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.

Various triphormic dioctahedral minerals and their dehydroxylation and rehydroxylation products have been examined by chemical, thermal, X-ray and infra-red absorption techniques. Since minerals with high Al-for-Si substitution do not rehydroxylate readily under the mild conditions used, attention was focused largely upon pyrophyllite and montmorillonite. Rehydroxylation occurs rapidly at about 400–500°C. Infra-red spectra indicate that there is a structural resemblance between the layers of all the dehydroxylates and that the rehydroxylates possess regions similar to untreated pyrophyllite. X-ray results show that stacking of the layers cannot be identical in the various samples. Although the structures could not be determined, the scheme proposed by Jonas for the rehydroxylate could not be reconciled with experimental results: nor does rehydroxylated montmorillonite resemble the abnormal form.

A theoretical mechanism for the oxidation of ferrous iron in hydroxylated silicates is described. The reaction is represented by the general equation 4Fe2+ + 2nOH− + O2→4Fe3+ + (n + 2)O2− + nH2O, where n can vary. Oxidation occurs only at the surface of the mineral and is sustained by the migration of protons and electrons through the crystal. Non-oxidizable ions, such as Mg2+, and water formed during oxidation inhibit the reaction. The experimental evidence for the theory is derived, from a study of the oxidation of amphiboles.

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 effect of the firing schedule on some physical properties of a group of Etraria marls has been studied. Of particular interest are the thermal expansion and moisture expansion of the clays which reflect mineralogical changes occurring during the firing process.

Certain minerals in soil clays may be directly estimated from diffraction line intensity and the mass absorption coefficient of the sample to the radiation used. The main advantages of the method described are that no internal standards or calibration charts are required, and any diffraction line of the component to be estimated may be chosen. The results obtained compare favourably with chemical determinations. Orientation of particles during sample preparation makes the estimation of some minerals very difficult. The uncertainty involved in choosing the background level under a diffraction line is the chief source of error in estimating small amounts of very fine-grained minerals.

Pure samples of the micaceous component of Yorkshire fireclays have been separated. Examination of these by chemical, optical, X-ray, and thermal methods indicates that the material is in all likelihood an intimate mixture (but not an interstratification) of mica and a kaolintype mineral—probably the b/3 disordered kaolinite commonly found in fireclays. The evidence also suggests that this kaolin-type mineral is the end-product of weathering of the mica, and a possible mechanism is suggested.

Examination of clays from surface and basal horizons of four Scottish soils by differential thermal analysis, before and after heating to 600°C and by infra-red spectroscopy indicates that these contain a proportion of highly hydrated amorphous material, resembling allophane in its properties, and that this component is particularly high in clays from the surface horizons with the greatest organic matter content. This conclusion is consistent with the cation-exchange capacities of the clays and with the amounts of silica and alumina extractable from them by hot sodium carbonate solutions. The ratio of silica to alumina in these extracts indicates that the amorphous material is highly siliceous.

Complex formation enhances the sorption of various diamines from aqueous solutions by montmorillonite in the presence of copper and vice versa. A distinct difference was observed between chelating and nonchelating diamines. The effects were measured quantitatively, and the resulting structures were investigated by X-ray diffraction tedhniques. No direct correlation was observed between cell height and the configuration of the interlayer complexions; ‘keying’ of organic and organometallic cations into the oxygen surfaces is inferred.

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.