Samples of ‘blue clay’ were distributed among nine laboratories in various countries to assess the possibility of quantitative mineralogical determination. The disordered form kaolinite commonly found in fireclays and illite, or mica, were found by all laboratories, as was a material variously described as montmorillonite, swelling material (vermiculite, beidellite or open illite), or intermediate or swelling illite. The next most abundant mineral was quartz. Opal, chlorite, felspar, anatase, and organic matter, all in small quantities, were not all found by any laboratory and traces of calcite and dolomite were observed in one only. The most effective procedure is discussed, and it is concluded that the results are of a semi-quantitative rather than a quantitative nature.

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.

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.

After a brief account of the work currently in progress on fine-grained sedimentary rocks in the U.S.S.R., and particularly in Azerbaijan, the concept of finely-dispersed minerals as against clay minerals is considered and some comments made on classification schemes. Conditions of formation and deposition determine whether sedimentation or diagenesis is predominant and hence the mineralogy of the deposit. Methods of mineralogical investigation are as yet ill-developed and much remains to be done. Finally, some of the wider problems in clay mineralogy are discussed and some suggestions made regarding the organization of suitable studies, particularly in the U.S.S.R.

The synthesis of kandites in the granites of south-west England has resulted from the breakdown of albite or oligoclase by hydrothermal solutions using joints and faults as channels. The crystallinity of the clay mineral and the extent of argillization are related to the distance from these channels. Intermediate stages between felspar and kaolinite remain obscure but recent evidence suggests that they may be represented by montmorillonite. Little is known about the composition of hydrothermal solutions: fluorine and boron determinations on rocks in various stages of argillization suggest that neither of these elements was an important constituent.

A hydrothermal synthetic study on the relationship between the stability and the composition of sodium saponite solid solutions revealed that a special trioctahedral sodium smectite can persist stably up to temperatures as high as 850°C under a water pressure of 1 kb. This smectite has a composition close to Na0·55(Mg2·70Al0·30) (Si3·15Al0·85)O10(OH)2. nH2O as estimated from the chemical composition of gels that yield this smectite in essentially pure form at the highest temperatures, and from other phase evidence. The composition range of these trioctahedral sodium smectites and phase relationships around the smectite field in the system Na2O-MgO-Al2O3-SiO2-H2O are also briefly described in the temperature range 600–800°C under a pressure of 1 kb.

Tonsteins, which are kaolinite-rich rocks with an amorphous groundmass containing much organic material, occur widely in European Coal Measure deposits. The nature of these rocks and their peculiar fabric is considered in detail and evidence is adduced which suggests that they were derived from soils and that the original organic components (micro-organisms and fungi) played a significant part in the formation of the kaolinite macrocrystals.

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.

A montmorillonite mineral occurs in the marginal facies of a biotite-granite in the younger granite ring-complex at Ropp, Plateau Province, Northern Nigeria. The mineral is associated with the development of silexitic textures (pegmatitic knots and miarolitic cavities) filling the spaces between euhedral crystals of quartz, perthite and biotite protruding from the walls. Chemical, X-ray and differential thermal results suggest that the mineral belongs in the beidellite-nontronite series; it is thought to be primary in origin.

Difficulties experienced in the preparation of organo-clay derivatives from an English montmorillonite led to an investigation of its properties. The mineralogical composition, cation-exchange capacity, surface area, and relative particle size, of this and two other montmorillonites, were compared. The results indicated that the large differences which were occasionally found could be attributed to the exchangeable cation on the clay.

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.

The current position with respect to low temperature synthesis of minerals is reviewed, and attention is directed particularly to the mechanism of synthesis. Possible compositions of the hydrated aluminium ion are considered and the significance of the fact that aluminate ions are necessary to introduce tetrahedral aluminium is discussed. It is concluded that present evidence is insufficient to define the processes exactly.

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.

An extension of earlier work on synthesis is described in which silica and alumina are provided in the form of gels instead of as solutions of salts. The systems with nickel and magnesium at different concentrations have been investigated in the presence and absence of sodium chloride and the products obtained are described.

Clay veins are present in deep weathering profiles in basic and ultra-basic rock beneath glacial drift at two localities on the Island of Rhum, Inverness-shire. Veins at one section are of saponite, at the other of the serpentine mineral, lizardite, with subordinate saponite. It is suggested that the veins were substantially formed during the interglacial period of development of the weathering profile, and that the lizardite formed in the lower horizons is replaced by saponite in the more intensely weathered upper horizons.

A study has been made of the mineralogical transformations that have accompanied the chemical weathering of some Palaeozoic clay-slates from New South Wales. The weathered sequences occur within a limited range of climatic and topographical conditions. Chlorite, interlayered with other expanded-lattice silicate minerals, is the least chemically stable of the primary constituents. Montmorillonite is a little more resistant to the percolating waters, whereas illite, present in all primary materials as the 2M1 polymorph, is relatively chemically stable. In the zone of maximum leaching, however, partial degradation and loss of illite is apparent. Kaolinite is the end-product of the breakdown of these clay minerals.

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.

It is suggested that piperidine may be useful in the identification of clay minerals during differential thermal analysis, particularly in the differentiation of expanding and non-expanding lattice minerals. Differential thermal analysis of two montmorillonites showing similar X-ray powder photographs indicated large differences in the distribution of cation-exchange sites.

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.

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.