Mineralogical and chemical variation in a 24 ft wide argillized zone in basic hornfelses from Gwavas quarry, Cornwall, is described and compared with variation in unaltered wall rock hornfelses.

Hydrothermal minerals developed are well-ordered kaolinite, sericite, chlorite, quartz, and limonite. Biotite has replaced hornblende as the dominant mafic constituent in the argillized zone. With increasing argillization kaolinite progressively replaces plagioclase, biotite, chloritized biotite, quartz, and finally Fe ore. No montmorillonite is present.

From the argillized zone, Si, Na, Ca, Mg, and Fe were extensively leached, although an approximate equivalent of H was added to compensate for the loss. Al and K are relatively constant throughout the zone. F was expelled from the most heavily argillized areas and is now concentrated in the biotite of the zone's margin as a marked ‘fluorine front’.

Two tenuous kaolinite-sericite zones are observed which are thought to be due to local variation in a single hydrothermal pulse with time.

Hydrothermal alteration of the hornfelses was effected in two stages (1) K+ metasomatism (biotitization) and (2) H+ metasomatism (kaolinization and sericitization).

Calcareous volcanic ashes (pumice–tuffs) occur among Ordovician rocks in Snowdonia, North Wales. The clays of these rocks and of the ecologically important soils derived from them are found to be almost mono-mineralic and to consist of chlorites. Chemical, differential thermal and X-ray analyses are given and discussed with particular reference to a clay-size chlorite from weathered pumice–tuff. There is no evidence for pedogenetic clay mineral transformations in the range of soils studied.

The matric potential of water in gels of five-sixths neutralized aluminium montmorillonite (bentonite) and aluminium kaolinite changes during periods of shear and periods of rest. Shear increases the potential of water in bentonite gels, and in kaolinite gels containing little water, but decreases the potential in kaolinite gels containing more water. All these effects can be explained by changes in the curvature of water films at the gel-air interface. The difference between thixotropic and dilatant behaviour is the increasing and decreasing of the water potential on shearing. In dilatant pastes of silt-size particles, and for which the effects of electric charges are small, the pressure deficiency can reach large values after an increase in pore space, and this suffices to explain the characteristic features of dilatant behaviour.

Varty & White's application of multivariate analysis to Grim & Kulbicki's measurements on montmorillonites has been re-examined and extended. Inconsistencies between their table of scored data and derived similarity table, and some unexplained errors in the similarity table, have only a small effect on their results. Similarities calculated from Grim & Kulbicki's data, using a different similarity coefficient, lead to a two dimensional representation which separates the groups of montmorillonites more clearly. The groups can be clearly separated even in a one dimensional representation, by changing the relationship between distance and similarity.

The surface properties of wet-ground muscovite were studied by thermodynamics of water adsorption. The surface appears to be covered by contaminating material, probably organic, which is strongly held; it was not removed at 350° C and under a vacuum of 10−4 torr. Attention is directed to the difficulty of maintaining freshly made silicate surfaces without contamination and to the importance of surface conditions in the interpretation of silicate properties which involve surface phenomena.

An X-ray diffraction technique has been used to determine the orientation pattern of the (001) planes of 10 Å micaceous minerals in slate. The guiding principle of this technique is the direct measurement of the diffracted radiation from the pre-selected family of crystallographic planes as a function of the orientation of the specimen.

Results obtained indicated that the basal planes of these minerals are mostly oriented in a direction parallel to the cleavage planes and the possibility of another orientation was considered to be slight.

The water gained by two micas during a wet-grinding process (extra-water) has been evaluated. It was found to be proportional to the surface area and the amount was about 29 µmoles/m2 for a fluorphlogopite and about 16 µmoles/m2 for a muscovite. This estimation was made possible by eliminating the physically adsorbed water. The material contaminating the surfaces and the constitution water of muscovite were taken into account by the comparison of the two minerals and by the use of infrared spectroscopy in addition to thermovolumetric data. It is thought that the extra-water is related to a disordered and chemically modified surface layer of the particles. The bearing of these results on the nature of illite is indicated.

A white-burning, highly-plastic clay from Jamaica, containing mainly disordered kaolinite, was examined recently. It was unusual in that it was practically free from fine mica, and therefore could be purified readily by standard sedimentation techniques.

The cation exchange capacity was abnormally high, and the purified clay mineral, on analysis and calculation of the ionic composition, was found to be deficient in aluminium, but contained magnesium and iron.

The results confirmed the view, previously expressed, that disorder in kaolinites is associated with a high cation exchange capacity and a substituted lattice.

Complexes are formed between a Ca-saturated fluorhectorite and organic molecules derived from propane by substituting benzene, pyridine, and piperidine rings in the 1,3 positions. The complexes are mainly of the A- and B-types described earlier by Greene-Kelly. Only one of the substances studied forms long-spacing complexes; in this case, the organic molecule is terminated at one end by a benzene ring and at the other by an active =NH group. Neutral forms of this molecule develop double-layer complexes, cationic forms develop single-layer complexes. The results are consistent with data for alcohols, alkyl chlorides and bromides, and alkyl amines.

Numerous exposures of highly altered kimberlites in exploratory trenches and pits have afforded an opportunity of studying the very great changes which take place chemically and mineralogically under conditions of tropical weathering in Sierra Leone.

Clay mineral analyses of Caribbean soils illustrate the differences in composition that can arise from differences in topography and parent material, and emphasize the differences between the groups of proposed new engineering classifications of the soils based on these factors. Within one set of environmental conditions the clay mineral assemblage was reasonably constant. Examples are given of local differences in clay type which were correlated with variations in climate and mode of formation.