Illite-smectite Minerals Research Articles

The Effect of K-Fertilization and Irrigation on the Composition of Cultivated Soils: Examples from Israel

Evaluation of soil sustainability should take into account chemical and mineralogical changes due to cultivation. Potassium (K) application is a key farming practice that may potentially affect phyllosilicate composition and soil properties. Irrigation is another agent that affects soil composition. This study aims to evaluate the imprints of cultivation on the chemical and mineralogical compositions of lowland soils under semi-arid and Mediterranean climate regimes and to assess them with the natural pedogenesis. The sites examined include two permanent plot experiments with crop rotation, and three single-plant plots. Cultivated, control, and fallow soils were analyzed for their chemical and mineralogical composition. The X-ray diffraction patterns of the clay fraction were decomposed to achieve quantitative detection of changes among the illite–smectite (IS) phases and in kaolinite amounts. In loamy and clayey soils dominated by IS phases, cultivation caused minor changes and diverse behavior of the IS phases. Yet, the K balance was negative under high fertilization levels and higher plant mass production. Fertilization enhanced the natural process of transforming IS into kaolinite and illite in the sandy soil, leading to a positive K balance. This study emphasizes the importance of IS minerals as a dynamic K pool that responds to plant needs.

Compositional-structural characterization of the Opalinus Clay and Passwang Formation: New insights from Rietveld refinement (Mont Terri URL, Switzerland)

Middle Jurassic Opalinus Clay (OPA) in Switzerland and Southern Germany is considered as host rock for the disposal of heat-generating radioactive waste. For the present study, core samples from the Mont Terri rock laboratory (Switzerland) were investigated using a facies-based approach including mineralogical and geochemical analyses. Special focus was on the analysis of the degree of ordering of irregular illite-smectite interstratified clay minerals (I-S) responsible for sorption of radionuclides and swelling properties of the rocks.The investigations support the classification of OPA into five main facies (instead of the established threefold division) and into further subfacies. The facies concept was extended to the Passwang Formation (PWF). The amount of clay fraction varies in samples from the different facies, which is consistent with variations in cation exchange capacity (CEC). X-ray diffraction (XRD) analyses of the <2 μm fraction revealed a homogenous composition of the main constituents, namely R1-ordered I-S, kaolinite, and illite. Crystal structure based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified illite-smectite minerals for all investigated facies. The I-S-phase can be described with the R1 model, which is together with the amount of illitic layers in the I-S an indicator for a maximum burial temperature between 90 and 105°C. The amount of illitic layer in the I-S varies from 73%–85% for all refinements and ordering types, and in all OPA and PW facies and subfacies. This, together with the homogenous composition of the clay size fraction points to (1) a homogenous overprint of the clay assemblage during burial and/or (2) a uniform siliciclastic sedimentary supply throughout the sedimentation period of the OPA. The results of this study highlight the need to combine lithofacial studies with mineralogical investigations in order to adequately capture the variability of clay-rich host rocks under consideration for radioactive waste disposal.

An integrated petrophysical analysis based on NMR, organic geochemistry and mineralogy. The Vaca Muerta source rock-unconventional play at different thermal maturities

The Vaca Muerta Formation (Upper Jurassic–Lower Cretaceous) of the Neuquén Basin, Argentina is a world-class source rock and renowned as an unconventional reservoir for both oil and gas. The present study examines rotary sidewall cores representative of the oil and gas windows to analyze correlations between nuclear magnetic resonance (NMR T2 and T1-T2 maps at 2 MHz), Gas Filled Porosity (GFP), Rock-Eval programmed pyrolysis and quantitative X-ray diffraction (XRD) mineralogy. Shale samples are characterized by high present-day total organic carbon contents ranging from 3 to 7.20% (mean: 4.30%) for Well A at the oil window and, 0.72 to 11.77% (mean: 5.86%) for Well B at the gas window. The analyzed set also covers a wide compositional spectrum from carbonate-rich (&gt; 50% of calcite and minor dolomite), mixed carbonate-siliciclastic (30 to 50% of calcite), to siliciclastic-rich (&lt; 30% of calcite) samples. Notably, the clay fraction is dominated by interstratified illite-smectite minerals with less than 10% of expandable layers. Total porosity, calculated from Gas Filled Porosity plus NMR T2 Cumulative Distribution, is in the range of 10 to 20 porosity units. In this work we also present a new NMR sequence to detect solid-like organic matter with NMR at 2 MHz in an integrated workflow to characterize petrophysical properties of the Vaca Muerta Formation.

Sorption and Spatial Distribution of 137Cs, 90Sr and 241Am on Mineral Phases of Fractured Rocks of Nizhnekansky Granitoid Massif

The sorption behavior and spatial microdistribution of Cs-137, Sr-90 and Am-241 onto the surface of a fractured rock sample from the R-11 borehole of the exocontact zone of the Nizhnekansky granitoid massif were studied. The sorption efficiency of the fractured sample increases in the row of Sr &lt; Cs &lt; Am, where americium is the most retained radionuclide. Based on the image processing of radiograms and scanning electron microscopy data, the mineral relative sorption efficiency (RSE) values were determined to quantify the contribution of the mineral phases of the fractured sample to radionuclide retention. It was established that cesium and strontium are predominantly retained in cracks filled with secondary mineral chlorite. Zeolite is a less effective sorbent with respect to cesium and strontium. Americium sorption is uniform over the whole surface of the fractured sample, with close RSE values for all mineral phases (RSE ~1). The behavior of cesium in heterogeneous and monomineral systems of crushed mineral phases of quartz, biotite and zeolite NaA imitating minerals of the fractured rock sample R-11 was determined. It was shown that the fraction of the sorbed cesium in a heterogeneous system of two mineral phases—biotite and quartz—was larger than the sum of sorption values for the same separated mineral phases. Based on the models of radionuclide sorption on illite–smectite minerals, it was able to estimate the depth of the penetration of solution into the fractured rock sample R-11. The variations of penetration depths for solutions of specific radionuclide into the fractured rock sample were established.

Serpentinization and H2 production during an iron-clay interaction experiment at 90C under low CO2 pressure

Interactions between steel canisters and clay-rich material in deep geological repositories for high-level nuclear waste were studied by reacting metallic iron and a claystone from the Callovo-Oxfordian formation of the Paris Basin (COx). The experiment ran at 90 °C for 14 weeks in the presence of water and under 20 mbar of initial CO2 partial pressure (p(CO2)). At the end of the experiment, the pressure in the autoclave reached 50 bar due to H2 production. Water was almost entirely consumed and poorly crystallized 7 Å-greenalite-type clays were formed at the expense of illite and mixed-layered illite-smectite minerals (I/S). The dissolution of quartz and pyrite and the crystallization of pyrrhotite were also observed. Considering the results of Mössbauer and Fourier Transformed Infrared (FTIR) spectroscopies, iron carbonate and and/or ferrous hydroxycarbonate were suspected to precipitate but in very small amounts. A follow-up of the mineralogical evolution by XRD (X-ray diffraction), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) shows that serpentinization of clays (formation of greenalite) is accompanied by a significant consumption of water and the production of H2 gas due to the oxidation and dissolution of metallic iron. Experimental results are in good agreement with geochemical modelling with the PHREEQC code. The kinetics of the iron corrosion was evaluated from H2 production and shows that diffusion processes are the rate limiting step.

Potassium Adsorption and Release Characteristics on Vertisols of North Western Ethiopian Highlands

ABSTRACTThe availability of plant nutrients in soils is governed by adsorption, desorption, and other chemical and physical processes. The adsorption characteristics of applied potassium (K) and the release kinetics of nonexchangeable fraction of soil K for plant nutrition on Vertisols of East Gojjam, North West Ethiopian highlands, were evaluated using the batch equilibrium technique and calcium chloride (CaCl2) solution, respectively. The mineralogy was assessed by X-ray diffraction (XRD). The result indicated that the adsorption data well fitted with the Freundlich and Langmuir models. The adsorption of K was controlled by cooperative adsorption ( > 1) and a partition factor ( < 1). Adsorption parameters were significantly (P< .05) correlated with the cation exchange capacity (CEC) of soils. The highest and lowest external requirement of K optimized from Freundlich isotherm were 1798.15 kg ha−1 and 73.18 kg ha−1 K, respectively, in which the requirement of K was a function of the adsorption capacity and adsorption maxima of soils. Based on the coefficient of determination (R2), the parabolic diffusion and the power function equations displayed the best fit of the kinetic data. The kinetic model confirmed that the release of K is diffusion controlled and indicated that the release of K was from interlayer positions. The rate constants of the parabolic diffusion and the power function models were related (r = 0.62* and 0.66*, respectively) to the uptake of K, suggesting the contribution of nonexchangeable K to plant nutrition. Considerable differences in the release rates were due to the supplying power of K by soils associated with the random ordered illite-smectite minerals and CEC of soils.

Dual liquid porosimetry: A porosity measurement technique for oil- and gas-bearing shales

Measuring and differentiating effective and ineffective porosity in oil- and gas-shale remains a challenging step in the calculation of hydrocarbon reserves. Shale composition, typically dominated by clay minerals and organic matter (OM), forms a complex pore system, making the proportion between effective and ineffective porosity variable and difficult to determine in the course of porosity measurement.In this study, total porosity was measured from a set of rock chips using both light kerosene (kerosene immersion porosimetry-KIP) and deionized (DI) water (water immersion porosimetry-WIP) as saturation-immersion fluids. Both KIP and WIP measurements were performed on samples equilibrated at 40% and 80% relative humidity (RH) and combined with water adsorption isotherms to estimate the range of clay-bound water (CBW). A combination of these procedures, and the use of kerosene and water as the saturation-immersion fluids, is hereafter called dual-liquid porosimetry (DLP). To test the method, two different burial diagenetic sequences of mudstone with different mineralogy and OM content were used.The results demonstrate that all the porosity values show a consistent declining trend with depth regardless of sample origin and type of saturation-immersion fluid used in the measurement process. Samples saturated with DI water were susceptible to swelling and the swelling decreased with sample depth, maturation and cementation, and increased with illite-smectite mineral content and degree of expandability. A consistently lower calculated grain density for KIP samples compared with WIP samples reflected incomplete pore saturation by kerosene. Kerosene saturation can be restricted by the presence of residual water that blocks kerosene pathways to small pores. This limitation, however, reflects geologic formation conditions making KIP measurement a good approximation for maximum liquid hydrocarbon-available porosity. Equilibrating the rock chips at 40% and 80% RH provides a rapid evaluation of the range of CBW that may represent ineffective porosity under various formation conditions. CBWmin (40% RH) constitutes roughly 1W–2W smectite equivalent layers and gives an estimation of bound water close to high hydrocarbon saturation. CBWmax (80% RH) represents the maximum possible bound water content in formations with higher water saturation. For samples, with low porosity and a significant content of expandable clay minerals and cation exchange capacity, CBW can constitute up to 100% of rock porosity.In order to avoid the limitations and assumptions of WIP (swelling) and KIP (incomplete saturation of nanopores), a combination of grain density and bulk density measured by WIP and KIP, respectively, was proposed. The paper further tests and discusses the pitfalls of saturation-immersion techniques.

Stress release and suction generation in the Callovo-Oxfordian claystone

Stress release effects in a specimen of claystone extracted at great depth (465 m) in the layer of the Callovo-Oxfordian claystone in Eastern France is examined by mimicking the stress change during extraction in the oedometer. To do so, a specimen first submitted to hydration with swelling impeded is afterwards unloaded at constant water content. Comparable mercury intrusion pore size distribution curves are obtained before and after stress release, showing little change in the configuration of inter-platelets pores within the clay matrix of the claystone. The swelling observed at constant water content is then related, to a water transfer from inter-platelets pores to intra-platelets pores, resulting in crystalline swelling within the smectite fraction of the inter-layer illite-smectite minerals.

Diagenetic differences of the Zhenzhuchong Member of Ziliujing Formation in the Jiulongshan–Jiange area, Sichuan Basin

The rocks of the Zhenzhuchong Member of the Ziliujing Formation, Jiulongshan–Jiange area in the Sichuan Basin, were analyzed by petrography, XRD, and SEM techniques to investigate their diagenetic history and properties, such as authigenic mineral types, evolution of mixed-layer illite–smectite minerals, the clay assembly, and the fraction of mixed-layer clay minerals. The results revealed that the Zhenzhuchong Member has experienced several important episodes of diagenetic alteration since the deposition, including compaction (pressure-solution), cementation, metasomatism, dissolution, fracturing, and infilling of caves and cracks. It was also observed that diagenetic properties of the Jiulongshan area were significantly different from those of the Jiange area. The rock samples from the Jiulongshan area were characterized by the composition of siliceous and calcareous cements, varying amounts of detrital grains, clay matrix and kaolinite replaced by calcites, a certain amount of rarely dissolved early-stage kaolinite, dickite, and infillings by late-stage calcite. On the other hand, for the rock samples from the Jiange area, the dissolution is a common phenomenon with features of abundant aluminosilicates-dissolution pores or components, but the replacement phenomenon has rarely been seen. These rock samples were characterized by the presence of clay mineral cements, quartz, and dolomite infillings. It indicated that there was a great difference of diagenesis between the two areas in the types, phase, and temperature of diagenetic fluids. Revealing the difference would provide theoretical and practical implications for the exploration of high quality oil and gas reservoirs.

The fifth paleosol layer in the southern part of China's Loess Plateau and its environmental significance

Through field investigations, dark red–brown lumpy ferruginous argillans (LFAs) and an underlying weathered loess layer under the fifth layer (S5 paleosol) were discovered in the Guanzhong Plain in the southern part of the Loess Plateau in northwest China's Shaanxi Province. The ultramicrotexture of the LFAs and their mineral and chemical compositions were analyzed. Experimental results showed that mixed illite–smectite minerals comprise about 90% of the LFAs in mineral composition, with 40% SiO2 content and high Fe2O3 (about 10.0%) and Al2O3 (about 22.5%) contents in chemical composition. Neo-formed microcrystalline grains of clay minerals dominated the LFAs. The micromineral crystal grains in the LFAs were mainly plates with some degree of alignment, giving the argillans crystalline optical properties. Red LFAs and weathered loess layers under the S5, and the depth to which Fe2O3 and CaCO3 from the S5 have been removed, indicate that chemical weathering was much stronger when the paleosol layers formed than that at present. While the S5 paleosol was developed, a subtropical climate prevailed in the southern Loess Plateau, with a mean annual temperature and rainfall about 4 °C and 500 mm higher respectively than that at present. The adequacy in moisture in at least 4.2 m of the gravitational water zone during the formation of S5 was suitable for forest growth. During the warmest and moistest climate span of growth for 2.5 million years when S5 was formed, the Qinling Mountains lost its function as a climatic boundary between the subtropical and temperate zones of East Asia, when the summer air masses crossed the Qinling Mountains and reached the Guanzhong Plain, bringing rich precipitation.

Microbial reduction of Fe(III) in illite–smectite minerals by methanogen Methanosarcina mazei

Clay minerals are ubiquitous in soils, sediments and sedimentary rocks, and could coexist with methanogens in anoxic environments, but whether or not methanogens can reduce structural Fe(III) in clay minerals other than smectite is not known. The objective of this study was to understand the capability of a mesophilic methanogen, Methanosarcina mazei in reducing structural Fe(III) in illite–smectite clay minerals and its potential role in inducing mineralogical changes. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu-2, mixed-layer illite–smectite RAr-1 and ISCz-1, and illite IMt-1) were exposed to M. mazei in growth medium with and without anthraquinone-2,6-disulfonate (AQDS) as electron shuttle. The extent and rate of bioreduction were quantified via wet chemistry and mineralogical transformed by X-ray diffraction and scanning and transmission electron microscopy. Our results demonstrated that M. mazei was able to reduce structural Fe(III) in clay minerals with Fe(III) in smectite being the most reducible and illite the least. AQDS enhanced the reduction rate and extent. The bioavailability of structural Fe(III) to M. mazei was correlated to the smectite proportion in each clay mineral . Methanogenesis was inhibited by Fe(III) bioreduction, possibly due to diversion of electrons from the methanogenesis pathway to structural Fe(III) in clay minerals. Bioreduction of Fe(III) in clay minerals induced formation of biogenic mixed-layer illite–smectite, silica, and vivianite. These data collectively showed that mesophilic methanogen is capable of reducing structural Fe(III) in illite–smectite minerals and of inducing a number of mineralogical changes.

Geochemistry of Late Triassic coals in the Changhe Mine, Sichuan Basin, southwestern China: Evidence for authigenic lanthanide enrichment

Abstract Five thin coals of Late Triassic age occur in the Changhe Mine, Sichuan Basin, southwestern China. Petrology and geochemistry of these coals and their roof, parting, and floor samples were studied. The results show that the Changhe coals are enriched in As (8.9–11.8 μg/g), W (2.2–11.2 μg/g), Pb (20.8–48.2 μg/g), and Th (7.5–18.8 μg/g). The source of these elements is possibly the Xuefeng old land which is an early Paleozoic source area southeast of the Sichuan Basin. Two tonsteins were found in the Changhe Mine. One is the roof (K8R) of the K8 coal which is a pure kaolinitic claystone. The other is the claystone (K9C) above the roof of the K9 coal which is mainly made up of mixed illite–smectite minerals. The REE content (287.9 μg/g) of the K8 coal is much higher than that of its roof (153.6 μg/g). SEM-EDX results show that authigenic monazite and kaolinite are the main carriers of REE in the K8 coal. On the basis of coal chemistry and occurrence of the precipitated minerals, it is likely that leaching of the K8R tonstein occurred. It is possible that trace elements including Be (8.4 μg/g), Zr (302 μg/g), Nb (26 μg/g), Hf (9.7 μg/g), Th (18.8 μg/g), U (5.8 μg/g), Ga (16.1 μg/g), and Pb (48.2 μg/g) in the K8 coal were also elevated by the leachate of its overlying tonstein (K8R). The K9C tonstein with a total thickness of 16 cm is widely distributed in the Changhe Mine and serves as a distinct marker bed for correlation of coal seams. Besides mixed illite–smectite minerals, trace amounts of euhedral zircon, anatase, apatite, and massive silica with vesicular structure were also examined, these constitute strong evidence for its felsic volcanic ash origin.

Surface properties of illite-smectite minerals as detected by interactions with rhodamine 6G dye

AbstractInteractions between smectite clay minerals and various organic dyes have been studied extensively, but little information has accumulated from dye interactions with mixed-layer illite-smectite (I-S) minerals, especially regarding relationships with clay layer expandability, layer charge, particle size/shape, and molecular aggregation of organic dye molecules. The purpose of this study was to investigate the surface interactions of a set of mixed-layer illite-smectites from different geological environments with Rhodamine 6G dye. The samples used have different amounts of expandable smectite interlayers, different particle size and/or shape, and different layer-charge density at the surface. Five smectites with differences in layer charge and some non-expandable layer silicates were also tested. The interactions detected by UV-vis spectroscopy show no reaction between R6G and non-expandable minerals (kaolinite, mica), and intense reactions forming H-aggregates and monomers with smectites and illite-smectites. The intensity of H-aggregate formation increases with increase in the layer charge of smectites. Mixed-layer illite-smectites interact with R6G more intensely than do smectites. H-aggregate and monomer formation increases with the illitization process for randomly ordered illite-smectites (R = 0) and decreases in the course of illitization for the ordered illite-smectites (R &gt; 0).

Cation Exchange Selectivity Coefficient Values on Smectite and Mixed‐Layer Illite/Smectite Minerals

Sodium‐K, Na‐Ca, Na‐Mg, and Na‐Sr exchange isotherms were performed at a total Cl concentration of about 0.005 mol L−1 on the fine fraction (&lt;2 μm) of MX80 montmorillonite, mixed‐layer illite smectite minerals (IS), and clayey stones from European underground research laboratories. The derived selectivity coefficients for Na‐K, Na‐Ca, Na‐Mg, and Na‐Sr were found to vary very significantly as a function of the exchanger composition. A model was built to reproduce the data, taking into account the change of selectivity coefficient as a function of the exchanger composition of smectite. Model parameters were fitted using data from the present study and data from the literature obtained on smectite minerals. The addition of parameters for cation–anion pairs was found to improve the fitting results. The model was then successfully tested to reproduce published cation‐exchange features on natural clayey materials and soils suspended in fresh or saline water. Finally, it is shown that the simple combination of this smectite exchange model with a previously published illite exchange model is quite successful although perfectible in reproducing exchange data on IS.

Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England

AbstractThe Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.

Three-Dimensional Crystal Structures of Illite-Smectite Minerals in Paleozoic K-Bentonites from the Appalachian Basin

AbstractThe three-dimensional crystal structures of illite-smectite (I-S) in K-bentonite samples from the Appalachian Basin are characterized by rotational disorder in the stacking sequence of 2:1 illite layers, different proportions of n60° rotations (as opposed to n120°) in the rotated layers, and layers with centrosymmetric trans-vacant (tv) octahedral sites that are randomly interstratified with noncentric cis-vacant (cv) layers. The proportion of cv interstratification in the I-S increases with tetrahedral A1 and decreases with octahedral Mg and Fe content. The I-S minerals in the northern Appalachian basin K-bentonites are characterized by high (79% average) proportions of cv (Pcv) layers. In contrast, I-S from equivalent K- bentonites from the southern Appalachian basin has low Pcv values (38% average). These values do not correlate with expandability or rotational disorder. The geographic distribution of these I-S structural parameters may have resulted from possibly short-term, hot, and advective fluid migrations that differed in Mg concentrations and/or other physical and chemical parameters.

Illitization of smectite as seen by high-resolution transmission electron microscopy

High-resolution transmission electron microscopy observations have been performed on a sample which represents an early stage of the transformation of smectite into illite (by hydrothermal alteration). Pure smectite (with uncollapsed layers of 1.25 nm thick), pure illite and interstratified illite-smectite have been distinguished. Detailed observations suggest the existence of two transformation mechanisms : (i) dissolution of smectite and crystallization of illite; (ii) solid-state transformation of one smectite layer into one illite layer. Interstratified illite-smectite minerals are of random type and probably formed by a solid-state mechanism. Pure illite crystals, generally of 1 M polytype, are mainly produced by dissolution-crystallization

Influence des faibles métamorphismes sur les variations du chimisme global des sédiments et sur les paramètres de l'interstratification des illite˗smectites

It has been noted that, in general, sequences of sediments do not change composition as a function of burial depth. However, analysis of chemical variation between elements indicate that small changes can take place in a regular manner. This chemical change, although often masked by large compositional variation due to the detrital origin of the sediments, can be seen by using the statistical method of « sliding averages ». The initial bulk composition has no influence on the increase or decrease of a given element and one can thus conclude that the chemistry is controlled by factors exterior to the rock itself, pressure or temperature for example. Since it is known that these physical factors control the type of pelitic mineral present (illite-smectite mixed layered minerals) it was found that a linear correlation could be made between interstratification parameters of illite-smectite minerals and the bulk chemical composition of the sample.