Because chlorite represents the highest order of mineral having a montmorillonite-illite like structure, it has been studied by the d.t.a. technique in water vapour atmospheres at pressures (absolute) in the range 0·001 mm of Hg to 2300 mm of Hg. The range of water vapour pressure makes profound changes in the thermograms, showing the very complex nature of the breakdown of the brucite-like layer which was not anticipated by other tests. The chlorite exotherm at 870°C can be completely suppressed by high pressure water vapour or can be greatly intensified by running in vacuum. The great differences in thermograms cannot be explained by chemical analyses or optical properties.

A new method for the determination of the base exchange capacity of small samples of clay minerals has been described by H. van Olphen (1950, 1951). The method is based on the exchange of the exchangeable cations against large organic cations; for the latter, cetyltrimethyl ammonium bromide (lissolamine) was used. The point of complete exchange can be detected by an emulsification test: at the point of equivalence the clay becomes an excellent emulsifier for oilwater systems; this is supposed to be due to its being at one and the same time oleophilic at the surface and hydrophilic at the edges of the particles. It was considered of interest to study the adsorption of lissolamine on clay, as this would contribute to our understanding of the method.

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.

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.

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.

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.

The work outlined in this paper was undertaken to show how, by a consideration of the energy changes involved, some insight may be gained into the reactions occurring when kaolin minerals are heated.

Activation energies have been determined experimentally for the dehydration of various kaolin minerals. The fireclay mineral is distinctive and it is suggested that it does not form part of a halloysitekaolinite series. It is further thought possible that there is a series of fireclay minerals.

From thermochemical and thermodynamic considerations the heat of formation of kaolinite has been calculated approximately and enthalpy and free energy changes have been estimated for the exothermic reaction at 980°C. These indicate that, if a compound is produced during dehydration, the most probable explanation of the exothermic reaction is the simultaneous production of mullite, γAl2O3 and SiO2. Finally the lattice energies of kaolinite and its thermal decomposition products have been calculated.

Samples of the argille scagliose from various localities in the Modenese Apennines have been studied by optical, X-ray and d.t.a, methods as part of an investigation into the composition and origin of this remarkable allochthonous formation. Illite, chlorite and a kaolinitic mineral of “fireclay” type are widespread. Samples from one locality contain also a conspicuous 22–28 Å swelling chlorite, the origin of which is discussed. It is as yet uncertain whether this mineral is confined to clays containing exotic blocks of ophiolite (greenstone) or whether it is more widely distributed.

A set of seven soils representative of types of considerable pedological importance in the Umbrian region of Italy have been examined by petrological and clay mineralogical methods in order to supplement previous data upon field and chemical characteristics (Lippi-Boncambi, 1950). Some basic data upon colour, locality of the sample examined, parent material, rainfall (P), temperature (T), rain factor (P/T—see Lang, 1915) and pH are summarised in Table 1. It will be noted that (a) the soils form an interesting colour group ranging from red through intermediate colours to yellow and then through brown to black, (b) all except one are derived from calcareous rocks, and (c) all except one are neutral to alkaline in reaction.

Clay minerals occurring in bituminous and brown coals have been identified by X-rays as illites and kaolinites. On burning coals reactions take place between the clays, lime, ferric oxides and sulphur. The clays set free the sulphur as sulphur dioxide rather than form anhydrite. The influence of clays on slag formation is described. Clays raise the viscosity of slags. In bituminous coal ashes the melting point is raised but in the ashes of brown coals lowered by the presence of clays. Examples in fluxed-ash and cyclone firing are given.

This sub-committee consisting of Mr G. Brown (Chairman), Dr M. H. Hey, Dr D. M. C. MacEwan and Dr R. C. Mackenzie was asked to make proposals for the nomenclature of clay minerals and these proposals were presented in part at a C.I.P.E.A. meeting in Paris in July, 1954, where some useful discussion ensued. The outcome of that meeting was a suggestion that the proposals be published more fully. It is important to remember that these are proposals for consideration and not a set of rules. The Clay Minerals Group would welcome comments on these proposals (adverse or otherwise) or the comments may equally well be sent to C.I.P.E.A.