The SEM, XRD, FFIR and DTA analyses of different size-fractions of clay material from sandstone reservoirs which have experienced a large range of burial conditions have been used to examine the different steps of the depth-related kaolinite-dickite reaction. Dickite progressively replaced kaolinite within a range of burial depths estimated between about 2500 m and 5000 m. The kaolinite-to-dickite reaction proceeds by gradual structural changes concomitant to crystal coarsening and change from booklet to blocky morphology. The crystallization of dickite proceeds by two distinct paths: (1) Accretion of new material from either dissolution of smaller unstable kaolinite crystals and/or detrital minerals (chiefly feldspars), on early-formed coarser metastable kaolinite crystals which exert extended morphological control on the growing crystals. (2) Neoformation of ordered dickite which will continue to grow by a dissolution-crystallization process. The kaolinite-todickite reaction is kinetically controlled and anomalies in the kaolinite/dickite ratio observed in certain sandstone reservoirs may be used to assess the timing of invasion by hydrocarbons.

Grain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.

The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.

Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

The clay mineralogy of soils and of the main calcareous sedimentary parent rocks of southern Iran were investigated to determine their origin and factors controlling their distribution pattern in soils. The results revealed that the soil-available moisture plays the major role in the distribution pattern of palygorskite and smectite clay minerals in the arid and semi-arid areas studied. There is an inverse correlation between palygorskite and smectite with regard to the soil-available moisture as expressed by P/ETº (ratio of mean annual precipitation to mean annual reference crop evapotranspiration). At P/ETº values >0.4 palygorskite transforms to smectite. Smectite is thought to be mainly of ‘transformed’ origin. It is detected in trace amounts in soils of more arid areas and increases in soils having greater available moisture. The general decrease in illite content with depth is related mainly to its transformation to smectite under favourable moisture conditions of the deeper horizons. Palygorskite is considered to be inherited in plateau soils of the arid regions whereas in saline and alkaline soils and soils with high gypsum, it is mainly of authigenic origin. The P/ETº and gypsum content show a significant correlation with the palygorskite content. The occurrence of kaolinite in some soils is due to its inheritance from the surrounding kaolinite-bearing Cretaceous rocks. Illite and chlorite abundance in soils is also largely related to their presence in parent rocks. The rare occurrence of vermiculite in the studied calcareous soils is mainly related to its lower stability under high pH, low Al activity and the presence of large amounts of Si and Mg in soils.

One-dimensional swelling tests and hydraulic conductivity tests have been performed at vertical effective stresses up to 450 kPa on Na-bentonite powder and compacted sand/Na-bentonite mixtures (5, 10 and 20% bentonite by weight) to investigate the use of bentonite-improved soils for waste containment. It was found that bentonite powder swells to reach a final state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, regardless of preparation technique. Swelling of sand/bentonite mixtures expressed in terms of the clay void ratio show a deviation from bentonite behaviour above a stress which depends on the bentonite content. Hydraulic conductivity data for bentonite and sand/bentonite mixtures indicate an approximately linear relationship between logarithm of hydraulic conductivity and logarithm of void ratio. A design model based on the clay void ratio, and the sand porosity and tortuosity is presented enabling the hydraulic conductivity of a mixture to be estimated.

Empirical porosity/depth trends from the Norwegian Continental Shelf are discussed in the light of models on compactional processes during burial. Linear best-fit lines are useful as a first approximation, illustrating the general decline in porosity with depth. Porosity increases non-linearly with pore-pressure, and notably higher than normal porosity is found in very high overpressure reservoirs. Hydrostatic pressure sandstones from the Haltenbanken area are on average more porous than equivalents from the northern Viking Graben. This may be related to the burial history, as generally more subsidence occurred in Late Pliocene and Pleistocene in the Haltenbanken basins. The porosity/depth data show considerable scatter, and this is to a large degree related to the initial mineralogical composition. Correlated with mineralogical analyses, the reservoir sandstones show a trend towards lower porosity with increasing clay content at shallow burial, probably due to more extensive mechanical compaction. At depths >3.0–3.5 km, the porosity loss is highest in sandstones with low clay contents. This may be explained by a lower rate of quartz cementation in sandstones with abundant intergranular clays or clay coatings around quartz grains. These data show that pre-burial mineralogy is a very important factor in porosity prediction of sandstones, in many cases more important than the degree of overpressure.

X-ray powder diffraction studies have been made of the structure changes which occur on dehydrating the minerals diaspore and goethite, and also the synthetically-prepared “δ-Fe2O3.” For diaspore and goethite the changes are interpreted in terms of superstructures referred to a hexagonal, anion lattice sub-unit which is common to both the α-type oxyhydroxides and the anhydrous corundum or hematite structures. The ferromagnetic δ oxide, obtained by rapid oxidation of ferrous hydroxide, is shown to be a true oxyhydroxide of the form FeOOH. It transforms into goethite on heating at about 150°C but the change is incomplete due to a tendency to lose water directly to the atmosphere. An approximate structure analysis has been made, using the X-ray powder data for two relatively well-crystallized δ oxide preparations. It is concluded that the ferric ions occupy the interstices of the anion lattice in such a way that approximately 80 per cent. of the iron is randomly distributed in octahedral sites, while the remainder is similarly placed in tetrahedral sites. This cation distribution enables both the transformation to goethite and the magnetic properties of the oxide to be satisfactorily explained.

Calibration of the internal and external signals appearing at geff ∼4 in the electron paramagnetic resonance (EPR) spectra of twenty-four kaolinites, has led to the recognition of two types of kaolinite-iron. The amount of iron associated with the internal signal (I iron) shows good inverse correlations with parameters used to describe the degree of crystalline perfection in kaolinite. Iron associated with the external signal (E iron) is insensitive to crystalline order. By considering both the amount and nature of the iron, the kaolinites investigated are placed in three groups. The first contains well-crystallized varieties characterized by low total iron and EPR spectra consisting mainly of an E signal. Poorly-crystallized kaolinites contain more iron and are characterized by a fifty-fifty partition of this iron into E and I sites. The third group contains samples which are intermediate with respect to both the amount and nature of the iron.

Because of their large potential for agricultural, industrial and medicinal applications, nanomaterials have been the focus of much research during the past few decades. Nanoclays are natural nanomaterials that occur in the clay fraction of soil, among which montmorillonite and allophane are the most important species. Montmorillonite is a crystalline hydrous phyllosilicate (layer silicate). Organically-modified montmorillonites or ‘organoclays’, formed by intercalation of quaternary ammonium cations, have long been used as rheological modifiers and additives in paints, inks, greases and cosmetics and as carriers and delivery systems for the controlled release of drugs. Perhaps the largest single usage of organoclays over recent years has been in the manufacture of polymer-clay nanocomposites. These organic–inorganic hybrid materials show superior mechanical, thermal and gas-barrier properties. Organoclays are also useful in pollution control and water treatment. Allophane is a non-crystalline aluminosilicate derived from the weathering of volcanic ash. A large proportion of the agricultural land in Chile is covered by volcanic soils,the clay fraction of which is dominated by allophane. Consisting of nanosize (3.5–5.0 nm) hollow spherules, allophane is a suitable support material for enzyme immobilization. Allophane is also effective at adsorbing phenolic compounds and colour from kraft mill effluents and phosphate from water and wastewater.

The use of ground granulated blast furnace slag (ggbs) is well established in many cement applications where it provides enhanced durability, including high resistance to chloride penetration, resistance to sulphate attack and protection against alkali silica reaction (ASR). The use of ggbs in soil stabilization is, however, still a novel process in the UK although it has been used in South Africa. This paper reports on efforts to extend the use of ggbs to highway and other foundation layers by determining the beneficial effect which it has on the reduction of expansion due to the presence of sulphates. The paper describes the results of laboratory tests on lime-stabilized kaolinite containing different levels of added sulphate to which different amounts of ggbs have been added. The tests determine the strength development of compacted cylinders, moist cured in a humid environment at 30°C, and the linear expansion of these moist cured cylinders on soaking in water. The results illustrate that small additions of ggbs to sulphate containing clays which are stabilized with lime reduce substantially their expansion when exposed to water and have no significant deleterious effect on strength development.

Infrared reflectance spectra of Li-, Na-, K-, Rb- and Cs-saturated samples of montmorillonite Zenith-N (Milos, Greece) have been measured in the 400–1300 cm-1 region in an attempt to elucidate the behaviour and migration properties of alkali cations after heating the montmorillonites at 300°C for 24 h. Deconvolution of the complex Si–O stretching band reveals that the component band that arises from the asymmetric stretching vibrations of the silicon-apical oxygen units exhibits the biggest change upon heating the montmorillonites. The normalized absorption area of this band has been correlated with the decrease in the layer charge due to the cation migration. Analysis of the IR data for each alkali-saturated montmorillonite has shown that only Li+ migrates near, or to, vacant octahedral sites.

It is well known that illite-smectite can form from smectite at elevated temperatures in natural and experimental systems. However, the conversion of smectite to illite-smectite is also found in some natural systems that have never been heated. The present experiments show that illite layers can form from smectite by chemical reaction at 35° and 60°C at high solution pH. The rate of this reaction is accelerated by wetting and drying.

The increasing presence of pesticides in natural ecosystems has stimulated research to look for improved adsorbent materials which can be used to remediate and prevent soil and water contamination by these compounds. Among the different materials that have been assayed as adsorbents of pesticides are natural clay minerals, particularly 2:1 phyllosilicates and their structurally complementary synthetic analogues layered double hydroxides (LDHs). The great interest in natural clays and LDHs as adsorbent materials is mainly related to the large specific surface areas associated with their layered structure, the ease with which they are obtained or synthesized, and the possibility of modifying their surfaces to increase their affinity for specific adsorbates. This review summarizes the adsorptive properties of natural clays and LDHs for pesticides and related organic compounds. Particular emphasis is given to the surface modification of clay minerals and LDHs with organic ions as a strategy to improve the efficiency of these materials as pesticide adsorbents. Potential applications of unmodified and modified clays and LDHs as adsorbents to prevent and remediate soil and water contamination by pesticides are also discussed.

At 25°C, synthetic schwertmannite, a common iron compound in acid mine drainage, completely converted in water to a sulphate-containing goethite within ~100 days. The rate of transformation doubled as the pH increased from 4.0 to 7.2. In deviation from its normal acicular habit, the goethite appeared as spherical aggregates.

Both aluminium substitution and poor crystallinity reduce the magnetic hyperfine field of goethite. Mössbauer spectra taken at 4·2 K show that the effect of poor crystallinity is similar to that of Al substitution, i.e. it reduces the saturation hyperfine field. A multiple correlation was found to exist between the magnetic hyperfine field at 4·2 K as a dependent variable vs Al substitution and crystallinity as independent variables. If a hyperfine field is to be interpreted with respect to either Al substitution or crystallinity, it is therefore necessary to have knowledge of the other variable.

The exchangable cations present on certain montmorillonites can be replaced by long-chain quarternary ammonium cations, and the resulting clays have the important property of being able to swell and disperse in a variety of organic solvents. Organophilic clays have a wide range of industrial applications and are, for example, important constituents of oil-well drilling fluids, paints, plastics and greases; their role in these is discussed. Organophilic clays have been extensively studied for about 50 years, but neither the structure nor the mechanisms by which they swell and disperse in organic solvents are clearly understood. An attempt is made to interpret the industrial applicatons in terms of published ideas concerning the structure and swelling properties of organophilic clays.

Pore-lining chlorites are often responsible for the preservation of porosity in deeply buried sandstones because they inhibit the formation of quartz overgrowths, but little is understood about when and how they form. Chemical analyses and XRD data indicate that there are at least two common types: Fe-rich, and Mg-rich. The Fe-rich examples occur as individual euhedral crystals, are invariably interstratified with 7 Å layers (probably berthierine) and form as the Ib β = 90° polytype at low temperatures. With increasing temperature, their chemistry changes, 7 Å layers are lost, crystal size increases, and eventually they transform to the high temperature IIb β = 97° polytype. The Mg-rich examples occur as boxwork arrangements of crystals, are not interstratified with 7 Å layers and are exclusively the IIb β = 97° polytype. The Fe-rich examples occur most frequently in sandstones that were deposited at the transition between marine and non-marine environments and the presence of Fe-rich oolites in many samples suggests a link to the ironstone facies. They probably formed originally at surface or near-surface conditions as a 7 Å mineral, such as berthierine or even odinite, in a fresh water/marine water mixing zone in tropical regions. The Mg-rich varieties tend to be found in aeolian or sabkha sandstones in close association with evaporites. They are probably replacements of Mg-rich smectites via the intermediate mineral corrensite. Precursor Mg-rich smectites formed originally from evaporite brines at near-surface conditions; chlorite itself was not formed until temperatures were high enough to crystallize the IIb β = 97° polytype.

Small amounts of Fe(III) induce precipitation of Fe(II) from carbonate solutions around neutral pH values. The reaction involves the formation of Fe(II)Fe(III) hydroxycarbonate, a green, layer-structured compound of the pyroaurite group of minerals. Variations in the concentration of Fe(II) in the solution phase, and in the form and amounts of the total Fe(III) in the system, may cause other phases such as magnetite and siderite to form also. Freeze-dried Fe(II)Fe(III) hydroxy-carbonate undergoes topotactic alteration to a magnetic phase, presumably maghemite, when stored under vacuum. The compound is readily oxidized in air in either the moist or dry state, rapidly becoming yellow or yellow-brown in colour. Oxidation leads to the formation of either goethite, lepidocrocite, ferrihydrite or mixtures of these phases, depending on the mode of oxidation and the impurities present. The similarity between the rapid colour changes and the oxidation products of the blue-green precipitates in gleyed soil horizons and this Fe(II)Fe(III) hydroxy-carbonate leads to the speculation that this and related compounds may be the meta-stable precipitates in such soils, rather than a previously postulated hydro-magnetite phase. Eh and pH conditions for the formation of Fe(II)Fe(III) hydroxy-carbonate are given, and are in the range encountered in the soil environment.

The instability of shales in drilled formations leads to serious operational problems with major economic consequences for petroleum exploration and production. It is generally agreed that the nature of the clay minerals in shale formations is a primary causative factor leading to their instability, although the exact mechanism involved is more debateable. Currently, the principal cause of shale instability is considered to be volume expansion following the osmotic swelling of Nasmectite. However, illitic and kaolinitic shales may also be unstable, so that interlayer expansion cannot therefore be considered as a universal causative mechanism of shale instability. This review considers alternative scenarios of shale instability where the major clay minerals are smectite, illite, mixed-layer illite-smectite (I/S) and kaolinite respectively. The influence of interacting factors that relate to shale clay mineralogy such as texture, structure and fabric are discussed, as are the pore size distribution and the nature of water in clays and shales and how these change with increasing depth of burial. It is found from the literature that the thickness of the diffuse double layer (DDL) of the aqueous solutions associated with the charged external surfaces of clay minerals is probably of the same order or even thicker than the sizes of a significant proportion of the pores found in shales. In these circumstances, overlap of the DDLs associated with exposed outer surfaces of clay minerals on opposing sides of micropores (<2 nm in diameter) and mesopores (2–50 nm in diameter) in a lithostatically compressed shale would bring about electrostatic repulsion and lead to increased pore/ hydration pressure in smectitic, illitic and even kaolinitic shales. This pressure would be inhibited by the use of more concentrated K-based fluids which effectively shrink the thickness of the DDL towards the clay mineral surfaces in the pore walls. The use of soluble polymers would also encapsulate these clay mineral surfaces and so inhibit their hydration. In this scenario, the locus of action with respect to shale instability and its inhibition is moved from the interlamellar space of the smectitic clays to the charged external surfaces of the various clay minerals bounding the walls of the shale pores.

A series of experiments has been carried out to investigate the possible formation of magnetite, Fe3O4, under ambient soil-forming conditions. Rapid and easy synthesis of magnetite was achieved through controlled oxidation of Fe2+ solutions at room temperatures and near neutral pH values. The synthetic products were found to range in size between 0·01–0·07 µm (mean diameter) and hence span the theoretical superparamagnetic-single-domain grain-size boundary.

L' étude de la structure d' une argile sensible du Québec est abordée en porosimétrie au mercure après lyophilisation et en microscopie électronique à balayage sur une surface obtenue par fracture de l' échantillon congelé (système Cryoscan). On observe une structure en agrégats définissant deux classes de pores. A grande vitesse de congélation, la cristallisation de la glace semble surtout affecter les points faibles de la structure, qui sont les ponts de plaquettes reliant les agrégats les uns aux autres.