bluuebunny/crossref_metadata_2025 huggingface dataset:
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

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

The 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

Fluid 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

Pore-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

Fluid 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

Clay 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

Authigenic 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

Stable 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

Grain-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

The 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

The 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 − 7.7

Oxygen: 1000 ln αkaolinite-water = 2.76 × 106 × T−2 − 6.75

1000 ln αsmectite-water = 2.55 × 106 × T−2 − 4.05

1000 ln αillite-water = 2.39 × 106 × T−2 − 3.76

The 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

Pore-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

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

Kaolin 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

The 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.
The microstructure of humic acid-montmorillonite composites
Authors: Ohashi H. , Nakazawa H.
Published: December 2006

An organo-clay composite was prepared by contacting Na-montmorillonite and humic acid in aqueous solution which was of variable pH. After freeze-drying the precipitate of the solution, the detailed structure of the humic acid-montmorillonite composite was observed. Transmission electron micrographs and electron diffraction patterns indicate that the composite which was formed under acidic conditions has a characteristic microstructure. Humic acid, a large organic molecule, is combined with bent or curled thin foils of montmorillonite to form a spherical aggregate. The X-ray diffraction pattern supports this observation and suggests that the montmorillonite is cleaved into thin flakes as a silicate layer. The composite formed under alkaline conditions was in the form of a thin plate or lath and had a similar morphology to that of montmorillonite. The difference in the microstructure of the composite is reasonably attributed to the chemical flexibility of humic acid, i.e., conformation changes depending on the pH of the solution.
The implications of reworking on the mineralogy and chemistry of Lower Carboniferous K-bentonites
Authors: Clayton T. , Francis J. E. , Hillier S. J. , Hodson F. , Saunders R. A. , Stone J.
Published: December 2006

Potassium-bentonites have been found in the Courceyan Lower Limestone Shales near Burrington Combe and Oakhill, Somerset, consisting of thin, greenish yellow, plastic clays interbedded within a mudrock and limestone sequence. Mineralogically, the clay fraction is composed of virtually monomineralic interstratified illite-smectite containing 7–10% smectite layers. The clay fraction of the surrounding mudrocks, however, consists of an illite-chlorite dominated assemblage. Their mineral composition, trace element content, and the relative abundance of zircon crystals suggest an origin from burial of montmorillonite originally formed from volcanic ash. The presence of anomalously high trace element contents with both euhedral and rounded zircon grains in the Oakhill K-bentonites suggests a secondary or reworked origin for these samples. In contrast, the presence of a non-anomalous trace element content and large (>100 μm) euhedral zircon grains suggests that the Burrington K-bentonite is primary in origin. Modelling of whole-rock rare-earth element (REE) patterns shows that the OakhillREEpattern can be derived from the Burrington pattern by the addition of small contributions from zircon and monazite, two major heavy minerals present. These K-bentonites probably represent the oldest Carboniferous K-bentonites so far recorded in the British Isles.
Palaeogeographic controls on palygorskite occurrence in mid-Cretaceous sediments of Morocco and adjacent basins
Authors: Pletsch T. , Daoudi L. , Chamley H. , Deconinck J. F. , Charroud M.
Published: December 2006

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 situ determination of Brønsted/Lewis acidity on cation-exchanged clay mineral surfaces by ATR-IR
Authors: Billingham J. , Breen C. , Yarwood J.
Published: December 2006

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy has been used to determine the nature of acid sites present in Na+-, Ni2+- and Al3+-exchanged montmorillonite using the diagnostic base, pyridine. The pyridine additions were performed in situ at 25°C in the presence of liquid benzene or deuterated 1,4-dioxan to ascertain whether the acidity profile was influenced by the presence of either the non-polar or oxygenated solvent. Pyridine treated Ni2+-montmorillonite exhibited absorption bands characteristic of Lewis bound base in both benzene and deuterated 1,4-dioxan indicating that the Lewis acid sites were present at low temperature and were accessible in both solvents. The presence of a strong 1537 cm−1 band in pyridine-saturated Al3+-montmorillonite confirmed that this clay acted as a Brønsted acid in benzene. However, this diagnostic band was significantly reduced in the presence of deuterated 1,4-dioxan.
Aluminosilicate diagenesis in a Tertiary sandstone-mudrock sequence from the central North Sea, UK
Authors: Huggett J. M.
Published: December 2006

Mudrocks and sandstones from the Palaeocene of the central North Sea have been studied to assess the petrology, diagenesis and extent of any chemical interaction between the two lithologies. Authigenic and detrital minerals have been distinguished using a variety of electron microscope techniques. Small but significant quantities of authigenic minerals, which would not be detected by conventional petrographic tools, have been detected through the use of high-resolution electron beam techniques. Sandstone mineralogy has been quantified by point counting, and mudrock mineralogy semi-quantified by XRD. The detrital and authigenic mineralogy in the sandstone is almost identical to that found in the mudrock. The principal difference is in the relative proportions. Qualitative mass balance suggests that cross-formational flow has not been significant in either clay or quartz diagenesis.
Field emission scanning electron microscopy — a high-resolution technique for the study of clay minerals in sediments
Authors: Huggett J. M. , Shaw H. F.
Published: December 2006

The use of field emission electron microscopy for the study of clay mineral petrography in mudrocks and sandstones is discussed. The methodology including sample preparation is outlined and three examples of the application of the technique are described: the formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones and the effects of KCl drilling muds on shale fabrics. In the study of authigenic illite formation in Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea revealed at least three different habits representing different generation episodes rather than one illite formation event.This has important repercussions with regard to the interpretation of stable isotope and dating data for the illites. Significant petrographic changes in shales after treatment with KCl drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales and the KCl muds.
Compositional variations in smectites. Part II: alteration of acidic precursors, a case study from Milos Island, Greece
Authors: Christidis G. , Dunham A. C.
Published: December 2006

The smectites present in some bentonites from the Island of Milos, Greece, derived from acidic rocks, have been examined by XRD and electron microprobe. The possible influence of loss of alkalis during analysis and contamination by fine grained Si-phases on microanalyses has also been evaluated. Smectites are closely associated with opal-CT and their composition ranges from beidellite to Tatatilla-type montmorillonite. Their chemistry is dominated by negative relationships between Mg and AlVI and Si and total Al. However, it cannot be resolved whether there is a solid-solution between the smectites or whether the variation observed is due to mechanical mixing of domains with different composition.The role of Fe is of minor importance reflecting an inherited factor imposed by the parent rock, but the role of the fluid phase is also important. Beidellite occurs in the rims, while Tatatilla-type montmorillonite, often associated with free opal-CT, is formed in the interior of the altered shards, indicating a variable degree of Si release during glass dissolution. The formation of beidellite is not necessarily associated with Mg uptake.
L'halloysite karstique: comparaison des gisements types de Wallonie (Belgique) et du Perigord (France)
Authors: Perruchot A. , Dupuis C. , Brouard E. , Nicaise D. , Ertus R.
Published: December 2006

Two cryptokarstic (covered karsts) halloysitic deposits were compared: the Entre Sambre et Meuse in Belgium and the Périgord in France. Both belong to a continental margin subject to marine transgression and regression cycles which enhance the karstic activity. This geodynamic context appears to be very favourable to the formation of halloysite. The similarity of the geodynamic context of both sites leads to a genetic convergence linked to specific properties of the cryptokarstic system, especially, the ability to collect the percolating waters and to juxtapose the conditions of precipitation, accumulation and maturation of the halloysite silico-aluminous gel precursors. However, each site also displays pronounced specificities relevant to: the nature of the host rock of the halloysite (a partly silicified limestone in the first, a smectite-kaolinite argilite in the second); the aluminium source (allochtonous in the first, relatively autochtonous in the second); and the halloysite content of the deposits (of ∼ 100% in the first, and 40% maximum in the second).
Clay mineralogy and surface charge characteristics of basaltic soils from Western Samoa
Authors: Naidu R. , Morrison R. J. , Janik L. , Asghar M.
Published: December 2006

The clay mineralogy and surface charge characteristics of four basaltic soils from Western Samoa have been studied. The soils contained subordinate to dominant amounts of poorly ordered allophanic material in addition to varying amounts of crystalline free iron oxide minerals. Infrared studies revealed the presence of trace to subordinate amounts of kaolin minerals in all the soils. The surface charge-pH curves followed a constant potential model indicating the presence of substantial amounts of pH-dependent charge. Some negative charge was present, however, at pH values as low as 3.0 and small quantities of positive charge were detected at pH values as high as 9. Values for PZC ranged from 2.2 to 3.9 and these were generally higher than the pHo determined by the ΔpH method.
X-ray photoelectron spectroscopic characterization of Silica Springs allophane
Authors: Childs C. W. , Inoue K. , Seyama H. , Soma M. , Theng B. K. G. , Yuan G.
Published: December 2006

A range of allophane samples (atomic AI/Si bulk ratios 1.1-1.9) from Silica Springs, New Zealand, has been characterized by X-ray photoelectron spectroscopy (XPS). Binding energies of Si 2s, Si 2p, Al 2p, O 1s, C 1s, and N 1s electrons, together with the kinetic energies of Al KL23L23 Auger electrons, at or near the surface of allophane aggregates, have been derived. The values for Al, Si and O electrons are similar to those for kaolinite but also to those for some framework silicates (feldspars) having 4-coordinate Al. Values for N electrons suggest that N occurs in organic structures. Comparison of XPS and bulk Al/Si ratios shows an enrichment of Al at or near the surface of allophane aggregates. The same is true for C and N. Extraction with citrate-dithionite-bicarbonate (CDB) reagent leaves the surfaces depleted in Al. The CDB extracts have higher Al/Si ratios than the bulk allophanes. Similarly, CDB treatment reduces the degree of surface enrichment of C and N. Small increases in the binding energies of Si electrons following CDB treatment suggest partial dissolution of the bulk structure though a concomitant removal of a separate phase or species cannot be ruled out. The results may be accounted for in terms of the structure previously suggested for the primary spherules of Silica Springs allophane (Childs et al., 1990) though the composition of the spherules at or near the surface of the allophane aggregates is different from those of the bulk.
Mechanisms of oxidation of Ni(II)-Fe(II) hydroxides in chloride-containing aqueous media: role of the pyroaurite-type Ni-Fe hydroxychlorides
Authors: Refait Ph. , Genin J.-M. R.
Published: December 2006

Ni-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P≥ 1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(IIl)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of NiII3-xFeIIxFeIII(OH)8Cl.nH2O (with 0 ≤ x ≤ 3), were obtained at the end of the first stage of oxidation, In a second reaction stage, these hydroxychlorides oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining FeII(OH)2 groups are transformed into FeIIIOOH groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the FeIIIOOH groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/33/2. On the other side of the domain, when P<1/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a lower chloride content, that is with an average composition of NiII3+yFeIII1-y(OH)8Cl1-y.nH2O where y = {[4/(1+P)] - 3}, down to minimum Fe(III) and Cl contents corresponding to y = 1/3 (P = 1/5). The in situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(H)-Fe(II)-Fe(III) hydroxychlorides are discussed.
Structural variation in wollastonite and bustamite
Authors: Angel R. J.
Published: December 2006

The structures of bustamite and the poly-types of wollastonite are shown to be derived from variations in the stacking sequence of a single type of structural unit. This structural unit is a column of unit cells of wollastonite along the b-axis and is bounded by (100) and (001) planes (P1̄ cell); these units may be stacked together along [100] and [001] with either zero displacement between successive units, or a relative displacement of 1/2[010]. Regular stacking sequences give rise to the ordered structures of wollastonite polytypes and of bustamite, while the transformations between these structures proceed by the propagation of line defects with Burgers vectors of 1/2[010] which thus change the stacking sequence of the structural units.
Apatite composition and the fugacities of HF and HC1 in metamorphic fluids
Authors: Yardley Bruce W. D.
Published: December 2006

Microprobe analyses of the halogen contents of apatites from two samples of amphibolite-facies schist from Connemara, Ireland, have been used to calculate the fugacity ratios fHCl/fH2O and fHF/fH2O using the experimental data of Korzhinskiy. The results imply fugacities for both acids in the range 0.03 to 0.1 bars, but whereas for the lower grade rock fHF > fHCl, the migmatitic sample gives fHF⋍fHCl. An independent estimate of fHF/fH2O from the biotite composition in one sample is in acceptable agreement with the result obtained from apatite.
X-ray diffraction studies of vivianite, metavivianite, and barićite
Authors: Sameshima T. , Henderson G. S. , Black P. M. , Rodgers K. A.
Published: December 2006

Vivianite specimens from various world localities yield X-ray powder patterns of two types: one corresponds with that shown by synthetic Fe3(PO4)2 · 8H2O and is not readily distinguished from that of barićite; the second shows reflections of monoclinic vivianite and triclinic metavivianite along with reflections of a bobierrite-type phase. The triclinic phase occurs as two twin-related lattices with twin plane 110 being the structural equivalent of 010 in the monoclinic phase. The relationship of the bobierrite-type lattice to the other two has not been established. The ternary pattern is produced by some coarse-grained vivianites on natural oxidation. Finer grained vivianites oxidise to an X-ray amorphous state without passing through a triclinic intermediate.
The structure of junitoite, CaZn2Si2O2 · H2O
Authors: Hamilton R. D. , Finney J. J.
Published: December 2006

Junitoite, CaZn2Si2O7·H2O, is orthorhombic, space group Area2, with a = 12.510, b = 6.318, c = 8.561 Å, Z = 4. The structure, determined on the basis of three-dimensional Mo-Kα intensity data for 745 reflections refined to R = 0.10. The structure consists of ZnO4 tetrahedral chains along b joined by Si2O4 groups along a and c to form a three-dimensional network. Ca occurs in distorted CaO5(H2O) octahedra. The structure shows similarities to hemimorphite and Ca-Zn silicates but represents a unique tetrahedral framework.
The determination of ferrous and ferric iron in rocks and minerals–a comment
Authors: Gillard R. D.
Published: December 2006

Hey's observation of reduction of ferric ion bound to 1,10-phenanthroline in water is explained in terms of the analogy between quaternization of the ligand and its coordination by iron(III).
Rare earth and other trace element mobility accompanying albitization in a Proterozoic granite, W. Bergslagen, Sweden
Authors: Baker J. H.
Published: December 2006

Zones of albitization 20 m wide are developed in the peraluminous, undeformed Proterozoic Bastfallshöjden granite, W. Bergslagen, central Sweden. During albitization Na, Si, Mg, Ni, Zn, and Ga are added, while Fe2+, Fe3+, Mn, K, Sc, Rb, Cs, Ba, Pb, U, and F are lost, together with the rare earth elements (REE) in decreasing amounts with increasing atomic number. Ti, Al, P, and Y were immobile. Trace element data for chlorites separated from hydrothermally altered country rocks and from a quartz-chlorite vein in the albitized granite show similar REE patterns indicating a common origin: the most altered granite has a similar REE pattern, probably resulting from interaction with the same hydrothermal fluid which produced the chlorites, in which seawater is thought to have been an important constituent.
Mg-Fe-Mn distribution in amphiboles, pyroxenes, and garnets and implications for conditions of metamorphism of high-grade early Archaean iron-formation, southern West Greenland
Authors: Hall R. P.
Published: December 2006

Remnants of early Archaean (Akilia) iron-formation occur as enclaves within the c. 3700 Ma Amitsoq gneisses in the eastern Godthåb-Sukkertoppen region of southern West Greenland. Amphibole- and pyroxene-bearing assemblages are preserved respectively in these two areas. The amphibole-facies rocks comprise one, two, and three-amphibole-bearing assemblages, coexisting with magnetite and quartz (and locally garnet). Experimentally determined phase equilibria suggest that the amphibole-bearing types have not exceeded c. 700 °C, while distribution coefficients in the two-pyroxene ironformation indicate a maximum temperature of around 625 °C.

Mg-Fe distribution relationships between the phases are:

and

where the numbers are generalized distribution coefficients and in phase a). Mn distribution is:

and

The distribution coefficients are variable both from one rock to another and internally from one set of grain pairs to another. The development of the observed variable silicate phase assemblages is dependent upon a complex interaction of not only metamorphic grade (P and T) and element availability, but more importantly aH2o and fo2. In particular, the possibility of reactions between observed assemblages containing cummingtonite + actinolite, magnesian actinotite + magnetite and ferrohypersthene+ferrosalite suggest lesser or greater degrees of dehydration under oxidizing or reducing conditions. For example:

and
Disposal of high-level nuclear wastes: a geological perspective
Authors: Ringwood A. E.
Published: December 2006

Most countries intend to dispose of their high-level radioactive wastes by converting them into a solidified wasteform which is to be buried within the earth. SYNROC is a titanate ceramic wasteform which has been designed for this purpose on the basis of geochemical principles. It comprises essentially rutile TiO2, ‘hollandite’ Ba(Al,Ti)Ti6O16, zirconolite CaZrTi2O7, and perovskite CaTiO3. The latter three phases have the capacity to accept the great majority of radioactive elements occurring in high-level wastes into their crystal lattice sites. These minerals (or their close relatives) also occur in nature, where they have demonstrated their capacity to survive for many millions of years in a wide range of geological environments. The properties of SYNROC and the crystal chemistry of its constituent minerals are reviewed in some detail and current formulations of SYNROC are summarized. A notable property of SYNROC it its extremely high resistance to leaching by groundwaters, particularly above 100°C. In addition, it can be shown that the capacity of SYNROC minerals to immobilize high-level waste elements is not markedly impaired by high levels of radiation damage. Current investigations are focused on developing a satisfactory production technology for SYNROC and progress towards this objective is described. The high leach resistance of SYNROC at elevated temperatures increases the range of geological environments in which the waste may be finally interred; in particular, SYNROC is well adapted for disposal in deep drill-holes, both in continental and marine environments. The fact that SYNROC is comprised of minerals which have demonstrated long-term geological stability is significant in establishing public confidence in the ability of the nuclear industry to immobilize high-level wastes for the very long periods required.
Cements in radioactive waste disposal: some mineralogical considerations
Authors: McCulloch C. E. , Angus M. J. , Crawford R. W. , Rahman A. A. , Glasser F. P.
Published: December 2006

Cementitious matrices are being assessed for immobilization of radioactive wastes. This paper discusses some mineralogical aspects of cement chemistry and the uses of siliceous minerals as selective sorbants to enhance immobilization potential.

Studies of sorption and leaching of caesium from pulverized fuel ash (PFA), blast furnace slag, tobermorite, xonotlite, and clinoptilolite are reported. The role of incorporation of these additives in cement and the effect on the nature of the composite matrix on caesium behaviour has been investigated. Specific mechanisms of the interaction of additives with highly alkaline cement environment are described. While slags, PFA, and clinoptilolite undergo reaction at different rates, tobermorite and xonotlite appear to be stable in cement.
Rock–matrix diffusion and its implications for radionuclide migration
Authors: Lever D. A. , Bradbury M. H.
Published: December 2006

Diffusion into the rock matrix is potentially an important retardation mechanism for nuclides leached from an underground radioactive waste repository in a fractured hard rock. A technique for measuring the intrinsic diffusion coefficient and rock capacity factor is briefly described. Simple solutions to migration model equations are used, together with diffusion results and typical hydrogeological parameters, to estimate the impact of matrix diffusion on radionuclide migration. It is shown that retardation factors in excess of 100 and reductions in the peak concentration by 3–4 orders of magnitude are possible for non-sorbed ions, which would otherwise be carried by the flow and not retarded at all.
Uranium-series disequilibrium studies of drillcore Km3 from the Kamlunge test-site, northern Sweden
Authors: Smellie John A. T.
Published: December 2006

Studies of the U decay series (238U-234U-230Th) have been carried out on samples from unaltered bedrock and highly altered fracture/crush zones from drillcore Km3 (Kamlunge test-site). The fracture zones are characterized by abundant iron oxide coatings (hematite and hydroxy iron oxides) resulting from the passage of hydrothermal solutions coeval with the Lina granite intrusion. Enrichments of U and Th, thought to be due to co-precipitation (or preferential sorption) processes together with the iron-oxides, are also present.

The isotopic results show that out of a total of twelve rock samples measured, six indicate isotopic disequilibrium mostly due to unequal depletions of 234U and 238U; one near-surface sample indicated some minor assimilation of U. The major fracture zones generally indicate removal of total U. This has resulted from interaction with groundwaters which are still marginally oxidizing, even at depths of 375 m.

Isotopic disequilibrium has occurred within recent geological times, i.e. during the last 0.5 Ma as imposed by the half-lives of 234U and 230Th. In terms of radioactive disposal considerations, the results are important in that: (1) the investigated bedrock environment (100–500 m) is generally reducing; however (2) there is some evidence to indicate that rock/water interactions, leading to the removal of total U, have resulted from the presence of less reducing groundwaters within those large-scale fracture/crush zones which intersect the bedrock surface.
Thermal groundwater movement and radionuclide transport in SW England
Authors: Durrance E. M. , Heath M. J.
Published: December 2006

Heat flow in SW England is well above average for the UK as a whole, but northwards towards Bath and Bristol the values decrease rapidly. However, hot springs occur both in the Bath-Bristol area and in mines in Cornwall. The development of hydrothermal circulation systems is thus not controlled entirely by geothermal gradient: the presence of a suitable fracture permeability is the main requirement. The thermal ‘head’ produced, which nevertheless depends upon the temperature and volume of water in the system, theoretically can exceed local topographic heads even in areas of low geothermal gradient.

Thermal groundwaters usually carry above average quantities of radioelements in solution because of the long residence times involved. 222Rn values are often particularly high. High concentrations of 222Rn in surface waters arise from the discharge of groundwater. The results of a survey of 222Rn in streams in SW England have established areas of high values which are interpreted as rising limbs of convection cells with dimensions in the order of 5–10 km.

In SE Devon γ-ray spectrometry of soils shows two E.-W. belts of high activity. The northern is coincident with the faulted southern margin of the Crediton Trough, while the southern is coincident with the westerly extension of the Abbotsbury fault system. Groundwater movement along deep-seated fracture systems is considered to be the explanation of these features. The horizontal scale of the area involved suggests that a thermal rather than local topographic head is the driving force. Groundwater circulation within fractures, driven by a thermal head, may therefore occur even in areas of low geothermal gradient and should be considered when selecting waste disposal sites.