound. Taking this concentration as an indication of quantitative exchange, the b.e.c, of the micelle may be easily calculated. Results of preliminary experiments are in good agreement with those obtained by application of conventional methods.

The structures of vermiculite and montmorillonite are very similar in that they consist of talc-like or pyrophyllite-like layers interleaved with layers of water molecules, the latter carrying exchangeable cations. In vermiculite, the exchange positions are normally filled by Mg ions, and the water layers consist of double sheets of water molecules with the exchangeable Mg ions presumably centrally placed. The water layers in natural Mg-saturated vermiculite are 5.1 Å high and it seems probable that this distance is fixed by the size of the hydration shells surrounding the exchangeable Mg ions as one of us (G.F.W.) has previously suggested.

Although vermiculite is found in particles of clay size, it also occurs as large flakes produced by the hydrothermal alteration of mica. This seemed a good reason for making a Fourier curve of the mineral in the c-direction with the object of examining the organization of the water layers. It was hoped that greater detail might be obtained from single crystals of vermiculite than would be possible in the case of montmorillonite, where oriented aggregates consisting of large numbers of very small crystals must of necessity be used.

A talc or talc-like unit forms part of the structure of mica, illite and hydro-mica, vermiculite, montmorillonite and chlorite as well as tale and pyrophyllite. Transitions should be possible between these structures by chemical and/or thermal methods. Many such transitions have been observed in nature but comparatively few successful transitions have been carried out in the laboratory.

One aspect that has recently been under investigation at Leeds University Physics Department is the transition chlorite→mica. Treatment of a penninite by various acid media either with or without heat treatment failed to remove the brucite layer from the chlorite without also attacking the talc layers. No mica or talc-type of mineral was observed as an intermediate product, and a decrease of only 0.4 kX occurred in the spacing of the layers during acid attack.

The combination of optical and elastic studies on sols of Wyoming bentonite containing the smallest particles sizes (15 to 20 μ equivalent spherical diameter) have led to the conclusion that the gelation of this system is due to the formation of a fibrillar network in which the plates are aggregated end-to-end in the form of flat, ribbon-like, filaments. The investigation of the effect of the addition of uni- and divalent electrolytes upon the rigidity is found to be consistent with the hypothesis that the particles are not in close contact, held by chemical bonds, but are situated at equilibrium distances which are determined by the combination of repulsive forces between their electrical double layers and attractive van der Waals forces.

The theory of the light scattering by a system of thin rectangular plates has been developed, and applied, to establish the existence of ribbon-like aggregates in undialysed sols(pH 8.9) at concentrations as low as 0.05 per cent.

Many analyses of montmorillonite indicate liberation of water above 300°C in excess of the amount required by the structure proposed by Hofmann, Endell and Wilm (1933). The structure of Edelman and Favejee (1940) will account for a larger quantity of water, but other inconsistencies with experimental results seem to arise.

Using as an analogy the hydrogarnets (Flint et al.; Pabst ; McConnell), it is suggested that hydroxyl ions can occur as tetrahedral groups within the silica sheets in substitution for SiO4 groups. Liberation of this "tetrahedral water" is believed to be associated with the second high-temperature endotherm which is occasionally a small deflection on the D.T.A. curve but is, nevertheless, characteristic of montmorillonite.

An earlier paper (McConnell, 1950), outlines a structural hypothesis according to which high-temperature water occurs in the tetrahedral layers of montmorillonite. In real crystals of montmorillonite, hydroxyl ions are presumed to be distributed at random within the Si2O5 layers in such a manner that four hydrogens substitute statistically for one silicon—the number of oxygens remaining unaltered. In an idealized model the hydroxyls are visualized as forming discrete (OH)4 tetrahedra which are linked at three vertices to other tetrahedra of the sheet in the same manner as the SiO4 tetrahedra for which they substitute.

Liberation of tetrahedral water was employed as a basis for partially explaining the second high-temperature endotherm of the differential thermal analysis, whereas it was indicated that the quantity of water liberated during the first high-temperature endotherm appears to be essentially equal to the theoretical hydroxyls required for octahedral co-ordination. The number of hydroxyl ions that occur in the silica sheets is presumed to be small but, nevertheless, sufficient to cause anomalies in rigorous calculation of the contents of the unit cell.