Poorly crystalline inorganic materials were removed from soil clays of different origin and mineral composition by successive treatments with sodium dithionite, 2 % Na2CO3 and 0·5 N NaOH. Techniques are described whereby difference spectra representing the infrared absorption of the materials removed by the treatments can be obtained. These spectra indicate that the materials dissolved are related to the mineral composition of the soil clay. Dissolution of allophanes of different composition was only proved for two volcanic ash soil clays in which these dominated. Layer silicates, probably including kaolin, were dissolved from the clay fractions of the red-brown earth and krasnozem studied. In addition, alumina-rich gel-like material and gibbsite, but no allophanes, were dissolved from another volcanic ash soil clay in which gibbsite and layer silicates are present in considerable amounts.

The influence of sorbed cations and hydration on the a and b dimensions of montmorillonite, has been established by the selected area diffraction technique (S.A.D.) and by X-ray diffraction analysis. Concepts on the bonds and interaction mechanism of aluminosilicate layers with sorbed cations and water molecules (which form hydrate-ionic layers) are discussed. These concepts also indicate the basic stages of the sorption process and the formation of the montmorillonite structure. Assuming that aluminosilicate and hydrate-ionic layers represent a single crystal-chemical structure, the change of configuration of the aluminosilicate layers and dimensions of the montmorillonite lattice are explained.

The sorption of potassium by mica partially depleted of potassium was studied by adding potassium to a solution of sodium chloride in quasi-equilibrium with the mica. The concentration of potassium in the solution was increased 1·5 times before reverse exchange was initiated. Potassium resorbed by the depleted mica was extracted more readily than potassium from the original mica. Potassium exchange from mica is not reversible; there is irreversible change in the mica when potassium is exchanged and also an additional hysteresis in the forward and reverse exchange.

Two phlogopites depleted of potassium by extraction with sodium tetraphenyl boron released resorbed potassium more readily than the original micas, but the large difference in the exchange parameters of the two micas were paralleled by those of their potassium-saturated alteration products. The apparent basal spacings of the potassium-saturated alteration products were about 0·03Å larger than those of the corresponding untreated micas; their broader lines indicate structural irregularities.

The electron microscope has been used to study the mechanisms of thermal decomposition of kaolinite in the temperature range 800-1350°C. Three main reaction mechanisms appear to be important in this temperature range. At 850°C metakaolinite breaks down to produce an amorphous defect oxide phase which is homogeneous and finely porous. When heated at 900°C the reaction product is a defect spinel with strongly preferred orientation and microporous structure. This defect spinel phase is observed in the temperature range 900-1150°C and shows little change in microstructure throughout this temperature range where the secondary development of muUite also occurs to a limited extent. Above 1150°C mullite develops in quantity and appears to represent the bulk of the reaction product at 1200°C.

The weathering of biotite in a sedentary soil profile developed on biotitehornblende- rock near Rehiran, Inverness-shire has been investigated by optical, X-ray, chemical, infrared and differential thermal methods. In the C horizon a direct transformation to hydrobiotite was observed. This involves oxidation of all the ferrous iron present and subsequent movement of ferric ions from the octahedral sheet. In the B horizon weathering proceeds to a fully expanded 14A phase and this was identified as interstratified vermiculite-chlorite in a 1:1 ratio. Chemical analysis indicates that this stage is accompanied by a substantial addition of magnesium, much of which goes to form the brucite-like interlayers. In the more acid surface horizon these interlayers tend to break down thus yielding a more vermiculitic product. The possible general implications of this type of biotite weathering are briefly discussed.

Poly(ethylene glycols) of molecular weights 200 to 20000 were strongly adsorbed on montmorillonite from aqueous solution. Free energies of adsorption calculated from the adsorption isotherms became more negative with increase in molecular weight and the entropy change became increasingly positive, probably due to a net desorption of water from the clay surface as the organic compound was adsorbed. The adsorption of the higher molecular weight polymers caused an increase in the interlamellar separation. The effect of the exchangeable cation on the adsorption followed the order Cs > Na > Ca > Al, indicating that the cation retained its hydration shell and did not form a direct association with the adsorbed organic molecules. Some evidence was obtained that 'water bridges' were formed between exchangeable calcium an aluminium ions and the ether groups of the polymers.

Water adsorption by interlamellar complexes of poly(ethylene glycols) and montmorillonite has been determined using a silica spiral spring, and supplementary data obtained by X-ray and infrared techniques. The adsorbed water enters the interlamellar region, where it is associated with the exchangeable cation. The lattice swells to accommodate the intercalated water. Only weak hydrogen bonds are formed between the surface of the clay and the adsorbed water or between adsorbed PEG molecules and adsorbed water. The presence of PEG did not prevent extensive swelling of sodium montmorillonite.

The usefulness of various techniques for characterizing poorlyordered inorganic gel systems in clays is considered. Many established instrumental techniques are unsuitable, but thermal methods, chemical dissolution techniques, and specific surface reactions are more appropriate. The reaction of fluoride with clays is used as an example of the latter and methods for detection and monitoring of the fluoride uptake are described. Its value is demonstrated for studying gel materials in soils and relating them to synthetic alumina-silica gels.

An investigation of the acid-leaching of a calcined kaolin was made to compare the reaction kinetics of hydrochloric, nitric, and sulphuric acids. The acid concentrations used were 5·9 and 8·6 normal. Reaction temperatures used were 95°, 80° and 60° C.

Particular attention was given to finding a reaction equation which would mathematically represent the kinetic data. A nucleation rate equation of the general form, was found to represent these data. Values of m varied from 1·10 to 1·55 depending on the experimental conditions.

Under the conditions employed the rate of alumina leaching is fastest with hydrochloric acid, slower with sulphuric acid, and slowest with nitric acid. The rate controlling steps are proposed from the experimental evidence.