The results obtained from this study of kaolinite dehydroxylation explain why different investigators have ascribed both first-order kinetics and a diffusion mechanism to this reaction. The fact that activation energies reported by these workers agree well, in spite of the different kinetics assumed when performing the calculations, is also explained. From a comparison of the results obtained by isothermal and non-isothermal methods it is concluded that, for reacted fractions, α, <0·6, kaolinite dehydroxylation is controlled by a diffusion process. A reaction mechanism explaining this behaviour is proposed.

Although the kinetics of the dehydroxylation of kaolinite have been widely studied, there is no definitive explanation of its mechanism due to its dependence on a variety of parameters. In this study, the dehydroxylation of kaolinite has been studied using controlled-transformation rate thermal analysis (CRTA), allowing precise control of the reaction rate, and thus of both the temperature and pressure above the sample. Modelling of the experimental results obtained by CRTA as well as those from TEM and MAS-NMR show that two elementary processes (diffusion and firstorder) can occur in competition. At the start of the decomposition the diffusion mechanism is predominant, but as the reaction progresses, the first-order mechanism prevails. It would seem that the importance of each process depends, in particular, on the presence of defects as well as on the local vapour pressure.

Sepiolite is an Mg-silicate with fibrous morphology. Its structure consists of talc-like ribbons parallel to the fibre axis. Although the tetrahedral sheets are continuous, the apices in adjacent ribbons point in opposite directions, each ribbon alternating with an open channel along the fibre axis. Tetrahedral positions are normally filled with Si atoms. The width of the ribbons corresponds to eight octahedral positions, which are, normally occupied by Mg atoms, and the channel cross-section is 3·8 x 9·4 Å. This structure gives the mineral a high adsorptive capacity and molecular sieve capability. The ideal mineralogical formula is (OH2)4(OH)4Mg8Si12O30.8H2O (Brauner & Preisinger, 1956).

Activation of sepiolite by acid attack has been extensively studied (Fernández-Alvarez, 1972; Jiménez-López et al., 1978; Rodríguez-Reinoso et al., 1981). This note describes the changes undergone by sepiolite during dissolution of the octahedral Mg-sheet by HCl attack, silica gel being the end-product. This process was followed by IR spectroscopy, study of Mg-extraction kinetics, and morphological observations by SEM and TEM.

Nickel crystal chemistry was systematically studied in various phyllosilicates, mainly the natural phases selected from the ‘garnierites’ of the supergene ore deposits of New Caledonia. Minerals which do not usually occur in New Caledonian parageneses were synthesised, as they could represent intermediate phases of genetic importance. In the kerolite-pimelite series, a linear relationship occurred between the ratio I(13,20)/I(02,11) of the hk bands and Ni-content. Diffuse reflectance spectra were used to derive the crystal chemical parameters of Ni. These confirmed its divalent character and its occupation of octahedral sites; the resulting structural distortion was slight and could not be detected in some minerals. There was no optical evidence for Ni atoms in 4-fold coordination. The two main parameters which showed significant variations among the studied phases were site distortion and crystal field stabilization energy (CFSE). Site distortion was at a maximum in trioctahedral smectites and sepiolite. CFSE depended on the mineralogy, crystallinity and chemical composition (Al-content) of the phase. Finally, clay minerals are classified according to the increasing stability of Ni in the octahedral sheet, which has been tentatively related to the geochemical distribution of this element. Secondary minerals are usually enriched vs. primary ones and among them are nepouite and kerolite which exhibit a high CFSE in contrast to sepiolite.

Opaline silica residue accumulates on the surface and in the near surface of the Te Kopia, Tikitere and Rotokawa geothermal fields, where rhyolitic tuffs are attacked by steam condensate, made acid (pH 2–3) by sulphuric acid produced by oxidation of H2S that accompanies steam discharge. Silica residue is one product of this alteration process that also yields kaolinite, sulphur, sulphide and aluminous sulphates, including alunite and alunogen, as pH, Eh and available moisture fluctuate across the field surface. Coagulation of colloidal polymeric silica or, possibly, direct deposition of monomeric silica can occur from the acid solutions of the digested country rock, depending on pH, concentration, temperature and the presence and concentration of other species. As with silica sinter, the first-formed silica phase consists of disordered opal-A microspheroids, commonly 0.1–5 μm in diameter. These coalesce and become overgrown by further opaline silica to yield a mass resembling gelatinous ‘frog spawn’ that lines cavities and envelops surfaces. This mass is the principle component of botryoidal, transparent to translucent hyalite that comprises much residue. Following deposition, this juvenile residue may crystallize to opal-CT lepispheres, 1–3 μm across and, subsequently, to chalcedonic quartz. Both the opal-A and opal-CT of the New Zealand residues are more disordered than those occurring in typical moderate- to low-temperature sinters. The opaline silica of silica residues enjoys a reaction relationship with both kaolinite and aluminium sulphates, including alunite and alunogen. These phases and the silica precipitate continuously and undergo dissolution at the surface of all three localities. The precise pathway followed depends upon the prevailing surface conditions, including humidity, pH, Eh, and Al and K activities. As Al is flushed from the system, the ultimate stage of alteration that may result is the dissolution of the silica itself in acidified rainwater, fogdrip or further steam condensate.

Although numerous models for the formation of glauconite have been presented, the precise process and micro-environment of glauconitization are still poorly constrained. We characterize the special micromilieu of glauconitization developed during early diagenesis and present a model for glauconite formation in fecal pellets.

Glauconitization at Oker (Central Germany) occurred predominantly in fecal pellets deposited in a shallow marine-lagoonal environment during the Kimmeridgian. Within the fecal pellets, rapid oxidation of organic matter provides the post-depositional, physicochemical conditions favourable for glauconitization. Replacements of matrix calcite, dissolution of detrital quartz, K-feldspar, and clay minerals, and Fe redox reactions were observed within the early micro-environment, followed by the precipitation of euhedral pyrite, matrix-replacive dolomite, and megaquartz accompanied by I-S formation as thin section analyses and SEM observations show. Carbonate geochemical compositions based on ICP-OES and stable oxygen and carbon isotope signatures demonstrate that glauconite formation started in a suboxic environment at a pH of 7–8 and a temperature of 22±3°C to 37±2°C at maximum.

TEM-EDX-SAED and XRD analyses on separated glauconite fecal pellets and on the <2 μm clay mineral fraction reveal the predominance of authigenic 1Md-glauconite, 1Md-glauconite-smectite, and 1Mdcis-vacant I-S, besides accessory detrital 2M1-illite and montmorillonite. Kinetic modelling of the glauconite (93–94% Fe-illite layers and 6–7% Fe-smectite layers, R3) and of I-S (66–68% Al-illite layers and 32–34% Al-smectite layers, R1) leads us to conclude that the I-S formed solely by slow burial diagenesis, whereas the glauconite formed close to the seafloor, suggesting significantly faster kinetics of the glauconitization reaction compared with smectite-illitization related to burial diagenesis. Thermodynamically, the substitution of octahedral Al3+ for Fe3+ and Mg2+ during the Fe-Mg-smectite to glauconite reaction via the formation of glauconite-smectite mixed-layered clay minerals may have resulted in a higher reaction rate for this low-temperature glauconitization process.

The Bridport Sands is a widespread marine sandstone of Lower Jurassic age found in much of southern England. It is a very fine grained, moderately sorted quartz-arenite and is characterized by the alternation of friable and hard calcareous-cemented layers. The sands form the upper reservoir in the Wytch Farm Field, Dorset, which is currently producing at the rate of around 4000 barrels per day. Investigation of core material to assess the suitability of water injection for gas/oil recovery has shown that significant reductions of liquid permeability compared to air permeability occur. These reductions vary from 30% or less in the best quality reservoir to more than 70% in low-permeability sandstones. Clay minerals in the Bridport Sands comprise mainly kaolinite and mixed-layer clays of both the illite-chlorite and illite-smectite types. Small amounts of vermiculite and chlorite also occur. The kaolinite is found as loosely-attached, discrete particles, whilst the mixed-layer clays form patchy pore linings. The permeability reductions may be explained by: (i) the adsorption of water and expansion of poorly-crystalline mixed-layer illite-smectites causing blockage of pore space (this reduction is largely reversible) and (ii) the physical movement of authigenic kaolinite crystal aggregates blocking pore-throats (this reduction is largely non-reversible). The pore-size distribution, clay particle sizes, the distribution of the clays within the pore space, and the composition of the clays are all important factors in controlling porosity/permeability relationships and permeability reductions in the friable reservoir intervals in the Bridport Sands.

Bentonite samples from Choushan treated at various acid concentrations were studied using chemical analysis, XRD, 29Si and 27Al MAS NMR to investigate the microstructure of the activated montmorillonites. Two types of structural units were formed during the activation: (1) (SiO)3SiOH, in which up to 15% of Si is bound; (2) Q4(0Al), a major building unit when the acid concentration is >10%. A significant 27Al signal at 55.0 ppm was recorded for both untreated montmorillonite and (to a much greater extent) in acid-treated montmorillonites. This was interpreted as arising from four-fold coordinated Al located in the ‘bulk’ octahedral sheet of montmorillonite. In the course of activation, the removal of one of a pair of octahedral Al ions from montmorillonite removes two hydroxyl groups and leaves the other Al of the pair in four-fold coordination.

Intrinsic surface acidity constants (Kalintr, Ka2intr) and surface complexation constant for adsorption of orthosilicate onto synthetic ferrihydrite (Ksi for the complex ≡FeOSi(OH)3) have been determined from acid/base titrations in 0.001-0.1 M NaClO4 electrolytes and silicate adsorption experiments in 0.01 M NaNOi electrolyte (pH 3-6). The surface equilibrium constants were calculated according to the two-layer model by Dzombak ' Morel (1990). Near equilibrium between protons/hydroxyls in solution and the ferrihydrite surface was obtained within minutes while equilibration with silicate required days-weeks, both reactions probably being diffusion controlled. Applying the values for specific surface area and site densities for ferrihydrite used by Dzombak ' Morel (1990) (600 m2 g–1, 3.4 μmole m–2) the constants pKalintr = 6.93 ± 0.12, pKa2intr = 8.72 ± 0.17 and log Ksi = 3.62 were calculated by using the FITEQL optimization routine. Use of the specific surface area actually measured (269 m2 g-1) gave a poorer fit of the experimental data. Due to the slow adsorption of silicate and hence long shaking times, changes in the surface characteristics of the ferrihydrite seem to take place, probably a decrease in the concentration of surface sites. Adsorption isotherms calculated using the derived equilibrium constants showed that approximately twice the amount of silicate was adsorbed at pH 5 compared with pH 3.

Infrared spectroscopy of silica adsorbed to ferrihydrite showed Si-O stretching absorption maxima in the range 940-960 cm-1. The shift of the absorption maximum to higher wavenumbers with increasing amount of silicate adsorbed is probably due to an increase in the frequency of Si-O-Si bonds between orthosilicate adsorbed at adjacent sites. Small amounts of goethite were identified in the adsorption products.

The surface composition of some halloysites with different particle morphology has been investigated by X-ray photoelectron spectroscopy (XPS) before and after removal of external Fe. The Fe(III) 2p3/2 binding energy of external Fe is appreciably smaller than that of structural Fe. Particle morphology is influenced by structural Fe content. The long-tubular halloysite has very little surface Fe, and its concentration tends to increase with the proportion of non-tubular particles in the samples. The spheroidal sample contains the most structural Fe which, however, does not appear to influence particle shape directly. Study by XPS indicates that Fe substitutes for Al in octahedral positions in approximately 1 : 2 proportion. As a result, an increase in octahedral vacancies and cation exchange capacity would be predicted. Further, halloysite layers within a crystal are generally inhomogeneous in composition. Built up like “onion skins”, the surface layers would either be enriched or depleted in Fe depending on the chemical environment in which crystal growth occurs.

Dioktaedrische Smektite aus 19 Bentonitproben werden abgetrennt. Aus den chemischen Analysen errechnen sich unter Berücksichtigung der direkt gemessenen Zwischenschichtladungen (nach Lagaly & Weiss, 1971) kristallchemische Strukturformeln von exakt dioktaedrischen Mineralen. Die Tetraederzentren in der Struktur sind fast ausschliesslich von Si4+ besetzt; nur zwei der untersuchten Minerale zeigen eine beidellitische Komponente. Die gemessenen mittleren Zwischenschichtladungen der Montmorillonite (0·33 bis 0·39) ergeben sich allein aus der oktaedrischen Substitution von Me3+ durch Me2+. Im Gegensatz zu der grossen Variation der Strukturformeln von dioktaedrischen Smektiten in der älteren Literatur sind die kristallchemischen Unterschiede der hier untersuchten gereinigten Smektite sehr klein.

Im Vergleich zu den Rohbentoniten sind die Konzentrationen der meisten Spurenelemente in den abgetrennten Montmorilloniten erheblich verringert. Nur Mn, Zn, Co und Ni scheinen in die Kristallstruktur eingebaut zu sein. Die übrigen analysierten Spurenelemente sind adsorbtiv gebunden oder in akzessorischen Mineralen enthalten.

Illite-smectite (I-S) mixed-layer minerals from North Sea oil fields and a Danish outcrop were investigated to determine the detailed structure and the diagenetic clay transformation. Clay layers in the chalk and residues obtained by dissolution of the chalk matrix at pH 5 were investigated. The phase compositions and layer sequences were determined by X-ray diffraction (XRD) including simulation with a multicomponent program. The structural formulae were determined from chemical analysis, infrared (IR) and 27Al NMR spectroscopies and XRD, and the particle shape by atomic force microscopy (AFM). A high-smectitic (HS) I-S phase and a lowsmectitic (LS) illite-smectite-chlorite (I-S-Ch) phase, both dioctahedral, together constitute 80 – 90% of each sample. However, two samples contain significant amounts of tosudite and of Ch-Serpentine (Sr), respectively. Most of the clay layers have probably formed by dissolution of the chalk, but one Campanian and one Santonian clay layer in well Baron 2 may have a sedimentary origin. The HS and LS minerals are probably of detrital origin. Early diagenesis has taken place through a fixation of Mg in brucite interlayers in the LS phase, this solid-state process forming di-trioctahedral chlorite layers. During later diagenesis involving dissolution of the HS phase, neoformation of a tosudite or of a random mixed-layer trioctahedral chlorite-berthierine took place. In the tosudite, brucite-like sheets are regularly interstratified with smectite interlayers between dioctahedral 2:1 layers, resulting in ditrioctahedral chlorite layers.

The alteration and transformation behaviour of montmorillonite (Wyoming bentonite) was studied experimentally to simulate the mineralogical and chemical reaction of clays in contact with steel in a nuclear waste repository. Batch experiments were conducted at 80 and 300ºC, in low-salinity solutions (NaCl, CaCl2) and in the presence or otherwise of magnetite and hematite, over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, transmission Mössbauer spectroscopy and EDS-TEM. Experimental solutions were characterized by ICP-AES and ICP-MS. The main results are that no significant change in the crystal chemistry of the montmorillonite occurred at 80ºC, while at 300ºC, the presence of Fe oxides leads to a partial replacement of montmorillonite by high-charge trioctahedral Fe2+-rich smectite (saponite-like) together with the formation of feldspars, quartz and zeolites.

The Tinto river is one of the most polluted stream environments in the world, as a result of both acid mine drainage and natural acid rock drainage. Two representative samples from the phyllosilicate-rich rocks exposed in the drainage basin (Palaeozoic chlorite-bearing slates and Miocene smectite-rich marls) were treated with acid river water (pH = 2.2) for different times to constrain the effects of extreme hydrogeochemical conditions on clay mineral stability. Illite and kaolinite did not show appreciable variations in their crystal chemistry parameters upon treatment. Chlorite underwent an incipient chemical degradation evidenced by the progressive loss of Fe in octahedral positions coupled with a shortening of the b unit-cell parameter, although no weathering products of chlorite were observed. Smectite and calcite were rapidly and fully dissolved thus neutralizing the water acidity, and subsequently Fe and Al oxy-hydroxides and opaline silica precipitated from the aqueous solution, together with a neoformed amorphous silicate phase largely enriched in Al and Mg.

The verdine facies, previously confused with components of ancient ironstones, has been studied in relation to its distribution, mineralogy, genesis, environment, and geological significance. Several chemically specific components are present including (i) a ferric and magnesian dioctahedral-trioctahedral 1 : 1 clay mineral, (ii) a chemically equivalent chlorite, (iii) a component intermediate between smectite and swelling chlorite, and (iv) a problematic low-temperature pyrophyllite-like clay mineral. The genetic mechanism is a series of crystallogenetic reactions using cations from the previous phase; this model, where a mineral phase is shown to be a temporary state of arrangement of cations, suggests that authigenic “marine” minerals may reflect diagenetic geochemical factors different from those in the original sea.

The usefulness of various techniques for characterizing poorlyordered inorganic gel systems in clays is considered. Many established instrumental techniques are unsuitable, but thermal methods, chemical dissolution techniques, and specific surface reactions are more appropriate. The reaction of fluoride with clays is used as an example of the latter and methods for detection and monitoring of the fluoride uptake are described. Its value is demonstrated for studying gel materials in soils and relating them to synthetic alumina-silica gels.

Palygorskite has been identified as a major constituent of the clay fraction in mid-Cretaceous deposits of the Meseta, the Middle Atlas, and the Rif in central and northern Morocco. Two types of palygorskite are differentiated, based on the morphology of crystallites, bio- and lithofacies associations of the containing sediments. Type 1 occurs in sections of the Meseta and the Middle Atlas, where the mineral displays bundles of long fibres under the electron microscope and is associated with shallow marine carbonate and evaporite facies. Type 2 was found in the Rif, where palygorskite occurs mostly as bundles of shorter fibres in turbiditic sediments of a supposed deep-marine environment. Associated microfossils indicate reworking of the palygorskite-bearing sediment from shallower parts of the basin. Palygorskite apparently grew authigenically in the chemically restricted environments of the Meseta and the Middle Atlas, whereas it is of a detrital origin in the Rif section. We consider that sediments containing authigenically formed palygorskite (type 1) could be the source of detrital palygorskite (type 2). Type 1 deposits on the North African shelf were probably partly reworked and responsible for the supply of palygorskite to mid-Cretaceous Atlantic basins, where the mineral is well known from several DSDP sites. As in the case of various Recent deep-sea palygorskite occurrences, aeolian transport may have significantly contributed to palygorskite supply to the ocean basins.

In the quantitative X-ray diffraction analysis of a series of samples, the problems arising from the variable compositions and degrees of crystallinity of clay minerals were overcome to a great extent by the use of standards extracted from the samples. Procedures are given for separation of the montmorillonite standard by differential settling of Na-montmorillonite solvated in an ethanol solution and for isolation of the kaolinite, mica, and chlorite standards by density separation of their Na-forms in thallous formate.

Calibration curves were prepared from the X-ray diffractograms obtained for series of known mixtures of Ca-forms of the standards and the internal standard boehmite using both powder and smear-oriented mounting techniques.

Results from previous work on clay-mineral complexes with dimethyl sulphoxide are taken as a starting point for the development of a method to identify kaolin in the presence of chlorite. The use of this method with artificial mixtures of these minerals, and with soil clays allows the detection of kaolin when it is present in an amount higher than 5%, through the formation of a mono-layer complex with a basal spacing of 11.8 Å. Details for the use of the method are given.

: Smectite genesis in two granitic saprolites was studied, using qualitative and quantitative mineralogical methods and high resolution transmission electron microscopy (HRTEM). The primary minerals (plagioclase and mica) give rise to the formation of different kinds of smectites according to the weathering conditions (pH and drainage). Under a relatively basic, drained environment, a neoformed montmorillonite with a spheroidal organization exists in fillings of plagioclase cores. In the same conditions biotites are transformed into beidellites with a quasi-crystal organization. Under hydromorphic conditions, muscovites evolve incompletely towards smectites, whereas biotites lead to a montmorillonite with a tactoid organization.