The soils over about half of South Africa are developed from sedimentary rocks of the Karoo Supergroup. Some units within the Karoo strata display variations in their mineralogical composition which correlate with fluctuations in the environment of deposition and with palaeotemperature. The former trends are reflected in the ratio of kaolinite to 2:1 layer-silicates, range from kaolinite-dominated to kaolinite-free, and are particularly evident in rocks of the Vryheid Formation. Geothermal history is preserved in various stages of smectite illitization ranging from R = 0 structures with 25% illite to 100% illite. Compositional trends may be significant on a regional, local or topographic scale down to a distance equal to the depth of a soil profile or even an horizon.

The affect of competing surfactant molecules, n-decyltrimethylammonium bromide (C10TAB) and n-hexadecyltrimethylammonium bromide (C16TAB), on the adsorption equilibrium between methylene blue (MB) and Na+-montmorillonite has been studied using visible spectrophotometry. The MB competed effectively with both these surfactants and also tetramethylamntonium ions, which were used as a model for the surfactant head group. At loadings up to 50% of the CEC of the clay, all of the MB was associated with the clay or clay/surfactant system. Evidence for the presence of monomer, MB+, dimer, (MB+)2, trimer, (MB+)3, and protonated monomer, MBH2+, was obtained in each case, with the trimeric form dominating at loadings >5% of the CEC. The spectra of adsorbed MB reflected the difference in chain length between C10TAB and C16TAB and indicated that a significant amount of monomeric, MB+, was solubilized in the C16TAB surfactant clusters on the clay surface. Pre-adsorbed MB was not displaced by added C16TAB surfactant.

Compositional trends of the Cenomanian Belle Fourche Formation, a marine shale unit in the Western Canada Sedimentary Basin, have been investigated on a regional scale using bulk-rock geochemistry and mineralogy, clay mineral compositions and oxygen isotope geochemistry of shale and bentonite core samples. Smectitic illite-smectite found in the matrix of immature, hemipelagic samples is compositionally and isotopically consistent with an origin from low-temperature alteration of volcanic ash in the central Western Interior Seaway, where the basin received minimal detrital input. The origin of the more illitic matrix in the deeply buried, western, pro-deltaic shales can be interpreted in terms of either diagenetic ‘illitization’ of a smectitic precursor, or depositional mixing of abundant, detrital, illitic material with minor amounts of ashfall-derived smectite. It is concluded that: (1) documented silicate mineral reactions during deep diagenesis of the Belle Fourche Formation took place in a relatively closed system, with no significant import or export of mobile species at the formation scale; and (2) diagenesis and depositional mixing can have similar effects in terms of bulk-rock and oxygen isotope geochemistry, and mineral compositions and assemblages.

Petrographic, X-ray diffraction, and microprobe analyses have been used to assess the significance of illite K-Ar ages from sandstones of two North Sea wells. Three closely spaced samples in one well from the upper Statfjord Formation yield similar ages (69-79 Ma) although the illites formed from different precursor minerals. Pore-filling illite in the upper Brent and the Upper Skagerrak Formations from a second well formed by replacing groundmass clays and other detrital minerals. The average layer charge and K+ content increase slightly with depth (0.69-0.80 K+) due to minor reaction and crystal growth during burial diagenesis. These K-Ar ages increase from 15 to 33 Ma within a 500 m depth interval. The K-Ar age vs. depth relationship for these samples corresponds to the burial rate during the middle Tertiary. In examples of extensive illitization of pore-filling clays in sandstones with little subsequent evolution of the clay minerals, the K-Ar ages indicate the age of diagenetic events.

In contrast, illitic minerals in shales from the Skagerrak Formation in the second well yield an age (108 Ma) that is much older than the clays in the sandstones, but is still younger than stratigraphic age. The K-Ar ages from illitic clay in shales reported in the literature can get younger, older, or remain essentially unchanged with increasing depth. These age vs. depth trends reflect the complex interplay of crystal growth and dissolution during diagenesis, as well as probable contamination by non-recrystallized detrital illites. The K-Ar ages of illitic clays, therefore, evolve in a different manner in shales than in sandstones.

Interlayer K from two biotites of different origin was extracted with octadecylammonium chloride (ODA) and sodium chloride/sodium tetraphenylboron (STB) before and after oxidation with saturated bromine water (Br2). Potassium release, Fe oxidation and Fe ejection from octahedra were measured. With ODA and STB the untreated biotites released between 40 and 90% of initial interlayer K, the muscovite less than 5%. After Br2 treatment, substantially less K was extracted from both biotites. Suppression of K release was caused by the formation of octahedral vacancies formerly occupied by Fe. The extent of Fe ejection was not proportional to the extent of octahedral Fe oxidation. During Br2 treatment, the biotites transformed into hydrobiotite. The hydrobiotite sample, high in Fe ejection during preceding Br2 treatment, strongly resisted subsequent K extraction with STB, whereas the sample with minor Fe ejection transformed to vermiculite. Octadecylammonium chloride extracts K more effectively than STB from biotites intermediate between trioctahedral and dioctahedral structure and is considered to be suitable for the distinction between biotite K and muscovite K.

The formation of FeII,III hydoxy salts (green rusts) from FeII sulphate and chloride solutions and their transformation to lepidocrocite, γ-FeOOH, was studied at ambient temperature, constant pH of ∼7.0 and under controlled air flow. The formation of green rust and its subsequent transformation to lepidocrocite could be distinguished by the reaction rate and solution chemistry. The SO42− and Cl− concentrations in solution attain a minimum at the break between the two reaction steps and are completely restored to their initial values after complete transformation to lepidocrocite. While the green rust is being formed, its FeII/FeIII ratio shows a minor decrease and its FeIII/anion ratio a minor increase. These ratios, however, deviate from the ideal pyroaurite-type structure (M2+/M3+ = 3; M3+/A− = 1; M: metal; A−: anion) and possibilities to accommodate this deviation are discussed. A period during which no protons were produced (i.e. no base consumed) was found to separate the two reactions in the chloride, but not in the sulphate system. During this period a partial solid state oxidation of the green rust took place, leading to its destabilization and eventual decomposition.

Six Spanish kaolins were studied to determine the potential application of their crystallinity for pharmaceutical purposes. Particular attention was paid to the effect of crystallinity on sodium amylobarbitone release in water, when the drug was prepared as granulates with kaolin. Crystallinity was estimated empirically in various granulometric kaolin fractions by examining X-ray diffraction (XRD) patterns, infrared spectra and morphology as shown by scanning electron microscopy (SEM). Crystallinity was usually moderate with a mean Hinckley index of (1.69. Statistically significant relationships were found between the XRD indices and other kaolin characteristics such as total iron content and mean particle size. The different degrees of crystallinity also appear to be related to the fabric type and morphology of individual aggregates and particles (SEM). The release of sodium amylobarbitone increases with kaolin crystallinity, as estimated by the indices HB1 (Hughes & Brown, 1979) and RWI (Range & Weiss, 1969), according to the formula y = 43.668 + 3.631 HBI -84.711 RWI. Our findings suggest the possibility of preparing sustained-release formulations with controlled drug release by using kaolins of different degrees of crystallinity as excipients.

Two profiles in Miocene fluvio-lacustrine deposits consist of sandy, marly, lutitic and carbonatic levels constituted by variable percentages of quartz, calcite and clay minerals as major components, and feldspars, dolomite and occasionally gypsum and anhydrite as minor ones. The clay minerals are inherited and consist mostly of micas, with minor quantities of chlorites, pyrophyllites and kaolinites. The crystallochemical parameters of the micas indicate muscovitic compositions and their uniformity through both the different rocks and their silt and clay fractions suggest the same provenance source area, possibly located northward.

Clay minerals concentrate preferentially Li, Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Cs, Ba, Zr, Hf, Th, U and REE whereas the authigenic carbonates concentrate Mn and Sr. The Sc, Cr, Th, Y, Zr and REE values in clay minerals indicate that the provenance source area of these deposits was similar in composition to the average continental upper crust, probably as a result of sedimentary recycling processes.

Zeolitic levels constituted by different proportions of analcime and smectite as major components outcrop at the top of the profiles. The analcimes show anhedral to euhedral morphologies, with grain-size ranging between 1 and 20 μm, and Si/Al ratios ranging from 2.2 to 2.5. The smectites are dioctahedral and beidellitic in composition. The zeolitic levels present significant chemical differences relative to the other ones, such as higher overall REE contents, more pronounced negative Eu anomalies and higher (La/Yb)n, Th/Sc and La/Sc ratios, suggesting a different provenance source area. Their chondrite-normalized REE patterns reflect the possibility that the starting materials were pyroclastic eruptive rocks originating from intracrustal partial melting. The variable analcime and smectite percentages are attributed to variations in H+/(Na+ + K+) and K+/(Na + Ca2+ + Mg2+) activity ratios and silica and water activities in the pore-waters during diagenetic processes.

The use of electron microscopy to study clay microfabrics in thin-sections is discussed. A technique is described to isolate undisturbed microparts of pedofeatures from thin-sections, which are subsequently used for TEM analysis. Re-embedding with a polyester resin of undisturbed, in situ, neoformed clay microfabrics, obtained by microdrilling and preparation of ultrathin sections by microtoming with a diamond knife are emphasized; these steps enable micromorphology, clay mineralogy, microchemical and HRTEM analysis to be performed on one unique microsample of clay fabrics, with conserved micro-organization. Two examples on clay neoformation are presented to demonstrate that this technique can successfully be applied to unravel the impact of mineral alteration and clay neoformation in undisturbed soil samples on a micro- and a nanometer scale.

Claystones of Kimmeridgian-Volgian-Ryazanian age are the main source rock for oil in the Central Trough, North Sea. During oil generation, illite layers form in illite-smectite (I-S) and the illitesmectites become short-range (IS) ordered and eventually long-range (ISII) ordered. The formation of illite layers involves dehydration and fixation of interlayer cations, K+ and NH4+. The content of soluble NH4 and K in core samples and the content of NH4 and K in I-S were determined, the NH4 by an isotope dilution method. In the Central Trough, soluble K+ decreases with depth, whereas soluble NH4+ increases with depth. From ∼3 km, the ratio NH4/K increases with depth, to an average of ∼0.5 at 4.4 km. In I-S, the ratio NH4/K shows a similar increase with depth. The NH4/K ratio increases significantly in I-S during ordering. It is concluded that NH4 fixation in I-S may be of general importance during burial diagenesis of oil source rocks.

The most widely used procedure for determining the cation exhange capacity (CEC) of a clay involves ammonium ion exchange, followed by Kjeldahl measurement of the ammonia released on treatment with alkali (Grimshaw, 1971; Mackenzie, 1951). The method is experimentally quite difficult, prone to error and time consuming.

The rocks of the palygorskite deposit at Bercimuel (Segovia, Spain) have been studied by transmission electron microscopy and microanalysis. These rocks correspond to the zone of convergence of two alluvial fan systems that have filled the small basin of the River Riaza and would originally have been composed of illite and quartz silts. Among the accumulations of palygorskite it is possible to observe surrounded particles of micromicas that have undergone dissolution and opening thereby giving rise to disordered illite-smectite mixed-layer clays. This process continued up to the individualization of structural relics formed of units of 1–5 layers. At the same time, the chemical composition was modified with a loss of K and Al and a relative increase in Si and Mg, progressively evolving towards the composition of palygorskite. The paleogeographic position of the deposits, and the climatic conditions (arid environment), appear to be the dominant factors in the neoformation of palygorskite by alteration of the original sediments. The basic mineralogical process could be referred to as ‘early diagenesis’ in the formation of calcretes.

The smectite-to-illite reaction (illitization) was examined in a dilute suspension of Wyoming bentonite in the presence of oxalate and acetate. Experiments were performed using fluidsampling apparatus at 200°C, 50 MPa, in which progressive changes in both clay mineralogy and fluid composition were monitored. The presence of K-oxalate and K-acetate under neutral-alkaline conditions produced significant reaction of smectite to either R1 or R2 ordered illite-smectite with 60–70% illite layers. Experiments at the same temperature with KCl, or oxalic acid plus KCl, produced no reaction. Fluid chemistry showed that K-oxalate and K-acetate resulted in the establishment of aK+/aH+ conditions within the muscovite stability field, favouring the formation of illite. The extent of dissolution and precipitation indicated by fluid data is insufficient to account for the amount of illitization evident in the mineralogy. This implies that a localized dissolution-precipitation process occurs, perhaps on the scale of a single clay particle. These experiments highlight the importance of pH as a control over illitization and show that organic anions can buffer fluid composition into the optimum aK+/aH+ condition for illitization. These findings are highly significant for the smectite-to-illite reaction occurring in organic bearing sediments where organic acid anions are generated during kerogen maturation.

The turbiditic facies of the Aquitanian-Oligocene of the Algeciras and Bolonia Units consist of lutitic-marly beds alternating with varied sandstones. Early diagenetic processes and the development of secondary porosity in sandstones were controlled largely by sedimentary facies. Mineral dissolution processes were important in rocks with high primary porosity (quartzose sandstones) and led to the development of allophane and kaolinite/dickite in later diagenetic stages. In rocks with high contents of biotitic matrix, chlorites and iron oxides were produced, together with the development of secondary porosity. In feldspathic sandstones, the early diagenetic processes produced chlorite, illite, illite-smectite mixed-layers and smaller amounts of kaolinite. In calcareous sandstones, the presence of an early cement prevented, to a certain extent, the dissolution of unstable fragments and restricted the development of clay minerals. The movement of fluids was not important during this early diagenetic stage in controlling the variety or abundance of mineral cements; the controlling factors were the different chemical conditions developed in the pore-solution of the various sandstone types. In spite of similar sandstone burial conditions, the different sandstone lithologies continued to determine the diagenetic processes, especially the degrees of compaction and dissolution. Certain burial processes are common to all levels: the development of reducing conditions is indicated by the development of Fe and Mn carbonates, Fe-dolomites and Fe-chlorites. A later alkaline diagenetic phase was responsible for the development of calcite, feldspar and illite. The circulation of fluids in these stages remained limited, although there was probably some movement from the lutitic-level fluids to the overlying sandstones.

Aeolian sandstones of the Lower Permian Leman Formation (Rotliegend Group) provide the best gas reservoir in the southern North Sea, but permeability is greatly reduced by the presence of authigenic fibrous illites. New radiogenic (K/Ar) and stable (oxygen and hydrogen) isotope data are presented for fibrous illite cements (<0.1 µm), so that the absolute timing and controlling diagenetic factors for their formation can be more fully evaluated. Thus, the expected quality of gas reservoirs in the southern North Sea might be better predicted. Samples have been analysed from five wells in areas with contrasting structural evolution: the Sole Pit Basin, and the Indefatigable Shelf. The K/Ar ages of between 160 and 190 Ma have been obtained from the Indefatigable Shelf illites, and between 120 and 160 Ma for those from the Sole Pit Basin, reflecting different times of basin inversion. These K/Ar ages are interpreted by reference to burial/thermal models for each well. The temperature of illite precipitation falls between 88 and 140°C. Calculated pore-fluid compositions derived from oxygen and hydrogen isotopic analyses give values of ∼ + 1 to +9‰ (SMOW) δ18O and +1 to −50‰ (SMOW) δD. The illite δD values have probably been affected by isotopic exchange and fractionation with the surrounding gaseous hydrocarbon. The δ18O values reflect the degree to which evaporative concentration had affected Zechstein marine waters which subsequently invaded the Leman Sandstone. Comparisons between δ18O and δD values in clays and in formation water for the Leman Field suggest that oxygen isotope exchange might have taken place, and that the initial K+ and radiogenic 40Ar contents within illites may have been modified.

Corrugated mats of palygorskite occurring in regolith overlying a fissured limestone were examined by X-ray diffraction, infrared spectroscopy, and electron optical techniques after heating at 50°C intervals to 700°C. Palygorskite ‘anhydride’ formed at 400°C and sillimanite formed at 500°C. The palygorskite is believed to have formed in joints in limestone of Upper Oligocene age (Duntroonian) prior to uplift and subsequent weathering. It is highly crystalline, and appears not to have altered or weathered since precipitation.

The adsorption of Rhodamine 6G in aqueous suspension on Laponite B was investigated by electronic absorption and emission spectroscopies. Fluorescence spectra suggest that the monomer is adsorbed at two different surfaces, the external and the internal. A monomer is intercalated in the interlamellar space at low loading of dye (<3% CEC), whereas the monomeric state of the dye seems to be at the solid-aqueous interface in suspensions with high loading (>12% CEC). The metachromatic effect observed in the absorption spectra, for the loading interval between 1% and 15% CEC of Laponite B, is attributed to the dimerization of the dye, which seems, from X-ray diffraction measurements, to be formed at the clay interlayer. The formation constant and the absorption spectrum of the aggregate were obtained and the dimer was structurally characterized by applying the Exciton Theory. The observed fluorescence quenching for loadings lower than 15% CEC is attributed to energy transfer from monomer to the dimer, which obeys the Perrin model.