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.

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.

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.

In Scandinavian podzols a swelling dioctahedral mineral, probably derived from dioctahedral mica and iron-rich chlorite, has been found in the clay (<2µ) fractions from A2-horizons. This mineral, or group of minerals, is shown by X-ray examination and differential thermal analysis to be of a vermiculite-smectite type, thus making the differentiation between vermiculite and smectite still more problematical. The pretreatment of the samples and choice of interlayer cations is shown to be of great significance for the identification of soil clay minerals.

The inference of Gruner that unsatisfactory powder photographs of vermiculite resulted from distortion of the structural planes by grinding has not been substantiated. The unit cell parameters of a true vermiculite were found to be ao = 5·24± 0·02Å, bo = 9·17 ± 0·02Å, co = 28·60±0·05Å, β = 94° 36′ ± 24′, while the structural formula worked out as (Mg2·37, Fe3+0·37 X0·26) (Al1·28, Si2·72)O9 (OH)3·4H2O, where X represents the remaining octahedral cations. The determined density of the thin flakes was 2·28 g/cm3 and taking this value along with the volume of the unit cell (1370Å3) and the “molecular weight” calculated from the above formula, the number of formula units per unit cell worked out as Z=4·03. Some ‘vermiculite’ samples contain small groups of mica layers, but the non-basal mica reflections are not observable. The structures of these interstratifications and also of interstratifications containing chlorite are considered. Electron micrographs of vermiculite and the interstratified minerals are presented.

Gibbsite, the dominant mineral in red and yellow earths from Mount Zomba in southern Nyasaland, is derived directly from orthoclase and acid plagioclase (albite-oligoclase). The bauxitization apparently took place at an early stage in the history of the mountain. The evidence suggests that kaolinization succeeds bauxitization in the sequence of weathering, and is related to present-day weathering conditions.

The extraction of naturally occurring free iron oxides from soils and clays with oxalate solutions is distinctly photosensitive. Apparently wave-lengths around 366 mµ, the near ultra-violet region, are the most efficient; this radiation is predominant in the emission of a medium-pressure mercury arc lamp. A qualitative consideration of the dissolution process indicates that in all probability electron transfer from solute to solid via intermediary C2O4− is involved. Such a mechanism may explain the attack of near-neutral oxalate solutions upon natural ferric oxides and the decomposition of the quite resistant hematite in slightly acid media, when irradiated. The reaction appears to be autocatalytic. Electron transfer utilizing thermal electrons may also explain the attack of dithionite ions upon ferric oxides, which was shown to proceed in media above neutrality. Finally, a detailed description of the method is given, including the application for estimating free iron oxides in particle-size fractions obtained during the course of mechanical analysis.

The contribution of free iron oxides to positive and negative charge distribution has been studied on soils and synthetic kaolinite-iron oxide complexes over a wide range of pH values. The charge on the iron oxides is pH-dependent, being positive at low and negative at high pH values, respectively, and accounts for a considerable proportion of the pH-dependent negative charge in soils. Iron oxides increase the buffer capacities of all soils and kaolinite-iron oxide complexes. A considerable proportion of the surface of the clay fraction is covered by iron oxides, resulting in a decreased negative charge on the clay. The isoelectric points of iron oxides in soils are considerably higher than those reported for pure iron oxides. A number of the soils studied were isoelectric at their field pH values, so that under field conditions they would be expected to be extremely infertile. Evidence is presented to show that kaolinite has a permanent negative charge, confirming the view that isomorphous substitutions occur.

Some characteristics of the kaolin→metakaolin reaction have been studied by applying nuclear magnetic resonance, X-ray fluorescence, and acid dissolution techniques to samples at intermediate stages of dehydroxylation. Nuclear magnetic resonance spectra enabled the average distance between hydroxyl groups to be computed, while X-ray fluorescence provided information on changes in the co-ordination number of aluminium and acid dissolution demonstrated the reduction in cohesion between individual layers during dehydroxylation. The last technique appears to be useful in studying the development of new solid phases. The main conclusions are that the reaction proceeds in a stepwise manner by complete removal of constitutional water from octahedral sheets and not by the growth of randomly distributed nuclei, that six-fold coordinated aluminium is gradually transformed into four-fold coordinated aluminium, and that the internal surface becomes progressively accessible to the acid reagent as dehydroxylation proceeds. The ‘ring structure’ of the silica layer remains.

A Quaternary or Recent tuff or pumice has weathered to an allophanic soil reminiscent of some described by Sudo. The soil has interesting engineering properties. Electronmicrographs show the apparent coalescence of alophane ribbons into a thin platy mineral, probably an early form of hydrated halloysite. The formation of crystals by the coalescence of mosaic crystallites is discussed.

Particle-size analyses of brickearth and Clay-with-flints from Kent, England, are presented. A calcareous sample of brickearth is comparable to loess. In the clay fraction, there is more illite (or glauconite) than kaolin; and there is some chlorite. The clay fraction of the Clay-with-flints consists of smectite, illite and quartz and is probably derived directly from the Chalk. Black sand-grade nodules are manganiferous.