Sodium montmorillonite and pyridine form a regular complex of spacing 23·3 kX in the presence of traces of moisture. The minimum interlamellar composition necessary for the formation of the complex is approximately Na+ (C5H5N)4 (H2O)2. The intensities of the 00l reflections are considered incompatible with the complex having a five layer structure and an alternative arrangement is proposed.

A sericite from Unnan Mine, Shimane Prefecture, Japan, has a three-layer structure, perhaps similar to three-layer phlogopite and lepidolite, but with various degrees of disorder in inter-layer stacking, in different samples. A classification of polymorphic and disordered forms, showing their relationship, is proposed.

The use of preferentially oriented aggregate specimens of clay for X-ray diffraction has been widely adopted since its origination by Bradley, Grim and Clark (1937). It is an extremely useful technique giving greatly enhanced intensity of diffraction from the basal planes of platey minerals relative to that from the general planes; the amount of one particular clay mineral which can be identified in a mixture is very much less than by the normal powder method.

Data are presented for the effects of wet and dry grinding on dickite from Ouray, Colorado. With increase in dry grinding time the endothermic d.t.a. peak becomes broader and moves towards a lower temperature, while the exothermic reaction is greatly sharpened. Weight loss data, demonstrate greater water loss from ground material and over a longer temperature range. X-ray powder photographs show that after a relatively short period of grinding little further destruction of the lattice occurs, and this lattice destruction apparently proceeds only to a certain stage, for high order spacings of hkl are very persistent.

Differential thermal analysis has been used by Hendricks, Nelson and Alexander (1940) to study the sorption of water by montmorillonite. They measured the water uptake at a number of different relative humidities and then traced the low temperature endothermic peak of the resultant material on a differential thermal apparatus. They found that the resultant curves of Mg, Ca, Sr, Ba and Li-montmorillonite showed a division into two components at low relative humidities. Assuming that the area under the D.T.A. curve was proportional to the weight of water lost, a division of the total sorbed water was made between the two peaks. The authors found that for Mg2+, Ca2+, Sr2+ and Ba2+ the high temperature component represented approximately 5—6 water molecules per exchange ion, whilst for Li+ it corresponded to 3·3 water molecules per exchange ion.

Up to now no method seems to have been found for carrying out laboratory syntheses of clay minerals in conditions similar to those obtaining in soils. Such a method would be of especial value because it would lead to the possibility of determining the conditions of formation of clay minerals in the natural state.

A colloquium was held on the 8th-9th July, 1955, in the Zaidin experimental station. The object was to give an account of the work which has been carried on recently in the Granada Section of the Institute, and in the Section of Inorganic Chemistry of the University of Granada (Prof. E. Gutiérrez Rios), on clay minerals, and other substances related to clay minerals.

There follows a list of the themes treated, together with brief mentions of the nature of the work in each case.

Clay minerals have been prepared by synthesis in solutions of comparable strength to their solubility (10–20 mgm./l). Syntheses have been carried out in boiling water and at normal temperatures, at various pH's, and with various components; the products have been examined with X-rays and the electron microscope. In general it is found that the proportion of the components has only a small effect on the product, this being determined mainly by the pH. The low temperature gives less crystalline products. Some results are:

Si-Mg syntheses.—Montmorillonite: “crushed paper” type at pH 9 (Fig. 1); “star” type at pH 8 (Fig. 2 from boiling solutions, Fig. 3 at ordinary temperature).

Si-Mg-Al syntheses.—Al as aluminate at pH 8 gives montmorillonite; with more Al, “swelling chlorite” (and 7Å mineral?) (Fig. 4). With cationic Al at pH 8·5 a filmy, almost amorphous product (sepiolite?) (Fig. 5); at pH 7·7 boeh-mite (Fig. 6). Anionic Al with KCl in great excess gives micas (Fig. 7).

Syntheses with iron.—Iron-rich saponites, montmorillonite minerals, and Fe2O3+“swelling chlorite” have been obtained. In NaCl excess, mica-like (but isotropic, poorly crystalline) products (Fig. 8).

The authors consider that the brucite-gibbsite layer forms first in these syntheses, and the silicate layer attaches itself to it: analogous products have been made with aluminate and chromate in place of silicate.

From their isothermal experiments, Murray and White drew the conclusion that the dehydration reactions of clays are first-order with a rate factor of Arrhenius form. The object of this paper is to discuss whether differential thermal analysis can be used to confirm or refute this hypothesis. The problem, previously considered by Murray and White, of a sample heated at a constant rate throughout is recalled. Particular attention is drawn to the effect of heating rate on the temperature at which the maximum rate of reaction occurs.

To apply to differential thermal analysis, the treatment must be extended to take account of thermal gradients; as the mathematics involved is cumbersome, only results are considered. The most important result concerns the effects of sample size and dilution on the position of the turning-point. A disagreement with the results of experiments on kaolinite is pointed out; it is argued that only a small part of this disagreement is due to simplifications made in developing the theory, and it is concluded that the hypothesis that the clay reactions are first-order, with a rate factor of Arrhenius form, is only approximate.

Soil formation has been generally attributed to the interaction of five variables: parent material, vegetation, relief, time, and climate. This interaction may be considered as the impact of the latter four forces on the parent material and the resultant is a profile of weathering that varies with environment. The objective of this paper is to demonstrate how the mineralogical characteristics of soil profiles may be used to determine their genesis.

Vold's mathematical analysis of the d.t.a, curve has been applied to Murray's data on the dehydration of clays. Owing to experimental uncertainties, values for the heats of dehydration are lower than those determined by other methods. The first order law is obeyed and the activation energies of dehydration are of the same order as those derived from isothermal data; rates of dehydration are, however, higher. Furthermore, Smith's constant heat flow method has been used to determine specific heats and heats of dehydration of clays, and the effect of sample porosity upon the results has been determined. The thermal data has also been used to estimate the amount of clay mineral present. The kinetics of the dehydration process, evaluated from the apparent specific heat-temperature curves, follow a first order reaction curve. The activation energies obtained are, however, higher than those derived from other data.

The techniques previously employed in this laboratory for removing free iron oxide from soil clays were the acid ammonium oxalate and the aluminium-ammonium tartrate methods (Tamm, 1922; Dion, 1944). Since both had disadvantages, especially when determining the iron by sodium salicylate (Scott, 1941), the use of sodium hydrosulphite, Na2S2O4, (Galabutskaya and Govorova, 1934; Deb, 1950) was fully investigated in the hope that it might lead to the development of a satisfactory, completely inorganic procedure.

The mineral present in most clays used in the fine ceramics industries is kaolinite. Both china clays and ball clays consist essentially of this mineral in association with quartz and some form of mica. There is some evidence for the presence of traces of montmorillonite in certain of the Cornish china clays (Clark, 1950) and a method of estimating this has been reported (Parker, Warren and Morcom, 1951). Work at the British Ceramic Research Association has shown that many Dorset ball clays are largely composed of a disordered kaolinite similar to Brindley and Robinson's fireclay mineral (1947); such clays are frequently more plastic than other ball clays and show high mechanical strength. Other ball clays contain normal kaolinite of finer particle size than the average china clay and in exceptional cases may show strengths equalling those of the disordered kaolinite.

Careful (00l) fourier syntheses have been made on muscovite, pyrophyllite, and montmorillonite (dehydrated), in an effort to discover the cause of the different (00l) spacings of these minerals. The oxygens of the outer (tetrahedral) layer superpose exactly in these syntheses, those of the inner (octahedral) layer almost exactly. Thus the differences are entirely in the inter-layer spaces, and are probably due to electrostatic repulsion of the negatively charged multiple layers. As the negative charge increases (and with it the repulsion, and hence the inter-layer spacing), in the series pyrophyllite-montmorillonite-mica, the net energy of binding of the sheets increases also. The two variations are not in contradiction.

It follows from this work that the shifts in cell size due to ionic substitutions are entirely taken up in the plane of the layer, its thickness remaining the same.

In the last twenty years a number of workers have turned their attention to a study of the grinding of clay in an attempt to elucidate a mechanism of the process and the nature of the resultant products.

The pioneers in this field were Kelley et al. who investigated the grinding of bentonite in 1931 and the grinding of kaolinite in 1936. In the earlier results the product showed a slight fogging of X-ray lines and in the latter an increased loss of water when it was examined by controlled dehydration at elevated temperatures; in both systems an increase in base exchange capacity (B.E.C.) was observed. The authors suggested a purely physical disintegration theory, i.e. that decreased particle size could account for all the observations; they postulated fracture both parallel and perpendicular to the cleavage plane and this was supported by the electron microscopic investigations of Shaw (1942).

Kinetic studies have shown that the isothermal dehydration of the clay minerals proceeds according to a first order law enabling velocity constants to be evaluated for different temperatures. From these, the Arrhenius parameters have been determined and on this basis, the clay minerals may be classified into three main groups, viz., kaolinites, secondary mica clays and montmorillonites. No significant difference is shown by comparison of the Arrhenius plots for the kaolinites and halloysites but the secondary mica clays decompose more rapidly than the kaolinites at comparable temperatures. Evidence is provided to illustrate that the constants obtained are associated with the basic process involving interaction of hydroxyl groups in the clay mineral lattice. Indirect evidence also suggests that the irreversibility of the dehydration is due to lattice collapse resulting in the formation of highly stable configurations of Si4+, Al3−, and O2− atoms. The first order nature of the dehydration reaction is shown to be the underlying basis of the type of curve obtained in differential thermal analysis. Two important conclusions are that:—(a) The clay mineral is only approximately 70% decomposed at the peak on the thermal analysis curve. (b) Heating rate has a marked effect on the peak temperature. Mathematical equations have been developed to obtain the effect of heating at a constant rate on the progress of a reaction for which the isothermal velocity constants are known. The effect of sample weight and dilution factors are discussed, together with the kinetic problems associated with mixtures of clay minerals.