Experiments on a number of clays from different formations show that heating at 375°C for 16 hours removes 90 per cent. or more of the carbonaceous material without removing combined water from the clay mineral. A close relationship is found between percentage loss at 375°C and percentage carbon determined in the carbon-hydrogen furnace.

In attempts to postulate dehydroxylation mechanisms for the clay minerals, three assumptions have generally been made: that the elements of water are lost more or less uniformly from all over the crystal, that the tetrahedral parts of the structure are less likely to be disturbed than are the octahedral parts, and that silica-rich material often presumed to occur as a secondary product is in fact pure SiO2. It is shown that all of these assumptions are doubtful. Dehydroxylation mechanisms in which the first assumption holds good are described as homogeneous, and it is shown that certain calcium minerals undergo dehydroxylation in this way. In contrast, where magnesium or other cations of similar size are predominant, dehydroxylation occurs by an inhomogeneous mechanism, in which no oxygen is lost from those parts of the crystal where oriented conversion to a crystalline product occurs; the oxygen for the expelled water comes entirely from other regions of the crystal which are wholly or partly converted into pores, while various migrations of cations occur. The dehydroxylation behaviour of serpentine, talc, kaolinite and pyrophyllite is briefly reviewed. A new approach to the dehydroxylation of clay minerals is suggested, based on the concept of inhomogeneous mechanisms.

A systematic study has been made of the effects of aluminium fluoride on the thermal behaviour of kaolin, using differential thermal, thermogravimetric, X-ray and electron microscope techniques. It is shown that mullite can be formed at a temperature as low as 600°C; that in the presence of aluminium fluoride the exothermic reaction occurs at a lower temperature and is reduced in size; that the size of the mullite crystals is increased as a result of pretreatment with aluminium fluoride.

The exchange capacities, surface areas (both external and total), and heats of immersion in water were measured on a selection of clay minerals and a fine-grained muscovite. The charge density (expressed as 103 X m-eq/m2) of muscovite was 3·6. This was, as expected, much higher than those of clay minerals which tended to group together, the average values being: kaolinites, 2·0; hydrous micas, 1·7; halloysites and montmorillonites, 1.4. The heats of immersion (expressed as J/m2) were, in contrast, more widely spread: muscovite, 0·63; halloysites and kaolinites, 0·49; hydrous micas, 0·28; montmorillonites, 0·14. The explanation advanced for the differences is that they reflect changes in surface energy and cohesion of the silicate layers.

A new method is described for determining in shales the amount of sulphides that dissolve in dilute hydrochloric acid. The samples are treated with hydrochloric acid under vacuum, and the hydrogen sulphide evolved is absorbed in potassium hydroxide and titrated with potassium permanganate. The analytical results are expressed as FeS.

It has been known for some time that when a gel of montmorillonite is freeze-dried the residual clay retains the volume and shape of the gel, and forms a skeleton of low density and some rigidity. Because there is no apparent change in the gel structure during freeze-drying, the dry residues have been studied in the past in an effort to determine the structure of the gels. However, X-ray diffraction measurements have shown that, as a result of freeze-drying, the distance between the elementary layers of Na-montmorillonite drops from high spacings (>30 Å) to zero. The present investigation demonstrates that the main reduction in interlayer spacing takes place on freezing, with a further small collapse on drying. Na-montmorillonite gels, swollen oriented aggregates of Na-montmorillonite and Na-nontronite, and swollen single crystals of Li-vermiculite and butylammonium-vermiculite, all behave in the above manner.

Mineralogical compositions have been calculated from a number of average analyses of shales and clays using the scheme of Imbrie and Poldervaart (1959). The average shale comprises 59 per cent. clay minerals, 20 per cent. quartz and chert, 8 per cent. felspars, 7 per cent. carbonates, 3 per cent. iron oxide minerals, 1 per cent. organic matter and 2 per cent. other minerals. Illite predominates among the clay minerals.

A thin vein of a white powdery material situated in massive syenite was examined by various methods and found to consist of an association of a zeolite with calcium montmorillonite, which seems to have been formed by the slow but prolonged action of water on nepheline.

The relationships between the shapes of kaolinite and dickite differential thermal curves are described. The slope ratios of dickite are distinctly greater than those of kaolinite of equivalent particle size for the endothermic peak at about 550ºC and the same effect is observed for the exothermic peak at about 980ºC especially in diluted samples. The packing density of dickite does not differ markedly from that of kaolinite and the curve obtained for weight vs. area of the endothermic peak is in agreement with this. The dickite samples, however, give slightly lower areas than those of kaolinite, an effect that is dependent on the shape of the particles. The exothermic peak is shown to be influenced considerably in shape and size by thermal gradients.

The possibility of the identification by X-ray diffraction of montmorillonite and swelling chlorite, in the presence or absence of each other, is discussed. Swelling chlorite or montmorillonite, even in mixture with normal chlorite, but in the absence of mica, can be identified by heating to 500°C with immediate sealing of the tube, or by acid treatment followed by solvation with ethylene glycol. If mica is present, however, only the latter procedure will usually ensure positive identification. The same procedures will differentiate montmorillonite and swelling chlorite in mixture only when normal chlorite is absent.

The use of thermal and acid treatments, prior to X-ray diffraction examination, for the identification of kaolinite and chlorite in mixture is discussed. Thermal treatment, although useful in confirming the presence of chlorite, cannot give definite information on the presence of kaolinite, but acid treatment is conclusive in this respect.

The <2μ equivalent spherical diameter (e.s.d.) fraction of the Ap horizon of a Brown Earth from Salamanca developed on Cambrian shales under a semi-arid climate with predominantly physical weathering has been studied. From X-ray diffraction results the mineralogical composition of this clay is: illite—70 per cent. kaolinite—10 per cent. and a “dioctahedral clay vermiculite”—20 per cent. The properties of the last mineral are detailed.

Experiments carried out in the author's laboratory on the artificial weathering, in an open system, of potash felspar, albite, leucite, tremolite, olivine, sideromelane, muscovite, and clay minerals are reviewed. The results are assessed, and eventually compared on the basis of the amount of SiO2 removed after leaching with 4 1 solvent.

It is suggested that the source materials of the English Aptian fuller’s earths were soils of the Anglo-Belgian Land and that montmorillonite is the characteristic clay mineral of marine deposits formed when old land surfaces of low relief are transgressed.