bluuebunny/crossref_metadata_2025_split huggingface dataset:
A reassessment of the 29Si MAS-NMR spectra of sepiolite and aluminated sepiolite
Authors: D'Espinose de la Caillerie J.-B. , Fripiat J. J. 
Published: December 2006

AbstractA 29Si MAS-NMR one-pulse and ﹛1H﹜ CP comparative study of sepiolite, folded sepiolite, aluminated sepiolite, and palygorskite was conducted. The three main resonances of sepiolite at -92.0 ± 0.1, —94.3 + 0.1, and — 97.8 ± 0.1 p.p.m. were assigned with respect to the position of the Si atoms within the three pairs of symmetrical sites in the basal plane. The line at —97.8 p.p.m. was attributed to the central sites by comparison with the resonances and chemical environments of Si in talc and palygorskite. The line at —94.3 p.p.m. was attributed to the edge sites, because it is strongly reinforced by cross-polarization. Alumination of sepiolite led to a broadening of the peaks and new downfield resonances in line with what can be observed in other phyllosilicates and zeolites.
Infrared study of octahedral site populations in smectites
Authors: Madejová J. , Komadel P. , Číčel B. 
Published: December 2006

AbstractThe hydroxyl stretching region of two montmorillonites (SWy-1, Crook County, Wyoming, USA, and JP, Jelšový Potok, Slovakia) and a ferruginous smectite (SWa-1, Grant County, Washington, USA) have been analysed using decomposition and curve-fitting. The positions and integrated intensities of component bands were correlated with the nearest cationic environment of the OH groups, and the populations of the octahedral sheet were calculated. The differences between the octahedral coefficients obtained and those from chemical analyses were 0.07 or less. This method provides reasonable contents of octahedral cations for the smectites with various iron contents, containing no other admixtures absorbing in the 3700-3500 cm–1 region. The cation distribution was found to be not completely disordered. Though some preferential pairing has been observed, no general rule could be established.
Stability constants for silicate adsorbed to ferrihydrite
Authors: Hansen H. C. B. , Wetche T. P. , Raulund-Rasmussen K. , Borggaard O. K. 
Published: December 2006

AbstractIntrinsic 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.
Significance of K-Ar ages of authigenic illitic clay minerals in sandstones and shales from the North Sea
Authors: Matthews J. C. , Velde B. , Johansen H. 
Published: December 2006

AbstractPetrographic, 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.
Clay diagenesis, overpressure and reservoir quality: an introduction
Authors: Jeans C. V. 
Published: December 2006

AbstractThe interplay of overpressure and diagenesis is an important factor in developing reservoir quality and controlling the movement of pore-fluids and hydrocarbons within sedimentary basins. The first part of this introduction deals with petrographic features by which certain highly overpressured reservoirs can be recognised. The second part considers the 20 papers, presented at the 1993 Cambridge Conference, published in this special issue of Clay Minerals and how they fit into the broader context of the subject.
Physical and chemical aspects of the development of overpressuring in sedimentary environments
Authors: Hall P. L. 
Published: December 2006

AbstractFluid pressures in argillaceous sediments depend on, inter alia, mechanical stresses, temperature, diagenetic volume changes and permeability. However, the relative influence of the pressuring mechanisms depends critically upon the long time-scale compliance, C, of the overpressured layer.In sediments undergoing first-time burial and currently exposed to their historically maximum applied stresses, C can be relatively large. Here, fluid pressure increases are principally due to mechanical causes, and overpressuring will be associated with undercompaction. The tendency for undercompaction (compaction disequilibrium) depends on the sediment thickness, burial rate and vertical permeability. In other cases, when applied stresses have been reduced by uplift, or when impermeable hard caps or seals have been formed, C may be substantially smaller. Here pore pressures may be predominantly controlled by diagenetic and aquathermal processes, with mechanical (undercompaction) phenomena being relatively less significant.Three-dimensionally sealed overpressured zones may exhibit vertical fluid pressure discontinuities. Within a sealed aquifer, fluid pressures may rise to almost lithostatic values, relieved by episodic fracturing of the seal.
Nature and causes of overpressuring in mudrocks of the Gullfaks Area, North Sea
Authors: Wensaas L. , Shaw H. F. , Gibbons K. , Aagaard P. , Dypvik H. 
Published: December 2006

AbstractPore-fluid pressure gradients in Lower Tertiary to Upper Jurassic mudrocks in the Gullfaks area show significant variations, both between and within individual structural compartments. Detection and quantification of abnormal pore-pressures, based on drilling parameters and wire line logs, are significantly influenced by petrological variations in the mudrocks and do not provide adequate direct pressure indicators. Empirical models show that the degree of overpressuring in mudrocks above the Gullfaks reservoirs cannot be fully explained by processes such as shale dehydration, compaction disequilibrium, aquathermal effects or in situ hydrocarbon generation, and other processes, such as caprock failure and subsequent migration of hydrocarbons (mainly gas) into the overlying mudrocks, appear to be influential in generating the observed pattern of overpressure.
Reservoir diagenesis and hydrocarbon migration under hydrostatic palaeopressure conditions
Authors: Swarbrick R. E. 
Published: December 2006

AbstractFluid inclusion studies from two contrasting fields in the Northern North Sea reveal hydrostatic pressure conditions during the formation of secondary quartz overgrowths which trapped both brine and petroleum inclusions. In these fields, dating of authigenic illite, part of the diagenetic sequence and broadly coeval with quartz cementation, permits comparison with modelled pressure conditions. Pressure modelling indicates the likelihood of several periods of overpressure in the past, interspersed with normal pressure. The timing of diagenesis coincides with one of these periods of hydrostatic pressure. Furthermore, as both fields are now highly overpressured, the cause and timing of overpressure generation post-date the diagenesis. A model is proposed to link the pressure history and diagenesis.
Origin of the Permo-Triassic clay mica assemblage
Authors: Jeans C. V. , Mitchell J. G. , Scherer M. , Fisher M. J. 
Published: December 2006

AbstractClay mica is the predominant component of the fine-grained siliciclastic sediments of the Western European Permo-Trias and it may occur as the sole component of the clay assemblage. Its characteristics have been studied by chemical analysis, radioisotope (K/Ar) data, X-ray diffraction and electron microscopy in the clay assemblages from Triassic and Permian sediments in Spain, Western Approaches, South Devon and East Yorkshire. The clay mica is a ferric dioctahedral mineral containing on average 6.5% Fe2O3 and 7.5% K2O. Crystal thickness ranges from 8 × 10 Å to 115 × 10 Å, and varies with geological, stratigraphical and grain-size factors. Radioisotope data and geological considerations suggest that much of the Permo-Triassic clay mica was formed originally in coeval desert soils rather than being derived from pre-existing rocks. It was then eroded, sometimes mixed with much older material, and deposited as fine-grained detritus in adjacent areas. Upon deep burial, this detrital mica assemblage underwent recrystallization with the development of euhedral crystals and the alteration of the K/Ar values.
Palaeohydrodynamic fluid flow regimes during diagenesis of the Brent Group in the Hutton-NW Hutton reservoirs: constraints from oxygen isotope studies of authigenic kaolin and reverse flexural modelling
Authors: McAulay G. E. , Burley S. D. , Fallick A. E. , Kusznir N. J. 
Published: December 2006

AbstractAuthigenic kaolin is a major occluder of porosity at all depths in the Hutton-NW Hutton Brent Group reservoirs. Two polytypes, kaolinite and dickite, are present. Kaolinite occurs to a depth of ∼10,600 ft and is interpreted to be eogenetic. Dickite increases in abundance with burial depth, concomitant with increasing K-feldspar dissolution. Kaolin petrography indicates that kaolinite transforms to dickite via dissolution-reprecipitation at depths of ∼11,000 ft. Dickite has δ18O values characteristic of modified meteoric water for growth at temperatures of contemporaneous quartz authigenesis.Palaeohydrodynamic models involving meteoric water penetration down to 2 km through faults or Palaeozoic rocks of the East Shetland Platform are unlikely to provide the vigorous fluid throughput required for kaolin precipitation. Reverse post-rift modelling indicates that the crest of the Hutton-NW Hutton fault block was not sub-aerially exposed, precluding meteoric water ingress via this route. Eogenetic kaolinite formed in a meteoric water flush driven by topographic head on the Brent Delta. Dickite is, in part, derived from kaolinite dissolution, with additional sourcing from K-feldspar dissolution and Si-Al in pore-waters expelled from mudstones during burial. This model precludes the need for extensive fluid-flushing at intermediate burial depths.
Kaolinite growth during pore-water mixing: isotopic data from Palaeocene sands, North Sea, UK
Authors: Stewart R. N. T. , Fallick A. E. , Haszeldine R. S. 
Published: December 2006

AbstractStable isotopic and petrographic data have been used to interpret conditions for the formation of authigenic kaolinite within Lower Palaeocene sands, Central North Sea. Two wells within the Witch Ground Graben were sampled (1975 m to 2795 m). Texturally early calcite concretions have isotopic compositions (δ18O = 18.3–21.6‰ SMOW) which indicate that they were precipitated in predominantly meteoric waters. The isotopic composition of later vermiform kaolinite (δ18O = 14.8–17.7‰ SMOW and δD = −53 to −71‰ SMOW) indicates that kaolinite precipitated at around 45–70°C, from a mixed meteoric-marine pore-water (δ18O = −5 to −3‰ SMOW). These modelled precipitation temperatures are consistent with the paragenetic sequence and consequently post-precipitation hydrogen isotope exchange between kaolinite and the pore-waters is presumed not to have occurred. It is inferred that the original depositional marine pore-waters were flushed out during the late Palaeocene (54.8 Ma) by a head of meteoric water from the East Shetland Platform. The Lower Palaeocene aquifer became closed to meteoric influx after marine transgression during the late Palaeocene (54.0 Ma). The remaining meteoric pore-waters in the sandstones became mixed with water from compacting marine muds surrounding the hydrostatically pressured sandstones.
Origin of grain-coating chlorite by smectite transformation: an example from Miocene sandstones, North Sumatra back-arc basin, Indonesia
Authors: Humphreys B. , Kemp S. J. , Lott G. K. , Bermanto  , Dharmayanti D.A. , Samsori I. 
Published: December 2006

AbstractGrain-coating chlorite cements commonly occur within sandstones of late Middle and Upper Miocene age deposited in the North Sumatra back-arc basin. Chlorites from the Lower Keutapang Member contain Ca (maximum 0.75 wt% oxide) and show textural evidence for direct precipitation on grains. However, crystals are subhedral, showing curved faces and often ragged edges, and show a tendency to merge together. In overlying beds of the Upper Keutapang Member, grain-coating chlorite-smectite (20% smectite) cements display an identical morphology but are more siliceous, have a lower octahedral occupancy and contain higher total (Na + Ca + K). It is proposed that chlorite cements in the Keutapang Formation originated as smectite-rich cement rims whose initial precipitation was related to the breakdown of volcanic detritus in the sediments. Transformation to chlorite occurred subsequently during burial, facilitated by a high geothermal gradient in the back-arc basin.
Diagenesis of mudrocks and concretions from the London Clay Formation in the London Basin
Authors: Huggett J. M. 
Published: December 2006

AbstractThe suite of London Clay mudrock samples may be sub-divided into three lithofacies: laminated silty claystone, partially bioturbated silty claystone, and partially bioturbated claystone. Mineralogically the mudrocks show little bulk variation, but considerable differences in the suites of trace cements which characterize each sample. The concretions also show a range of cement types, even within a single concretion. Preservation of uncompacted burrows and minus cement porosities (MCP) indicate that the concretions formed during early diagenesis. These data have been used to constrain the timing of authigenic mineral formation in both the mudrocks and concretions. Petrographic study of the concretions has revealed that there are significant errors involved in the use of MCP for estimation of depth of formation, which make it of qualitative rather than quantitative use. Stable isotope and chemical data suggest concretion formation occurred towards the base of the sulphate reduction zone and in the methane oxidation zone.
Stable isotope geochemistry of clay minerals
Authors: Sheppard S. M. F. , Gilg H. A. 
Published: December 2006

AbstractThe equilibrium H- and O-isotope fractionations can be approximated by the following equations which are based on experimental, empirical and/or theoretical data:Hydrogen: 1000 ln αkaolinite-water = −2.2 × 106 × T−2 − 7.7Oxygen: 1000 ln αkaolinite-water = 2.76 × 106 × T−2 − 6.751000 ln αsmectite-water = 2.55 × 106 × T−2 − 4.051000 ln αillite-water = 2.39 × 106 × T−2 − 3.76The equilibrium H-isotope fractionation factors vs. 106 × T−2 for kaolinite and probably smectite and illite are monotonic curves between 350-0°C. More complex curves, with a minimum fractionation near 200°C, are probably influenced by surface effects and/or disequilibrium fractionations among the different hydrogen sites. The H-isotope fractionations between smectite-water increase by ~70‰ from Fe-poor montmorillonite to nontronite at low temperatures. The pore-interlayer water in smectite H-isotope fractionation at low temperatures is ~20±10‰. The presence of organic matter can modify both the δD value of the clay analysis and its ‘water’ content. Clays — kaolinite, illite, smectite and probably halloysite — tend to retain their D/H and 18O/16O ratios unless subjected to more extreme diagenetic or metamorphic conditions or special local processes. Kinetic information is still only qualitative: for comparable grain sizes, hydrogen exchanges more rapidly than oxygen in the absence of recrystallization. Low-temperature diffusion coefficients cannot be calculated with sufficient precision from the higher temperature exchange data. The H- and O-isotope studies of clays can provide useful information about their conditions of formation.
Origin of pore-lining chlorite in the aeolian Rotliegend of northern Germany
Authors: Hillier S. , Fallick A. E. , Matter A. 
Published: December 2006

AbstractPore-lining chlorite is common in Rotliegend lake shoreline aeolian sandstones from northern Germany and preserves abnormally high primary intergranular porosity. In the north of the study area, in a basinward direction, the chlorites are Mg-rich while towards the south they become Fe-rich over a distance of about 15 km. All are unusually rich in Mn. Magnesium-rich examples tend to be more abundant than illite, while Fe-rich examples cover framework grains less continuously and are admixed with more abundant illite. Oxygen isotope analysis of 30 chlorites in the 2–6 µm fraction gave δ18O (SMOW) values of 7 to 12‰ (mean 9.8‰). These data show no obvious trend across the study area, nor in relation to changes in chlorite composition, or burial depth.The honeycomb arrangement of chlorite crystals suggests chlorite formation via the sequence smectite-corrensite-chlorite. Chlorite distribution and the systematic changes in its composition suggest that formation of a precursor was related to lateral migration of Mg-rich fluids from basinal shales and/or evaporites during shallow burial. Interaction of these fluids with early formed oxyhydroxide coatings on the aeolian sand grains provided a source of Fe and the Mn. The isotope data suggest that the eventual formation of chlorite during deep burial occurred from waters with positive δ18O values, comparable to those present during the deep burial formation of illite.
Predicting the properties of bentonite-sand mixtures
Authors: Mollins L. H. , Stewart D. I. , Cousens T. W. 
Published: December 2006

AbstractOne-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.
Influence of metakaolinization temperature on the formation of zeolite 4A from kaolin
Authors: Chandrasekhar S. 
Published: December 2006

AbstractKaolin has been used as an alternative cheap raw material for the synthesis of zeolite 4A. Two steps are involved in the reaction: (1) dehydroxylation of kaolin at 550–900°C to form an activated X-ray amorphous material called metakaolin; (2) hydrothermal treatment of metakaolin with aqueous alkali to form the zeolite. The inherent colouring impurities in kaolin, especially the Fe minerals, affect the brightness of metakaolin and the resultant zeolite. The dehydroxylation temperature is found to have a significant influence on the kinetics of zeolite formation as well as on the brightness of zeolite. The present investigation deals with the preparation of metakaolins from a good quality kaolin at different temperatures and their characterization by XRD, IR, TGA, MAS NMR and brightness measurements. Hydrothermal reactions of these metakaolins with aqueous alkali have been conducted. The residual Fe in the mother liquor has been estimated. The improvement in brightness and change in reactivity of the metakaolin, difference in the kinetics of its conversion to zeolites and dissolution of Fe during zeolite formation have been correlated with the calcination temperature. A calcination temperature of 900°C (1 hr) is the optimum for this clay to change into a reactive metakaolin which gives detergent grade zeolite 4A of high crystallinity and maximum brightness.
Isotopic (K-Ar and oxygen) constraints on the extent and importance of the Liassic hydrothermal activity in western Europe
Authors: Clauer N. , Zwingmann H. , Chaudhuri S. 
Published: December 2006

AbstractThe K-Ar dates and oxygen isotope data of illite-to-mica mineral phases in Permian to Triassic sandstones from Germany and Libya have been compared to previously published results of similar minerals of Liassic ages taken from lithologically similar rocks in several sites in western Europe and northern Africa. The widespread Liassic age for the micaceous mineral phases can be related to abnormal heat pulses induced by pre-rifting conditions. The temperatures in most cases appeared to be in the 150–220°C range. Some of these hydrothermal activities were responsible for both gas generation and U and Pb-Zn concentrations.An approach combining K-Ar isotope dates and oxygen isotope signatures provides a framework for the characterization of the clay separates used in isotopic investigations and for depicting any crystallization conditions. The trend in decreases in δ18O values relative to increases in K-Ar dates, while the size of the clay fractions in a single sample increases, is indicative of detrital contamination. Varied trends in the δ18O values at constant K-Ar dates for clay fractions with increase in size might be indicative of changes in the pore volume of the host rocks.