A general expression for diffracted intensities from partially ordered layer structures with a ‘mistake’ has been calculated. The ‘mistake’ consists of a shift of a layer parallel to adjacent layers by any arbitrary fraction b/q along the b axis, q being any integer. The expression is free from any simplifying approximations.

Experimental transformations of feldspars and muscovites following additions of magnesite and dolomite have been studied at PH2O = 1 kbar, T = 200–600°C. Formation of layer silicates such as smectite, 7 Å (Mg,Al)-serpentine, some mixed-layer phases and other minerals is shown to be a function of the composition of the starting materials, temperature and run duration. It is established that 1 M- and 2 M1-phlogopites are formed from 1 M- and 2 M1-muscovites, respectively, under Mg-bearing hydrothermal conditions. Some causes of variations in the composition of 7 Å (Mg,Al)-serpentines at elevated temperature as a function of the composition of hydrothermal media are given. These data may be used to explain the main characteristics of clay mineral formation from feldspar- and muscovite-bearing sedimentary rocks during their alteration in postdiagenetic and metasomatic processes.

The clay mineralogy of a sequence of 36 andesitic tephra and associated paleosols dating from 320 to > 100,000 years BP has been studied using XRD, IR, DTA and TEM techniques. The mineralogy and weathering sequence is compared with that of rhyolitic tephra of central North Island, New Zealand.

Sediments from a Quaternary beach ridge complex along the coastal plain of north-west Egypt have been examined by X-ray diffraction and optical and scanning electron microscopy. Palygorskite was identified in the caliche capping the beach ridges and in the gypsiferous marls between the ridges; petrographic evidence indicated that this had formed authigenically in both environments. Examples are thus provided of the pedogenic and evaporative in situ formation of palygorskite.

The morphology of volcanic glass particles in rhyolitic and andesitic tephra of central North Island and Taranaki areas of New Zealand has been studied by scanning electron microscopy. Electron probe analyses of the glasses are compared with those of the clays to which they weather. Loss of silica characterizes the weathering of both glasses. The rapid rate of weathering of andesitic glass is attributed to its occurrence as fine, soft microlites and extensive substitution of Al for Si in the structure. Rhyolitic glass weathers more slowly because it occurs as hard and brittle particles containing relatively little alumina. It is suggested that the structure, chemical composition and chemical activity of allophane is governed largely by the chemical composition and bonding characteristics of the parent glass.

L'étude porte sur la décomposition du massif de bandes de vibration de valence O-D centré vers 2500 cm−1 que présentent les spectres infrarouge des hydrates à deux couches des vermiculites -Ca et -Mg partiellement deutérées. La détermination des bandes composantes a été effectuée à partir, d'une part de l'étude du dichroïsme des bandes de vibration infrarouge, d'autre part des données structurales disponibles sur les couches interfoliaires de ces minéraux. Ces deux méthodes conduisent—avec un accord satisfaisant—à une décomposition en 4 bandes composantes correspondant à 4 types de liaisons O-D...O. Parmi ces liaisons entre molécules d'eau, certaines sont relativement courtes, ce qui conduit à penser qu'il existe dans la couche interfoliaire de ces hydrates une charpente de molécules d'eau un peu analogue à une charpente anionique.

The Weissenberg method has been used to study crystalline fragments of hydrobiotites from Stupná near Křemže, Czechoslovakia. The intensity distribution for basal reflections recorded in zero-level photographs indicates that most ‘single crystals’ are nearly regular interstratifications of vermiculite and biotite. Proportions of vermiculite in these interstratifications lie between 12 and 60%.

A method for the preparation of Ln-montmorillonite (Ln = La, Ce, Sm, Gd, Er and Y) that ensures saturation of the CEC is described and the most favourable equilibrium conditions are discussed. The H+- Ln3+ exchange rate in acid montmorillonite and the effect of concentration and pH on the retention of Ln3+ ions by the clay are also investigated. The presence of montmorillonite modifies the pH value at which precipitation of hydroxides begins, a process that actually occurs at pH ∼ 5.

Two well-characterized soils containing substantial amounts of interstratified mica-smectite have been examined by a combination of techniques in an attempt to correlate the characteristics of the interlamellar surfaces with other properties. The techniques used included XRD and chemical analyses, determination of surface area by nitrogen sorption and by desorption of water and ethylene glycol monoethyl ether vapour, and determination of cation exchange capacity. Reasonable agreement was found between the results obtained for the proportion of expanding or smectite-like surfaces, although estimates by XRD, especially when coarser material (> 2/µm) was included, were generally lower than those obtained by the other methods. The mean density of charge on the surfaces of the soil clays appeared to be similar to that of smectite. The decrease in porosity observed for both soils dried by the critical point method was largely accounted for by the decrease in volume associated with the loss of interlamellar water from the smectite-like interlamellar surfaces included within the clay fraction.

Im klassischen Lössprofil von Stillfried an der March wurden in der oberen Humuszone des Stillfrieder Komplexes Mineralbestand und Mikrogefüge mittels Röntgendiffraktometeranalyse und Rasterelektronenmikroskopie untersucht. In der Schluffund Tonfraktion fand sich neben anderen Tonmineralen Palygorskit, der vor allem in Form feinstrukturierter Mineralanhäufungen über Karbonatknollen auftritt.

Die charakteristische Morphologie der dünnstengeligen und faserförmigen Einzelkristalle sowie die Bildungsbedingungen bei alkalischem Milieu und hohem Mg-Gehalt der Bodenlösung sprechen für eine pedogenetische Neubildung des Palygorskit.

L'interprétation des diffusions en dehors des taches de Bragg dans une vermiculite saturée par du baryum (d001 = 12·20 Å) permet de préciser l'organisation de l'espace interlamellaire. Le motif élémentaire est constitué par un groupement Ba-6 H2O où le baryum est engagé dans une cavité ditrigonale et les 6 molécules d'eau sont dans le plan médian, l'ensemble forme une pyramide très aplatie de base hexagonale. La répartition de ces motifs est bipériodique avec une maille 2 a,b centrée en projection. Cette organisation se développe dans des domaines identiques, orientés à 120° les uns des autres.

The vertical clay mineral distribution pattern in two sediment cores taken from two East African lakes has been studied in detail. In the core from Lake Albert (Mobutu Sese Seko) a clay assemblage consisting principally of smectite with some illite and kaolinite in the uppermost segment of the core changed with depth into first a mixed-layer illite-smectite dominated clay and then at 6·60 m into an iltite dominated clay. Lower in the core there was again a transition towards interlayer mixtures, and in the bottom segment towards smectite. These changes are interpreted as a diagenetic transformation of smectite into illite, effected by saline palaeo-lake water with an unusually high K/Na ratio. Three smectite→illite-smectite→illite transition cycles were recognized in a 56 m deep sediment core from Lake Manyara. The sections in which illite dominated contained silt-sized analcime, while those with interlayer mixtures contained alkaline zeolites. The diagenetic illitization of smectite appeared to parallel the process of analcime formation, the K necessary for illitization being released during K,Na-zeolite→analcime transformation. These occurrences suggest that high temperature and pressure may not constitute absolute prerequisites for the diagenetic illitization of smectite.

Hydration/dehydration behaviour and the effect of various thermal treatments on montmorillonites saturated with lanthanide ions have been investigated by X-ray diffraction, thermal analysis (DTA, TG, DTG), IR spectroscopy and sorption-desorption of water vapour techniques. Heating at 150°C under 10−5 torr did not eliminate all the interlayer water of the montmorillonite, neither did it affect the CEC. Heating above 160°C caused a reduction in CEC. At 25°C La-montmorillonite takes up a maximum of three water layers in the interlayer space, the water molecules adopting a nonacoordinated distribution around the La3+ cation.

The thermal behaviour of a ferrous doped kaolin has been studied by Mössbauer spectroscopy and electron paramagnetic resonance spectroscopy. From the observations it is concluded that the iron substitutes trioctahedrally as Fe2+ in the ‘gibbsite-like’ sheet in place of dioctahedral aluminium. The g = 2 EPR signal is shown to be associated with these ferrous ‘cells’ which appear to occur in clusters. It is suggested that these ferrous cells are trapped within the normal dioctahedral aluminium structure. Dehydroxylation of the ferrous iron cells takes place between 623 and 673 K leading to the formation of an iron-rich pyroxene and, by 723 K, a ferric oxide. At temperatures > 723 K the pyroxene itself oxidizes to a second ferric oxide. The EPR signal changes at 623 K and disappears at 723 K. The signal is attributed to a trapped hole induced by X-irradiation, located near a silicon atom on the boundary between normal dioctahedral cells and trioctahedral Fe2+ cells. It is possible to extend the model to explain some puzzling features concerning the g = 2 EPR signals reported by other authors and to propose other effects which might result from the presence of these cells.