In a study of sandstones from the Coal Measures (Westphalian A and B) of the East Midlands, UK, fibrous illites exhibiting random core and lath structures were observed during SEM examination of material dried in air, but not during TEM examination of ultrathin sections made from the same material. Previous studies have reported random core and lath structures during TEM examination of extracted clay material and it is suggested that they are an artefact of the specimen preparation technique.

Different montmorillonites (Otay, Chambers, Marnia, Camp Berteau, Moosburg, Greek White, Wyoming bentonite were deferrated with a dithionite/citrate solution; iron sulphide by-products were eliminated with an HCl washing and citrates with an H2O2 oxidation. The smaller CEC after deferration could not be assigned to either (i) the occurrence of a positive correlation between CEC and particle sizes smaller than 0.2 µm or (ii) to the mean charge density of these sub-fractions. All sub-fractions had a heterogeneous interlayer cation density distribution.

During weathering of forsterite and tephroite under the humid, tropical conditions of the Ivory Coast, intermediate Mg,Fe3+− or Mn2+-smectites appear prior to widespread development of iron or manganese oxyhydroxides. In forsterite, the smectite varies from an Mg-trioctahedral variety near the centres of altered crystals to an Mg,Fe3+,Al-di/trioctahedral variety near the edges. The Al-content of this latter smectite is supplied by dissolution of adjacent minerals, notably enstatite. An Mn2+-trioctahedral smectite forms in cracks in tephroite crystals.

The Southern Permian Basin of the North Sea represents an elongate E-W oriented depo-centre along the northern margin of the Variscan Mountains. During Rotliegend times, three roughly parallel facies belts of a Permian desert developed, these following the outline of the Variscan Mountains. These belts were, from south to north, the wadi facies, the dune and interdune facies, and the sabkha and desert lake facies. The bulk of the gas reservoirs of the Rotliegend occur in the aeolian dune sands. Their recognition, and the study of their geometry, is therefore important in hydrocarbon exploration. Equally important is the understanding of diagenesis, particularly of the diageneticaily-formed clay minerals, because they have an important influence on the reservoir quality of these sands. Clay minerals were introduced to the aeolian sands during or shortly after their deposition in the form of air-borne dust, which later formed thin clay films around the grains. During burial diagenesis, these clay films may have acted as crystallization nuclei for new clay minerals or for the transformation of existing ones. Depending on their crystallographic habit, the clay minerals can seriously affect the effective porosity and permeability of the sands.

La phase <2 µm issue de l'évolution géochimique superficielle d'une trachydolérite de Provence s'est révélée être presque exclusivement constituée d'une smectite. Son examen, grâce à plusieurs techniques, et notamment l'identification des produits de recristallisation à haute température, montre qu'il s'agit d'un minéral 2/1, surtout magnésien. L'établissement de sa formule structurale approchée, calculée sur la base de 11 oxygènes, en ferait un minéral intermédiaire entre les smectites diet trioctaédriques, avec une occupation de 2.55 cations en sites octaédriques. Cette smectite de Provence n'est pas une saponite au sens strict, compte-tenu du nombre des cations R3+ = 0.94 en position octaédrique, plus élevé que celui de ces minéraux, tandis que la part des cations R2+ en ces mêmes sites est inférieure au seuil de 85%.

X-ray photoelectron diffraction data for three cleavage surfaces from a crystal of a Norwegian lepidolite containing 2·3% Rb and 3% Mn are reported and interpreted. Rubidium is shown to occupy anhydrous interlayer sites equivalent to those of potassium, and to be distributed uniformly throughout the crystals. Cleavage occurs in Mn-rich regions in which the manganese(II) is located in octahedral sites essentially equivalent to those of lithium. The octahedral Al sites can readily be distinguished from the Li,Mn sites and it is concluded that Al occupies predominantly M(2), cis sites while Li and Mn(II) prefer M(1), trans sites. Photoelectron diffraction data also indicate that 40 ± 5% of the Al is tetrahedrally coordinated, compared with a figure of ∼34% deduced independently from a surface analysis.

The distribution of diagenetically-formed clay minerals in Rotliegend dune sandstones of the Southern North Sea Basin is closely related to the following factors: (i) the petrography of the sandstones, (ii) the paleoburial depth and tectonic setting of the area, (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes, and (iv) the facies distribution of the overlying Zechstein. The diagenetic clay minerals are mainly conversion products of feldspars and, to a lesser extent, of detrital clays and micaceous lithic fragments. Sandstones containing dominant drusy illitic and chloritic clay minerals have been buried to depths > 3000 m; if kaolinite is the dominant clay mineral, burial depths were less. Sandstones containing feldspars (detrital and authigenic) up to approximately 7% of bulk volume have permeabilities that are about four times higher than sandstones with similar amounts of kaolinite, and as much as 200 times higher than sandstones with similar amounts of illite and/or chlorite.

A number of Ni-hydroxy-montmorillonite complexes were prepared by titrating NiCl2 solutions with NaOH in the presence of charge-reduced and normal montmorillonite suspensions. These complexes were characterized by chemical analysis, CEC, XRD, IR and temperature-programmed reduction (TPR). The R (charge-reduced) Ni-hydroxy complexes proved to be merely mechanical mixtures of Ni(OH)2 and clay. The reducibility of Ni contained in these samples was not affected by preliminary heat treatments. The nature of the N (normal) Ni-hydroxy montmorillonites depended principally on the amount of Ni actually incorporated into the precipitates. Neither the OH/Ni ratio selected for the preparation or the quantity of Ni initially added completely determined the type of complex formed. TPR patterns of the N samples generally showed three reduction bands whose respective intensities varied with the quality and the amount of Ni present. One of these reduction bands, occurring at high temperature, measured the extent of incorporation of Ni into the silicate framework during the TPR run. The intensity of this high-temperature band with respect to lower temperature ones was increased by thermal pretreatments. The results emphasize that when TPR patterns of natural Ni-bearing clays are being interpreted it is important to recognize that thermal transformations may occur in the samples caused by the heat supplied during the TPR run itself.

La majorité des termites sont en rapport avec le sol et utilisent l'argile de ses horizons, pour édiffer leurs termitières. Pour les termites du sol, dont la plupart sont des maçons, l'argile est avant tout le matériau de base offrant les propriétés de rétention d'eau, de plasticité et d'ahésivité, leur permettant de confectionner les mortiers et les microbétons de leurs unités de constructions. Si les termites humivores absorbent toujours une quantité relativement importante d'éléments argileux avec leurs aliments, les termites lignivores n'ingérent qu'occasionnellement une fraction minime d'argile. Leur action sur l'évolution des silicates se traduit par des néoformations de minéraux argileux au sein de leurs constructions: illite ‘artificielle’, hydrobiotite, vermiculite, halloysite, montmorillonite, et parfoils sépiolite et attapulgite. Ces néoformations résultent de l'action conjugée du travail de broyage et de trituration salivaire de minéraux encore peu altérés de la profondeur du sol, et des conditions physico-chimiques de milieu particulières aux termites.

In the past, two factors have impeded the quantitative estimate of Fe-oxides in soils by X-ray diffraction. First, Fe-oxides are still quite often considered X-ray amorphous, although numerous results, e.g. a low ratio of oxalate- to dithionite-soluble Fe, have indicated the opposite. Second, even if crystalline, the cocentration of Fe-oxides in many soils is low, thereby complicating their identification by XRD. Recently, however, more sensitive methods such as Mössbauer spectroscopy and Differential-XRD (Schulze, 1981) have been introduced, which substantially reduce the lower limit of detection. Because these two methods are not generally available and, especially in the case of M&oum;ssbauer spectroscopy, are rather time consuming, ordinary XRD should be adapted for quantitative estimation of Fe-oxides.

Determination can be facilitated by using samples in which the Fe-oxides are concentrated by particle-size separation and a 5 M NaOH boiling treatment (Norrish & Taylor, 1961). The latter treatment is particularly suitable for kaolinitic soils as the Fe-oxides are unaffected―provided certain precautions are taken (Käimpf & Schwertmann, 1982a). This paper gives details of a procedure for the quantitative estimation of goethite (Gt) and hematite (Hm) by XRD.

La prise en compte simultanée des résultats de diffraction des RX et de spectrométrie infrarouge a permis de préciser la nature des interactions entre l'eau, les cations compensateurs et les feuillets dans les phases à 14.6, 12.2 et 10.1 Å d'une saponite-Li de synthèse. En effet, la diffraction des RX renseigne sur l'espace interfeuillet disponible à l'eau et aux cations compensateurs alors que les perturbations observées sur les fréquences de vibration de valence des hydroxyles de structure apportent des indications sur la nature et la position des atomes situés à l'aplomb des cavités hexagonales. Cette étude a permis de confirmer la présence de cations hydratés (Li-H2O)+ dans les cavités ditrigonales des feuillets de la saponite-Li amené àun état d'hydratation correspondant à la phase à 10.1 Å.

The Permo-Triassic red bed sequence penetrated in BNOC Well 72/10-1A, located in the Western Approaches, is divided into eight clay mineral zones. The zones are defined by variations in the clay assemblage in which iron- and magnesium-rich chlorite, corrensite and mica occur. This zonal sequence is matched mineralogically and stratigraphically with the pattern of clay mineral variation recorded from the Permo-Triassic sediments of the south Devon coast, some 500 km distant.

It is now generally accepted that species names can be given to regularly interstratified clay minerals, in accordance with the recommendation of the AIPEA Nomenclature Committee (Brindley & Pedro, 1970). In Part A of this report we suggest (i) criteria for defining the degree of regularity of alternation of different layer types that should merit a name, (ii) data that should be provided for documentation of a regular interstratification, and (iii) some examples of interstratifications that do not merit names. In Part B of the report we apply the suggested criteria to the analysis of regular interstratifications previously reported in the literature, and make recommendation regarding the usage of existing names.

Ferrihydrite transformed to crystalline Fe(III)-oxides in salt solutions but the degree of transformation generally decreased with increasing concentration (decreasing water activity) and haematite was favoured over goethite. More transformation occurred for chloride than for nitrate or sulphate and more for calcium than for magnesium or sodium. Calcium and magnesium favoured haematite over goethite. The results support the hypothesis that haematite not only can be formed but will be favoured over goethite in natural environments with low water activity, for instance dry or saline soils or sediments. The ionic environment modifies the general water activity effect.

Sediments of the Tertiary Akata and Agbada Formations have been studied by mineralogical and petrographic techniques. In the Akata and Agbada shales the clays are largely detrital, with only minimal alteration attributable to the effects of burial diagenesis. Clay minerals occur in minor (<10%) amounts in the reservoir sandstones of the Agbada Formation both as detrital and authigenic phases. There are significant differences between the clay assemblages of the water- and hydrocarbon-bearing sandstones. The latter have lower overall clay contents and authigenic kaolinite, whilst authigenic kaolinite and smectite are present in the water-saturated sandstones. These differences are thought to be related to the early migration of hydrocarbons into the reservoir sands. The causes of the poor degree of cementation in the Agbada sandstones are discussed along with the engineering problems posed by their very friable nature.

Amorphous iron oxides in soil are often extracted by an ammonium oxalate solution (Schwertmann, 1964). This treatment may, however, also dissolve crystalline iron oxides and iron silicates (McKeague & Day, 1966; Baril & Bitton, 1969; McKeague et al., 1971; Arshad et al., 1972; Pawluk, 1972; Schwertmann, 1973; Taylor & Schwertmann, 1974; Borggaard, 1976).

It has been shown that EDTA can selectively extract amorphous iron oxides from soils (Borggaard, 1979, 1981) and a synthetic mixture of amorphous iron oxide, goethite, and hematite (Borggaard, 1976). As pointed out previously (Borggaard, 1979), the EDTA method should also be tested on selected minerals to decide if it can serve as a reference method against which other less time-consuming methods may be tested.

The mineralogy, petrology and trace element geochemistry of volcanogenic glauconites and smectite-rich clays are described and related to clay assemblages in Lower and Upper Cretaceous sediments of southern England and Northern Ireland. Volcanogenic glauconite grains represent argillized lava particles of predominantly mafic composition and may have been derived from submarine basaltic magmatism; they occur in all the sediments examined (Aptian-Senonian), and are particularly abundant in the Cenomanian-Campanian Hibernian Greensand of Antrim. The smectite-rich clays in southern England have developed by the argillization of predominantly acid or alkaline ash during early diagenesis. Three types of volcanogenic deposit are recognized. Primary bentonites are thin ash-falls deposited in quiet, brackish and marine waters (Speeton Clay, Ryazanian; Weald Clay, Barremian). Secondary bentonites are local accumulations of ash transported into the Cretaceous seas by rivers draining ash-blanketed, local land areas (London Platform, Portsdown Axis). These deposits are well-developed in the Sandgate Beds, Folkestone Beds and their contiguous deposits, and the lower part of the Gault (Upper Aptian-Middle Albian). The ash originated from penecontemporaneous, subaerial vulcanism located in the southern part of the North Sea. The most conspicuous phase of activity occurred during late Aptian times and has been dated by 40Ar/39Ar isotope analysis at 112 m.y. Bentonitic clays and marls are widespread accumulations of argillized ash that occur as a fine-grained fringing facies to glauconitic quartz sand facies. They make up the upper part of the Atherfleld Clay (Aptian) and the upper part of the Gault (Upper Albian), and they are associated respectively with the Hythe Beds (Aptian) and the Upper Greensand (Upper Albian). They may also occur in the lower part of the Lower Chalk (Cenomanian). The distribution pattern of these smectite-rich clays in southern England is related to the changing palaeogeography of the area in Cretaceous times, and the general coincidence of extensive glaueonite deposits and smectite-rich clays in the Middle and Upper Cretaceous of western Europe and along the eastern seaboard of North America is briefly discussed.

The point of zero charge (pzc) of synthetic Fe-oxides is well documented and usually ranges between pH 7 and 9 (Parks, 1965; Schwertmann & Taylor, 1977). In contrast, the pzc of natural Fe-oxides has only rarely been determined. Using electrophoretic mobility, Van Schuylenborgh & Arens (1950) found that a natural goethite had a much lower pzc (∼3) than synthetic goethites. They attributed this to better crystallinity of the natural goethite caused by slower crystallization. Soils dominated by Fe- (or Al-) oxides rarely have pzc values as high as those of pure oxides. This is usually attributed to the presence of negatively charged impurities such as clay silicates and organic matter (Parfitt, 1981).

Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe2O3.9H2O, occurs in hydromorphic soils (Schwertmann et al., 1982) and is the main component in ochrous precipitates formed when Fe-bearing fresh waters come in contact with air (Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). Under these conditions the ferrihydrite is reasonably free of other charge-active minerals. The aim of this study was to find out if the pzc of these natural ferrihydrites differed from those of synthetic samples.

Green-blue Fe(II)Fe(III)-and Fe(II)Al(III)-hydroxycarbonates with a pyroaurite structure were prepared by Taylor & McKenzie (1980) and Taylor (1980). They were thought to be related to the unstable mineral phases which occur naturally in the dark green-blue mottles of wet, anaerobic horizons of gleyed soils. These synthetic compounds are unstable to oxidation, suffering structural and colour changes, with the ultimate products depending on the environmental conditions. This instability makes it difficult to identify them and study their characteristics by X-ray diffraction and other methods.

The technique described in this note has been found to stabilize the synthetic green-blue compounds satisfactorily. It involves handling and storage of the compounds in a CO2 atmosphere, such as might occur under natural anaerobic conditions in a soil below the ground-water level. The same principles can be applied to collect field samples and maintain them in a stable state.

Phosphorites occur throughout the Negev, southern Israel, within the upper part of the Mishash Formation of Late Campanian age. These phosphorites are part of the Upper Cretaceous-Eocene Tethys phosphatic province. Two units can be differentiated in the Mishash Formation: a lower chert unit and an upper phosphatic unit (Nathan et al., 1979). In the upper unit, rich, economic phosphorite beds with more than 25% P2O5 alternate with chert beds and so-called ‘sterile intercalations’ which contain about 15% P2O5.

Clinoptilohte (Mumpton, 1960), a zeolite of the heulandite-clinoptilolite family, was identified from XRD patterns of air-dried and heated samples of the clay fraction from the insoluble residue (1-20% of the rock) of selected phosphatic samples from the Hor Hahar field, Negev, southern Israel. Clinoptilohte occurred mainly in the ‘sterile intercalations’. Inasmuch as clinoptilohte has not been identified in older formations in the Negev, it was presumed to be authigenic in the Mishash Formation.