Allophane-like materials occur in the weathered zones of the phosphate-rich veins hosted in Silurian metasediments of the Catalonian Coastal Ranges. These metasediments also host sulphide and phosphate sedimentary mineralizations. Mineralogical and geochemical investigations of the allophanic samples indicate that they comprise Si-rich allophane, with a molar SiO2/Al2O3 ratio ranging between 1.19 and 2.23, with amorphous Al-(Ca) phosphate and hydroxylapatite as major minerals, and minor goethite and quartz. It is assumed that allophane, amorphous Al-(Ca) phosphate and hydroxylapatite come from the reaction of acid solutions, released during the weathering of sulphide interbedded in black shales, with phosphate-rich veins and volcano- sedimentary host rocks. Silica-alumina gels were deposited in fissures and cavities left by a previous dissolution of the phosphate-rich veins. Later the phosphate minerals filled the conchoidal microfractures and shrinkage microcracks of the allophane.

Petrographic study of the Frewens sandstone, Upper Cretaceous Frontier Formation, documents reservoir-scale diagenetic heterogeneity. Iron-bearing calcite cement occurs as large concretions that generally follow bedding and are most common near the top of the sandstone. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m; median volume is 5.2 m3. Concretions comprise 12% of the sandstone.

The minus-cement porosity of concretion samples is low, indicating that the calcite precipitated near maximum burial depth. Isotopic and burial history data suggest that the calcite precipitated at ~54°C from evolved meteoric water enriched in 18O or from a mixed meteoric±marine pore-water. Shell-bearing transgressive shales above the Frewens sandstone are interpreted to be the source of calcium carbonate. Concretions of this size and distribution would influence fluid flow in a reservoir and would reduce the amount of hydrocarbons in place.

The ‘Fischschiefer formation’ has been identified as the likely source rock for petroleum in the western Molasse Basin of Austria. The investigated cores originate from different depths (1242 m to 4294 m) of the Fischschiefer horizon. The illitization observed in the <0.2 μm fraction proceeds slowly, probably because of the low geothermal gradient (3°C/100 m) in the Molasse Basin. The illite content in illite-smectite ranges from 30% (randomly ordered) for the shallowest sample to 65% (R0+R1 ordering) for the deepest sample. The oxygen index (OI) vs. hydrogen index (HI) plot implies type I to II kerogen for the organic matter. Vitrinite reflectance and Rock Eval parameters indicate an immature stage in the diagenesis of kerogen. Only the deepest sample (4294 m, 0.6% Rr) reaches the early oil window. There is a good correlation of the illitization trend of the mixed-layer mineral I-S and the organic maturity parameters.

Metakaolins formed by total dehydroxylation of kaolinite, dickite and nacrite were hydrothermally treated at 240°C, for 2, 21 or 182 days, at initial pH = 5– 6 or 1. Kaolinite was the only mineral of the kaolin subgroup crystallized in these treatments. Kaolinite is less ordered for pH = 4– 6 than for pH = 1. This can be attributed to the influence of specific chemical, morphological, and/or crystalline properties of the metakaolins. Two different processes of kaolinite formation and crystal growth must be involved depending on the pH and metakaolin. The first process implies the dissolution of the metakaolin and crystallization of thin prismatic or dendritic kaolinite particles which curl, probably due to surface tension. The second process implies the rapid formation of small pseudohexagonal kaolinite particles which would aggregate and coalesce to form bigger particles.

Clay minerals from the tidal flat sediments along the coast of Youngjong Island, west of mainland Korea, were studied to ascertain their potential as an indicator of sea-level fluctuation. Semi-quantitative analysis of their clay-size fractions by XRD shows that the vertical distribution of clay minerals can be separated into four clay mineral assemblage units. Based on the relative contents of chlorite, four sedimentary units are distinguishable, and the abundance of chlorite is inversely related to that of smectite. The kaolinite content is constant, and so it is clear that chlorite originating from the Han River was transformed to smectite by pedogenesis during regression periods. Therefore, the relative contents of clay minerals from the unconsolidated materials in the tidal flat of Youngjong Island canbe used as indicators of transgression and regression in accordance with the sea-level fluctuation.

A three-fold zonation can be imaged within authigenic siderite from sandstones of the Brent Group using back-scatter SEM techniques. We interpret this zonation in terms of the biogeochemical zonation of shallow buried sediment. The innermost siderite crystal zone is very Fe rich (95.0±0.5 mol.% FeCO3), with high Mn levels relative to Ca and Mg. This is interpreted as forming within the Fe reduction zone, with Mn from the closely associated Mn reduction zone. The second siderite crystal zone is frequently represented either by an episode of dissolution, or is impure (80±1 mol.% FeCO3), and this corresponds to the sulphate reduction zone. The outer crystal zone is intermediate in composition, and is equated with the zone of methanogenesis (88±1 mol.% FeCO3). Isotopic values cannot be assigned to individual crystal zones. Bulk δ18O values (−2.7 to −13.0‰ V-PDB) are not consistent with precipitation from seawater at low temperatures, but suggest meteoric pore-waters. δ13C data (−4.3 to −15.7‰ V-PDB) are consistent with microbially-mediated precipitation.

Pyrite and siderite are usually mutually exclusive within a single sample. Sedimentary conditions which favour the development of a strong sulphate reduction zone, and hence the formation of pyrite, do not favour the formation of a strong sub-oxic zone, where siderite is preferentially precipitated, and vice versa. There is a strong facies control upon siderite formation, with ripple cross-laminated sands being most strongly siderite cemented.

A structural model is proposed for illite-smectite (I-S) from diagenetic environments which accounts for the presence of three different layer types which are defined as follows: montmorillonite (low-charge, octahedrally substituted, fully expandable), vermiculite (high-cha rge, octa- and tetrahed rally substitut ed, only partly expandabl e) and illite (K0.9Si3.3Al0.7R1.83+R0.22+O10(OH)2). All three layers may be found within the MacEwan crystallites, whereas external edges of the crystallites are only vermiculitic during the illitization process. In the proposed model, a layer is defined symmetrically on each side of the interlayer space, leading to the existence of polar 2:1 units. It is proposed that the I-S growth is a three step mechanism: (1) formation, from sediments of variable composition, of montmorillon ite crystallites; (2) vermiculitization of the montmorillonite crystallite interfaces and of inner montmorillonite layers; and (3) precipitation of illite of fixed chemical composition. The I-S crystal grows by addition of illite layers linked by K+ or NH4+ ions saturating the vermiculitic interfaces.

The Bertaut-Warren-Averbach (BWA) technique and high-resolution transmission electron microscopy (HRTEM) were used to characterize the products of dry-ground pyrophyllite. Mean crystallite thickness and crystallite thickness distributions were measured for each sample using the BWA technique. Mean crystallite thickness decreases during the treatment with respect to grinding time and energy applied per unit mass. The BWA data were checked by HRTEM measurements and good fits were obtained for samples having small mean particle thicknesses. Samples with thicker particles could not be measured properly by HRTEM because the number of particles counted from images is statistically insufficient. The shape of the crystallite and the particle-size distribution were used to determine the mechanism of pyrophyllite particle degradation. Particles initially having a lognormal size distribution are first delaminated randomly, then some are delaminated preferentially, thereby producing polymodal thickness distributions. Finally all particles undergo delamination yielding a lognormal thickness distribution.

Secondary clay minerals observed in the two uppermost basalt lava flows at ODP Site 1001, in the Caribbean Sea, drilled from the large igneous province of Cretaceous age, result from low-temperature alteration processes. Alteration mainly proceeds by circulation and diffusion of sea water. Six different types of clay mineral assemblage were recognized. Initial alteration with oxygenated sea water involves Fe and K fixation, creating visible oxidation halos parallel to the sides of cracks and fissures. A saponite/ beidellite mixture, interstratified smectite-glauconite, interstratified glauconite-nontronite and Fe oxyhydroxides are obtained depending on the distance from fluid conduits. The presence of beidellite may be due to enhanced Al mobilization resulting from high fluid flux. These early minerals are cross-cut by thin veins of pure celadonite or glauconite with further vesicle infill. Late-stage alteration is typified by the formation of saponite and takes place under closed reducing conditions resulting from deposition of the sedimentary overburden.

Observations of natural minerals and experimental run products show that I-S (illitesmectite mixed-layer minerals) with a high illite content as well as pure mica-illite crystallites can have a lath shape (aspect ratio >3). This shape is common to the I-S minerals under a variety of conditions of formation. The experiments presented indicate that the occurrence of lath-shaped micaillite is due to a slow growth rate from supersaturated solutions. This lath-shaped crystallite is metastable as the ultimate stable shape is hexagonal. A longer period of crystal growth will replace the mica-illite laths with crystallites of a hexagonal form. Thus the presence of lath-shaped micaillite indicates the existence of a metastable crystal form for this mineral.

Sandstones of the Palaeocene Montrose Group were deposited in a deepwater fan environment, and form a major oil reservoir in the North Sea. Calcite concretions occur commonly within thick-bedded and structureless sandstones. These concretions have been identified by sonic logs and well reports, and were cross-checked with available core data. Regionally, 101 wells have been examined and carbonate concretions form 0.6–7.2% of the core. Concretions are most abundant along the flank of the Fladen Ground Spur, the north Witch Ground Graben (WGG), the east south Viking Graben and East Central Graben (ECG). Concretions of the ECG formed at deep burial, with C from decarboxylation. Geochemical inheritance of Mn and Sr from Cretaceous chalk clasts may occur. Concretion growth may also have been influenced by vertical expulsion of fluids (leak-off) localized above salt tectonics. Isotopic and petrographic evidence indicates that much carbonate C in the WGG was derived from biodegradation of migrating oil in meteoric water at shallow depth. The locations of abundant carbonate with characteristic negative C isotope signatures can be used as shallow exploration guides to leak-off points located above deep overpressured structures.

Five palygorskite samples with different chemical compositions and specific surface areas (SSA) were used for this study. Batch experiments in dilute salt solutions under neutral conditions were conducted to study the kinetics of clay hydrolysis. The rates of release of Mg and Si differ significantly among the palygorskite samples. It was found that differences in release rate of Mg among the palygorskite samples are due to differences in both surface area and chemical composition. The rate of release of Mg was greater in palygorskites with high SSA and high Mg and Fe contents than that in palygorskites with low SSA and high Al content. The rate of release of Si depends on the SSA of the mineral and is not related to chemical composition. The initial amount of Si released increases with SSA, while the Si rate of release decreases with increasing SSA. These data suggest that the decomposition of palygorskite in soils and sediments also takes place under neutral conditions.

Kaolinite hydroxyls in dimethylsulphoxide-intercalated (DMSO-intercalated) kaolinites have been determined using Raman spectroscopy at 298 and 77 K. The inner surface hydroxyl frequencies at 3650, 3670, 3684 and 3693 cm-1 move to higher wavenumbers upon cooling to 77 K and are observed at 3659, 3676, 3692 and 3702 cm-1. The inner hydroxyl frequency is at 3620 cm-1 at 298 K and is at 3615 cm-1 at 77 K. Upon intercalation with DMSO, additional bands are found at 3660, 3536 and 3501 cm-1 for the low-defect kaolinite and at 3664, 3543 and 3509 cm-1 for the high-defect kaolinite at 298 K. The 3660 cm-1 band at 298 K is resolved into two bands at 3658 and 3663 cm-1 at 77 K for the low-defect kaolinite and these bands are assigned to the inner surface hydroxyl groups, hydrogen-bonded to the DMSO molecule. It is proposed that the DMSO molecule exists with two different orientations in the intercalate and these two molecular forms are differentiated by the OH-stretching bands of the inner surface hydroxyl groups. This band for the high-defect kaolinite is found at 3664 cm-1 at 298 K and resolves into two bands at 3664 and 3673 cm-1 at 77 K.

Illite age analysis (Pevear, 1992) and similar mixing methods demonstrate nearlinear behaviour when corrected for smectite content in illite-smectite and substitutions for K+ in illite layers. Srodon (1999) found that highly curved mixing lines result from theoretical calculations and challenged the validity of IAA. However, our theoretical calculations indicate near-linear mixing lines result when corrected for the amount of smectite and known K+ substitutions (Na+ or NH4+). We conclude that perfect linearity is unattainable because IAA attempts to unmix a system linearly, but the system is based on an exponential function (K-Ar equation). In practice, we believe IAA can successfully determine the age range of illite diagenesis, even though perfect linearity is not attainable because: (1) significant deviations from linearity occur only if >500 Ma separate the diagenetic and detrital ages; (2) IAA analytical uncertainties result in diagenetic and detrital ages that span a range of ages.

The Palaeocene Everest Complex, located in the Central Graben of the North Sea, comprises interbedded sandstones and mudrocks deposited in a submarine fan environment and overlies Cretaceous limestones. Depositional facies are the main influence on reservoir quality, although the latter is reduced significantly locally by ferroan calcite cementation, particularly in the lower part of the complex. Carbon and O stable isotope analyses have been made from cements in sandstones and mudrocks and from shear fractures and injection features within mudrock intervals. These indicate a marine source of detrital carbonate becoming mixed with HCO3− from decarboxylation higher in the succession. Petrographic observation and microprobe analyses indicate that dissolution of coccoliths and foraminifera in limestones are the principal sources of cementation. The similar composition of cements in sandstones, fractures and injection features suggests a common formation. Burial history modelling shows cement precipitation is synchronous with periods of excess pressure and therefore excess pressure release associated with rock failure is proposed as a prominent mechanism for initiating cementation.