Amorphous hydroxides of Al, Fe, Mn, Mg, Zn, Co, Ni, etc., are capable of coprecipitating SiO2 even from very dilute (weathering) solutions. Silica minerals form only in those precipitates from solutions undersaturated with respect to amorphous silica (100 ppm SiO2 at 20°C). With higher SiO2 concentrations the precipitates remain amorphous. The size of the cations should allow 6-fold coordination and giving a brucite-like layer. Most suitably sized octahedral ions are Mg, Zn, Ni, Co and Fe2+ (size 0·78–0·82 Å), and chemically pure three- and two-layer clay minerals with these ions are easily synthesized. With a relatively high content of silica in solution (60 ppm SiO2 with 1–2·5 ppm metal) several smectite minerals could be synthesized. With a low content of silica in solution (5–20 ppm SiO2 and ca. 2 ppm metal) the serpentine minerals, could be synthesized.

It is possible to crystallize the difficult to form Al-clay minerals in a solid solution with these more easily synthesized clay minerals.

Clay minerals with the heavy metal ions of Ni, Co, Zn, and with Cu, Cr, etc., can be found in the weathering zone of Gossan and in the lateritic weathering crust of ultrabasic rocks.

The densities of two allophane samples measured by displacement with water were 2·72–2·78 g/cm3, while those measured by a float-sink test in CH3OH-, CH3 COCH3-, C6H6- and CCl4-C2H2Br4 mixtures were 1·84, 1·94–1·98, 2·35 and 2·35–2·39 g/cm3, respectively. The results of the density determinations for montmorillonite, kaolinite, halloysite, imogolite and volcanic glass are also reported. The data were interpreted in terms of the accessibility of the displacement liquids to void spaces within and between the layers or ‘structure units’ of the minerals and a model of the ‘structure units’ of allophane is proposed.

The principal methods for studying clay minerals in andosoils differ especially by the succession of the pretreatments, i.e. differential dissolution of amorphous mineral cements before (first method) or after (second method) the dispersion and recovery of granulometric fractions.

The first method permits a rapid total quantitative estimation of amorphous components and an accurate identification of crystallized clay minerals by means of X-ray diffraction. The second method only proves the presence of allophane and imogolite.

The two methods were applied in two different laboratories on the same soils from the Massif Central. The comparison of the results emphasizes the profit of using the two methods, especially if hypotheses on the mechanisms of clay genesis are to be proposed.

These results, if placed in the context of a more comprehensive study, suggest the existence of two principal phases of clay genesis, strongly connected to the evolution of organic matter in andosoils, as follows: (i) an early, prepedologic phase during which halloysites and smectites would form; (ii) a late phase giving rise especially to some kaolinite and gibbsite.

La sépiolite chauffée à 300°C et 480°C sous vide a été étudiée avec un microscope électronique en haute-résolution. Les clichés donnent une photographie réelle des structures de la sépiolite naturelle et de la sépiolite anhydre, telles qu'elles ont été calculées par la cristallographie. Les résultats obtenus sont en accord avec ceux de la diffraction des rayons X et de la spectroscopie infrarouge.

A kaolin mineral in the form of long fibres, similar to the fibrous kaolin from Brazil (de Souza Santos et al., 1965), was found in the <2 μm fraction of an Ekiti series soil formed on biotite gneiss at the International Institute of Tropical Agriculture, Ibadan, Nigeria. The fibres, up to 6 µm long, appear tubular in electron micrographs. Scanning electron micrographs show that the mineral is formed by direct weathering of ort oclase and, perhaps, biotite.

Bastnaesite occurs in the < 2 µm fraction of the same weathering gneiss but not in the coarser fractions or the overlying soil. It was identified by indexing the X-ray powder pattern of a concentrate obtained by treating the clay with HF. The unit cell thus found as hexagonal with a = 7·056 and c = 9·730 Å, and was confirmed by electron diffraction from selected grains. Evidence of chemical composition was obtained by XRF and IR spectrometry. Bastnaesite can be mistaken for gibbsite because their strongest reflections have nearly the same spacing, d = 4·85 Å, when unheated oriented specimens are used. The occurrence of bastnaesite indicates that it is formed when the rock weathers, but is unstable in the soil.

This paper presents detailed mineralogical analyses of the non-carbonate material obtained from four cored boreholes in the Chalk of Berkshire and Oxfordshire, England. Consideration is given to the relationship between the non-carbonate material and the interstitial water chemistry.

Sample preparation was by means of acid digestion followed by vacuum sedimentation. Analyses were carried out using X-ray diffraction techniques.

Results are presented in a semi-quantitative form based upon integrated peak intensities and are compared to those of previous studies on the Chalk of southern England. It is concluded that the zones of occurrence of the major clay minerals as demonstrated in this study are in general agreement with those established by other authors. The non-carbonate content of the Chalk increases from the concealed beds at the Fair Cross site towards the outcrop Chalk of Lambourn and Harwell. The variation in mineralogy and purity is ascribed to changing conditions of deposition and to diagenesis related to groundwater circulation.

Comparison of the adsorptive properties of two samples of akaganéite prepared by different methods shows that a structural scheme based upon solid rods is much more likely than a previously proposed scheme involving tubes. Specific surface areas, qualitative and quantitative assessments of porosity' and electron micrographs strongly support this structure.