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.

Recent examination of clay-materials from several different sources has suggested to us the existence of a type of chlorite capable of the same kind of expansion along the c-axis by imbibition of glycerol, as is shown by montmorillonoids, vermiculites and halloysites. The expansion occurs within the range of 14 kX to 18 kX.

It is necessary to discuss the definition of chlorite in this connexion, since such expansion is not regarded as characteristic of any of the minerals to which the name has been given. The characteristics by which chlorites are generally identified from their X-ray diffraction patterns are twofold: (1) they give basal reflexions at approximately 14 kX and submultiples, the first four orders being usually strong and of comparable intensity, though differing in different species of chlorite; (2) these reflexions persist after heating at 500°C.

In a paper to the Faraday Society (1946), Mering showed that the temperature to which one required to heat montmorillonite to produce irreversible collapse of the c-axis depended on the exchangeable cation. With Na-montmorillonite irreversibility began at about 550°C but with Ca-montmorillonite below 350°C During a study of the liquid phase sorption of pyridine by montmorillonite, the writer obtained evidence that irreversible collapse occurred at 200°C with Li-montmorillonite.

The source of montmorillonite was a sample of Wyoming bentonite, which was purified by passing through a Sharples supercentrifuge. The resultant exchange capacity to ammonium acetate at pH 7 was about 90 m.e./100 gms. and the displaced ions chiefly consisted of Na+ and Ca++.

In the course of some studies on the effects of anions on the viscosities of clay suspensions, it was observed that anions such as pyro- and metaphosphate, oxalate and fluoride were much more effective in reducing the viscosity of kaolih suspensions than were chloride and nitrate. There is relatively little information available regarding the specific adsorption of anions by clay minerals; it is known, however, that soils take up considerable quantities of orthophosphate from acid solutions. Whether the active component of the soil, in this respect, is the clay mineral or hydrated iron and aluminium oxides, the adsorbed phosphate may be readily released by shaking the soil with neutral or acid solutions containing fluoride, oxalate, citrate or malonate. In addition, a soil which is normally capable of taking up appreciable quantities of phosphate will not do so if fluoride is present in the phosphate solution. It has been recognized that the ability of this group of anions to form soluble complexes with iron and aluminium is responsible for their effectiveness in this respect.

Both kaolinite and its thermal decomposition products are to be regarded as “active” solids in that they have a fairly large specific surface area, of the order of 10 square metres per gram. It was therefore thought worth while to apply some of the procedures which proved useful in the investigation of other surface active solids, to the problem of the dehydration of kaolinite. The techniques chosen were: (a) the resorption of water by the dehydration product; (b) the sorption isotherm of a vapour (carbon tetrachloride) on the dehydration product, in order to estimate its surface area; (c) the apparent density of the product in a suitable, inert, immersing liquid; (d) X-ray examination of the product (through the kindness of Dr Clark and Mr Parker of English Clays Lovering Pochin Ltd.).

We investigated two earths sent to the Institute for Glass and Ceramics at the Technical University in Prague by the Inspectorate for Research and Mining at Presov in Slovakia. The earth from the region of Kuzmice is a “metabentonite.”

In the literature there has as yet been no mention of the abnormal properties of suspensions of such clays. We found that suspensions with a concentration of over 15 per cent (measured by volume) behave as a gel with a very low limit of flow. This is the reason why no signs of thixotropy show themselves. Between certain limits the viscosity does not rise with increasing concentration above the 15 per cent mentioned above, which we explain as a consequence of structural flow of the gel. At lower concentrations the particles are rather distant from one another so that they can move freely (rotate) when flow takes place and the viscosity increases with the concentration of the suspension. Above 15 per cent the particles draw so near to one another that their swollen and easily deformable surface layers can touch one another. Such particles deform during their motion; they orientate themselves in the direction of the flow and they glide over one another so that the viscosity does not increase. The limit of flow being very low, the gel has not got such a strength as is necessary to solidify perfectly; this is the reason for the absence of signs of thixotropy.

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.

The arrangement of water molecules in the vicinity of clay mineral surfaces has been the subject of much speculation. Vermiculite offers a promising line of approach to this problem since, in addition to its occurrence in soil-clays, it is available as relatively large flakes which can be studied by single-crystal X-ray diffraction methods. This note reports an as yet incomplete structure analysis of vermiculite. A two-dimensional Fourier synthesis has been carried out and a determination of atomic parameters in the third dimension is in progress. The results obtained so far would appear to justify a preliminary statement.

The (h0l) set of reflexions for a natural Mg-saturated vermiculiet were recorded on two multiple-film packs exposed to MoKα radiation in a Weissenberg camera for 100 hours and 5 hours respectively. Intensities were estimated visually by comparison with a set of standards obtained from the (00·10) reflexion.

This paper reports a statistical study of clay rocks both according to their geological character and their petrographic characteristics. The study was concerned almost entirely with rocks and not with present-day sediments. Further, typical sedimentary environments have been chosen: marine lagunar (saline and supersaline) and lacustrine. Deposits from river estuaries and moraines have not been considered, their conditions of genesis being more difficult to determine.

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.

It has been estimated that nearly 50 per cent. of all known sedimentary rocks belong to the clay grade and it is therefore a hard fact that our knowledge of the mineralogical composition of approximately half the rocks of the stratigraphical column is still in a rudimentary state. The difficulties of petrographical work on clay grade sediments are all too familiar. The size of the individual particles is extremely small, often below the resolving power of the microscope. Most of the common clay minerals are biaxial and negative. Although minerals of the kaolinite group should be readily distinguishable from illitic and montmorillonoid minerals by their lower double refraction, films of iron hydroxide on individual particles or intimate admixtures of different minerals render observation difficult. In addition the presence of iron hydroxide films or changes in water content can give rise to variations in refractive indices. Petrographical work on clay minerals must be supplemented by other techniques which have been extensively developed within the last 20 years—X-ray analysis, thermal methods, the use of the electron microscope and certain chemical techniques. Despite the progress. which has been made, however, the day when quantitative estimations of the mineralogical composition of clay grade sediments can be rapidly and accurately made is still a long way off.