Aluminous hydroxy 2/1 clay minerals (‘intergrades’) can form in two ways: Al can either come externally from the solution, or internally from the mineral itself (octahedral or tetrahedral layers). This study is more concerned with the latter which seems the main source in acid soils.

The most common organic acids present under natural conditions were used on two trioctahedral micas (phlogopite and biotite), and their chemical and mineralogical weathering effects were determined.

Different kinds of mineralogical evolution are possible: mineral destruction by a few very complexing acids, possibly with intermediate stages (interstratified minerals); formation of ‘transformation’ smectite by acids derived from sugars; Al-hydroxy vermiculite formation (the 14 Å diffraction peak of which is stable under K saturation and vacuum). This evolution is the most frequent and is produced by the most numerous organic acids (even CO2), the acidity of which results in Al migration in the interlayer, but the complexing ability of which is not sufficient for Al removal.

Low temperature nitrogen adsorption has been used to study the external and internal structure of a number of allophanic soil clays. The results indicate that allophane, in addition to having pores in the intermediate size range (2–10 nm radius), contains micropores of < 1 nm radius. The occurrence of hitherto unreported micropores in allophanic clays has necessitated re-evaluation of the validity of specific surface area measurements.

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.

Chemical and X-ray analysis of eight samples of nontronite are consistent with the Hofmann et al. montmorillonite structure. The finest fraction of some fully hydrated (15·3 Å) samples is composed of crystals all about 6 or 7 unit cells thick, and having 12 H2O per 8 Si, arranged at three levels in the interlayer, possibly as upright octahedra. During dehydration, non-integral 00l sequences, and variations in d001 result from a range of layer thickness in one crystal, caused by loss of water from the crystal periphery with consequent margin collapse. At about 50% relative humidity, the H2O content of nontronite is determined by the tetrahedral aluminium, probably because Aliv controls maximum crystal size.

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.

Les hydrates à deux couches et à une couche d'une saponite calcique ont été étudiés par infrarouge. Les résultats expérimentaux indiquent que dans l'hydrate à deux couches les molécules d'eau directement coordinées à l'ion calcium sont liées par liaisons hydrogène fortes d'une part aux atomes d'oxygène de surface chargés négativement et d'autre part à d'autres molécules d'eau. En revanche, dans l'hydrate à une couche où chaque ion calcium est solvaté par un minimum de 2 molécules d'eau, il n'y a pas de liaison hydrogène intermoléculaire. La plupart des molécules d'eau donnent une seule liaison hydrogène forte. Chacune des molécules d'eau forme un pont hydrogène, l'une avec le feuillet supérieur et l'autre avec le feuillet inférieur.

Since digestion of imogolite or allophane in 5% Na2CO3 at 95°C for periods of 2–100 h leads to solid products different in structure and composition from the starting materials, the procedure is unsuitable for selective dissolution of reactive silica-alumina gels from complex clay mixtures. Cold 5% Na2CO3 does not alter these clays, but effects partial solution, the extent of which depends on the degree of dispersion of the clay and the ratio of solids to solution.

The controversy on the origin of the English Mesozoic fuller's earths is reviewed. Findings of (1) unaltered, devitrified, zeolitized and argillized glass, (2) fragments of holocrystalline and hypocrystalline igneous rocks, and (3) high temperature feldspar and sphene, conclusively demonstrate their volcanic origin. The bulk chemistries of the Bathonian and Lower Cretaceous earths are similar and suggest derivation from ash of trachytic composition. The two earths can be differentiated mineralogically and petrographically by: (a) the presence of sphene, and zeolites replacing glass in the Lower Cretaceous earths; (b) the presence of minor amounts of basaltic glass in the Bathonian earths.

K/Ar radioisotope dating on high-temperature alkali feldspar gives (1) 152–159 Ma for the Bathonian fuller's earth, and (2) 118–125 Ma and 142–148 Ma for the Lower Cretaceous fuller's earths. The distribution of the Mesozoic fuller's earths and the origin of their parent ash is discussed.

Die kristallchemischen Strukturformeln von 2:1-Schichtsilikaten werden unter Voraussetzung der idealen Struktur mit 22 negativen Ladungen von 10 Sauerstoffen und 2 Hydroxylgruppen berechnet. Für die Verteilung der 22 Kationenladungen auf Tetraeder-, Oktaeder- und Zwischenschichtpositionen werden unter Berücksichtigung der direkt gemessenen Zwischenschichtladungen (Lagaly & Weiss 1971) bzw. Kationenumtauschkapazitäten Voraussetzungen und Berechnungsregeln angegeben. Als Beispiele werden die Formeln eines Glaukonits und eines Nontronits berechnet.

Die Verteilung der Kationenladungen in der Kristallstruktur von 2:1-Schichtsilikaten wird am besten in einem Dreiecksdiagramm mit den Ecken Pyrophyllit-Seladonit-Muskowit dargestellt. Jeder Punkt im Dreieck stellt die gesamte Kationenladung in der Formeleinheit gleich 22 dar. Es ist die Summe der Ladungen aus Tetraeder-, Oktaeder- und Zwischenschichtkationen.

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.

A selective chemical dissolution method has been worked out in order to characterize quantitatively amorphous Al, Fe and Si oxides simultaneously in soils. Based on theoretical considerations an alkaline Tiron solution was tried out. Experiments on well-characterized synthetic and natural substrates were carried out in order to find the optimum set of conditions of pH, temperature and reaction time. These are: an alkaline medium consisting of 0·1 m Tiron and buffered at pH 10·5, an extraction temperature of 80°C and an extraction time of 1 h.

The < 1·0 µm fractions of three rhyolitic tephras are shown to contain multifaceted squat cylinders of halloysite at various stages of development. The < 1·0 µm fraction of one of the tephras, the Pahoia Tuff, is composed mainly of large well-ordered squat cylinders, together with amorphous silica flakes, and novel disk-shaped particles of halloysite. The disks always occur sandwiched between silica flakes. A spiral structure is proposed for the disks. The squat cylinders are interpreted as having formed by a similar spiral mechanism to that of the disks, but without the constraining influence of silica flakes.

Electric fields were imposed on aqueous suspensions of the rigid asymmetric particles of crocidolite. Orientational order resulted in the field and the intensity of the light scattered by the medium changed. Such intensity changes were recorded for pulsed electric fields of up to 500 V cm−1 amplitude and 1·5 kHz frequency. When combined with data for the scattered intensity prior to the application of the field, the average particle mass (8 × 10−15 g), length of the rod-like particles (1·3 µm), rotary diffusion coefficient (2·4 s−1) and electrical polarizability (22 × 10−29 Fm2) were all evaluated for this sample. The relationship of the polarizability to the electric double layer structure of the sols is briefly discussed. The transient scattering method offers a reliable and rapid means of characterizing mineral sols.

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.

Imogolite structures in allophanic clays can be recognized and semi-quantitatively estimated by an absorption band at 348 cm−1. Allowance can be made for interference by aluminous dioctahedral layer silicates present in small amounts.

This paper reports an electric birefringence study on dilute solutions of the clay mineral, celadonite. The results show that celadonite particles in solution have the form of rectangular platelets and are characterized by a permanent dipole-moment perpendicular to the plane of the platelet and an induced dipole-moment in the plane of the platelet along the longest dimension. It is suggested that the permanent dipole-moment is an inherent property of the atomic structure of the platelet, whilst the induced dipole-moment is a consequence of the ion-atmosphere surrounding the celadonite particles. The paper also demonstrates the usefulness of electro-optic techniques in studying problems in clay minerology.

X-ray diffraction and electron microscopy show that halloysite occurs widely in soils derived from a variety of parent rocks (granite, gabbro, schist and slate) in north-east Scotland. Both tubular and non-tubular forms are observed, the latter being characterized by electron diffraction patterns with 001 reflection either absent or very weak and diffuse. Clay fractions from a poorly drained profile separated without prior drying of the soil samples contain essentially dehydrated halloysite at the surface, this becoming progressively more hydrated with depth. Since halloysite occurs extensively in soils of widely varying drainage class the mineral is probably not the result of recent soilforming processes but may have originated during Tertiary or interglacial weathering.

Hydroxy-aluminium beidellites are prepared by treating Na-saturated beidellites from Taiwan, with compositions intermediate between end-member beidellite and end-member montmorillonite, with hydroxy-aluminium solutions having OH/Al ratios mainly between 2·00 and 2·50. Chemical analyses show that hydroxy-aluminium ions have compositions near Al6(OH,O)16 and charges near 1·0 in two cases, and near 2·2 in a third case. The basal spacing of the prepared clays is near 17 Å and remains without collapse up to 500°C. Argon adsorption measurements at liquid nitrogen temperatures, after heat-treatment up to 325°C in vacuo overnight, give surface areas near 500 m2/g when the exchangeable Na ions are about one-third replaced by hydroxy-aluminium, but near 200 m2/g when wholly replaced by hydroxy-aluminium. In the latter case the hydroxy-aluminium covers the internal surfaces of the clay almost completely prior to the heat-treatment.

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.

Alunite-kaolinite deposits occur in argillized zones in acid volcanic rocks at Tolfa. Over seventy samples were examined by one or more of the following methods: thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist; their relative amounts are variable but the dickite content tends to increase with depth. SEM studies show that dickite crystals—up to 30 µm—are bigger than kaolinite.

The D/H and 18O/16O ratios of six clays, a whole rock, two biotites from fresh host rock, two chalcedony veins and three local meteoric waters indicate that the clays formed in a meteoric-hydrothermal environment of acid hot spring type at about 80°C. The fresh volcanic host rocks are strongly enriched in 18O relative to ‘normal’ igneous rocks due to derivation from or exchange with 18O-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of this hot spring system was less than 4 m.y.