Transformation Of Smectite Research Articles

Lithological Control on the Occurrence of Chlorite in the Diagenetic Wealden Complex of the Bilbao Anticlinorium (Basco-Cantabrian Basin, Northern Spain)

AB ST R ACT : Sandstones and shales from the southern flank of the Bilbao anticline in the Gordexola and Orozko valleys, northwestern Spain, have been subjected to a detailed sedimentological, mineralogical and geochemical study. They are composed of proximal frontdeltaic sandbars interbedded with silty-clayey sediments and correspond to a prograding sequence of a deltaic system. The clay mineral assemblages of doublets of sandstones and shales are composed mainly of illite, chlorite and illite-smectite mixed-layers R3 (ISII). The clay diagenesis consists of the transformation of smectite and I-S mixed-layers to illite, the precipitation of Fe-rich chlorite in the pore spaces, and the alteration of micas to chlorite. According to petrographic, mineralogical and geochemical analyses, chlorite is more abundant in sandstones than in shales in both the <2 mm and coarser fractions. The relative abundance of chlorite increases in the sandstones located at the top of coarsening upward sandbars. Furthermore, chlorite formation occurs preferentially in the coarser grained sansdstones previously cemented by ankerite. The geochemical and petrological investigations suggest that chlorite formed during burial diagenesis in a relatively closed system.

The Role of Randomly Mixed-Layered Chlorite/Smectite in the Transformation of Smectite to Chlorite

AbstractVesicular and groundmass phyllosilicates in a hydrothermally altered basalt from the Point Sal ophiolite, California, have been studied using transmission electron microscopy (TEM). Pore-filling phyllosilicates are texturally characterized as having coherent, relatively thick and defect-free crystals of chlorite (14 Å) with occasional 24-Å periodicities. Groundmass phyllosilicates are texturally characterized as 1) randomly oriented crystals up to 200 Å in width and 2) larger, more coherent crystals up to 1000 Å in width. Small crystallites contain predominantly 14-Å layers with some 24-Å units. Large crystals show randomly interlayered chlorite/smectite (C/S), with approximately 50% chlorite on average. Adjacent smectite-like layers are not uncommon in the groundmass phyllosilicates. Electron microprobe analyses show that Fe/Mg ratios of both groundmass and vesicular phyllosilicates are fairly constant.Termination of brucite-like interlayers has been identified in some of the TEM images. The transformation mechanisms represented by these layer terminations are 1) growth of a brucite-like interlayer within smectite interlayer regions and 2) the dissolution and reprecipitation of elements to form chlorite layers. Both mechanisms require an increase in volume as smectite transforms to chlorite.The data, combined with that from previously published reports, suggest that randomly interlayered C/S is a metastable phase formed in microenvironments with low water/rock ratios. Chlorite forms in microenvironments in the same sample dominated by higher water/rock ratios. The relatively constant number of Mg's in the structure (Mg#) of both structures indicates that in both microenvironments the bulk rock composition has influence over the composition of phyllosilicates.

Transformation of Smectite to Illite In Bentonite and Associated Sediments from Kaka Point, New Zealand: Contrast in Rate and Mechanism

AbstractSmectite and mixed-layer illite/smectite (I/S) in Triassic heulandite-rich bentonite from Kaka Point, New Zealand, have been investigated by scanning electron microscopy (SEM), transmission electron microscopy/analytical electron microscopy (TEM/AEM) and X-raydiffraction (XRD) for comparison with matrix phyllosilicates in closely associated siltstones and analcimized tuff. Samples that were treated to achieve permanent expansion showed that some smectite in bentonite occurs as curved packets of wavy 10.5- to 13-Å layers enveloping relict glass shards, the centers of which consist of an amorphous clay precursor. The dominant clay minerals in bentonite are smectite-rich randomly disordered (R0) I/S with variable proportions of 10-Å illite-like interlayers, only locally organized as 1:1 ordered (R1) I/S. R0 I/S was also observed in separate packets retaining the detailed texture of packets that replaced shards. Such relations are consistent with a “solid-state”-like, layer-by-layer replacement of original smectite layers by illite-like layers with partial preservation of the primary smectite texture, in contrast to textures observed elsewhere, such as in Gulf Coast mudstones. The smectite, as in other examples in marine sediments, has K as the dominant interlayer cation, suggesting that precursor smectite may be a major K source for reaction to form illite.Only a small proportion of illite (35%) occurs in mixed-layer smectite-rich I/S in bentonite and the dominant trioctahedral phyllosilicate is disordered high-Fe berthierine, implying that little mineralogical change has occurred with burial. This contrasts with observations of closely associated siltstones and analcimized tuff, which contain well-defined packets of illite and chlorite but which have no detectable matrix smectite component. These data imply that the rate of transformation of smectite to illite is much slower in bentonites than in associated sediments of the same burial depth and age. Such relations emphasize the significance of factors other than temperature, (e.g., organic acids, permeability and pore fluid compositions) in affecting the rate and degree (and perhaps mechanism) of transformation of smectite to illite.

Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France

ABSTRACT The thermal history of the Ardeche paleo-margin (southeastern France) has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation. These methods comprise Tmax and vitrinite reflectance measurements on organic matter, determination of the smectite content in mixed-layer illite/smectite, fluid-inclusion microthermometry in quartz, barite, dolomite, and anhydrite, and fission-track analyses in apatite. The Balazuc (BA1) drill hole, 1730 m deep, intersects mainly Jurassic carbonate rocks, Triassic sandstones, and sulfate-rich claystones and ends in Carboniferous rocks. The present bottom-hole temperature is 70°C. Other drill holes, less than 6 km away, were studied when necessary. One of the main objectives of this paper is to ssess the agreement between the different geothermometric methods, and to check their limitations and convergence when used on carbonate- and sandstone-dominated reservoirs. In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C. Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic. The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene. These paleotemperatures, much higher than the present ones, can be explained by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation implies a high average sedimentation rate during the Cretaceous, in agreement with data determined on a regional scale. A time-temperature evolution is proposed for the formations present in the Balazuc drill hole and the eroded sedimentary rocks. The application of the dual reaction model of the transformation of smectite to illite (Velde and Vasseur 1992) shows that the data are in good agreement with the kinetic model. However, the vitrinite reflectance variation trend with depth, predicted from the Burnham and Sweeney model (1989), is different from the data variation. No definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the BA1 drill hole.

Occurrence and genesis of palygorskite and associated clay minerals in a Pleistocene calcrete complex, Sde Boqer, Negev Desert, Israel

AbstractPalygorskite and associated clay minerals have been studied in a Pleistocene calcrete complex from the Negev desert (Sde Boqer, Israel). This complex is divided into five main parts: the chalky and marly bedrock overlain by its weathered product, a brecciated calcrete hardpan; the laminar crust; loess pockets trapped in the calcrete; and the overlying soft surficial soil. The distribution of clay minerals is directly dependent on the position of the sample in the calcrete complex. Smectite is inherited from the bedrock in the calcrete with an aeolian enrichment in loess pockets and upper soft soil. The source of kaolinite is mainly detrital, related to desert dust. Illite is partly inherited and partly neoformed in loess pockets and desert surficial soil. The origin of palygorskite is clearly related to precipitation processes from soil solutions, Si and Al ions being provided by the slope and detrital grains trapped in the calcrete, with Mg being provided by the parent rock, the slope and aeolian detrital dolomite. This neoformation occurs mainly around detrital grains such as quartz in the hardpan and along textural transitions in loess pockets and is associated with gypsum in the lower part of the toposequence where the evaporation fluxes are the most important. The existence as coatings around grains and the delicate structure of the fibres are incompatible with a detrital origin. Moreover, no evidence was found to support any transformation of smectite into palygorskite in the solid state. Palygorskite is the last step of the cycle of mobile silica and seems to be a product of late diagenesis in Sde Boqer calcrete.

Evaluation of Kinetic Models for the Smectite to Illite Transformation

AbstractThree different models have been reported previously to describe the kinetics of the transformation of smectite to illite (Pytte 1982; Velde and Vasseur 1992; Huang et al. 1993). In order to evaluate the general utility of these models to calculate the timing and extent of this transformation, each model was applied to four different geologic settings (Denver Basin, Gulf Coast, the Salton Sea Geothermal System, and Paris Basin) in which the ages, geothermal gradients and potassium ion activities vary markedly. The model results are compared to the measured percentages of illite in illite/smectite (I/S) and the K/Ar ages of I/S (if available) to test the utility of a given model to a particular basin.Although individual models can be applied to study this transformation within a specific setting, none of these models was successful in simulating the transformation for all four basins. The Salton Sea was simulated best using the model by Huang et al. (1993), which incorporated an increased geothermal gradient during the last 20,000 years. These results indicate that a large fraction of illite formed due to this increased geothermal gradient, and underscores that temperature is a dominant kinetic factor in forming illite. The Denver Basin was simulated well by the models of Velde and Vasseur (1992) and Pytte (1982). The Gulf Coast was simulated very well by the model of Huang et al. (1993) using a term that terminates the transformation at 75% illite. For the Paris Basin, the results are mixed. The models can be refined by comparing the calculated and measured ages of illite such as the K/Ar ages of I/S to understand the thermal history of a particular basin. The calculated ages of illitization derived from these refined models can be used to indicate the time at which source rocks became thermally mature to form oil and gas.

An approach to the genesis of palygorskite in a Neogene-Quaternary continental basin using principal factor analysis

AbstractThe Carrión lacustrine basin (≈10 km2), central Spain, is filled with Neogene-Quaternary sediments (20–25 m thick) rich in palygorskite. Two clay units of ∼15 m thick are of particular interest in the basin. The lower one contains over 75% phyllosilicates (palygorskite up to 90%, smectites and illite) in marginal areas (facies A), and it is laterally bound by a distal facies (B) also rich in clay minerals (smectites and illite, and some palygorskite) and gypsum, and includes dolomitic marl intercalations. The composition at the top is primarily marly, with some gypsum (C). The upper clay unit, mainly clay-size material, (D) is rich in smectites, palygorskite and sepiolite, and contains carbonate intercalations near the top. A statistical treatment of the mineralogical and chemical composition data from 120 samples, using principal factor analysis, suggests that: (a) illite and smectite are detrital in origin; (b) palygorskite was formed by transformation of illite and smectite involving reaction with Mg; and (c) sepiolite was a neoformation product favoured by evaporation of silica- and Mg-bearing waters.

Some Evidence Supporting the Existence of Polar Layers in Mixed-Layer Illite/Smectite

AbstractThe fraction &lt;20 μm of a bentonitic material was selected to study the transformation of smectite into illite under hydrothermal conditions. Powder XRD showed that the studied material was composed of 97% dioctahedral smectite and 3% of quartz, cristobalite and plagioclase. The XRD analysis of the oriented, glycolated sample showed that the smectitic phase was actually a randomly interstratified illite/smectite with 11% illite. The chemical analysis of the sample yielded the structural formula: K0.25Na0.25Ca0.12Mg0.13(Al2.76Fe0.33Mg1.03)(Si7.66Al0.34)O20(OH)4. The reaction conditions of the hydrothermal experiments were: KCl concentrations 0.025, 0.05, 0.1, 0.3, 0.5, 1 M; temperature 60, 120, 175, 200°C; run time 1, 5, 15, 30, 90, 180 days; solid: solution ratio 1:5. XRD of run products showed an important transformation of smectite into illite, in contrast with all other techniques (DTA, IR, NMR), which did not detect any transformation of the original material. This suggests that illite quantification in I/S by means of XRD patterns after hydrothermal experiences can be much affected by other variables. The original material and the run products were analysed by means of DTA. The starting material displayed two dehydroxylation peaks, one at 560°C corresponding to illite, and the other at 650°C corresponding to smectite. Quantification of the two dehydroxylation peaks also yielded a content of 11% illite. The run products displayed DTA diagrams in which the smectitic peak remained unaltered, but the illitic one was reduced or eliminated by the presence of exchangeable K in the interlayer. When exchangeable K was removed the illitic dehydroxylation peak appeared again. The coherence of XRD and DTA in quantifying the proportion of illite layers and the effect of exchangeable K on dehydroxylation of illite layers in mixed-layer illite/smectite seem to be positive proofs of the existence of polar layers.

Origin of grain-coating chlorite by smectite transformation: an example from Miocene sandstones, North Sumatra back-arc basin, Indonesia

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.

鉱物の変化からみた堆積盆の熱履歴評価

It is one of the most important factor for exploration of energy resources to evaluate the thermal history of a sedimentary basin. A common method for the evaluation of thermal history is measurement of maturity of organic matter. The recent advances in the field of mineral geothermometry are remarkable. The outline of mineral geothermometer in low temperature is given in this paper. Two topics are mentioned in detail. The one is the application of fission-track dating to the thermal history of rocks in sedimentary basin. The other is the progressive transformation of smectite to illite via mixed-layer illite/semectite. The illitization is strongly controlled by temperature, and also influenced by several factors, such as kinetics, potassium availability, pressure, and fluid and rock composition. Some examples in the Niigata Neogene basin are briefly described.

Mineralogical characterization of the Garumnian subbituminous lignite from the central Pyrenees by SEM-EDX, X-ray diffraction and Mössbauer spectroscopy

A full mineralogical characterization of a Garumnian subbituminous lignite found in the Pyrenees is performed and a good correlation between the mineral diagenesis and the coal rank is obtained. The mineral matter is characterized in the different sequences and analysed by optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and transmission Mössbauer spectroscopy. Carbonates, quartz, clay minerals, sulfides and sulfates are identified and their presence is related to the diagenesis suffered by the mineral matter. Full transformation of smectite indicates temperatures > 360 °C, but the small formation of ankerite is related to low-grade metamorphism, in good agreement with the coal rank determination (Rock-Eval maximum temperature 430 °C).

Considerations and Applications of the Illite/Smectite Geothermometer in Hydrocarbon-Bearing Rocks of Miocene to Mississippian Age

AbstractEmpirical relationships between clay mineral transformations and temperature provide a basis for the use of clay minerals as geothermometers. Clay-mineral geothermometry has been applied mainly to diagenetic, hydrothermal, and contact- and burial-metamorphic settings to better understand the thermal histories of migrating fluids, hydrocarbon source beds, and ore and mineral formation.Quantitatively, the most important diagenetic clay mineral reaction in sedimentary rocks is the progressive transformation of smectite to illite via mixed-layer illite/smectite (I/S). Changes in both the illite/smectite ratio and ordering of I/S, as determined from X-ray powder diffraction profiles, correlate with changes in temperature due to burial depth. Although the smectite-to-illite reaction may be influenced by several factors, reaction progress appears to be strongly controlled by temperature. Studies show that the model proposed by Hoffman and Hower in 1979 is applicable in burial diagenetic settings from about 5 to 330 Ma, and includes most rocks about Miocene to Mississippian in age. Reliability of the I/S geothermometer is, however, dependent upon a good understanding of the rock's original clay-mineral composition.Changes in the ordering of I/S are particularly useful in the exploration for hydrocarbons because of the common coincidence between the temperatures for the conversion from random-to-ordered I/S and those for the onset of peak, or main phase, oil generation. Here, the utility of the I/S geothermometer is reviewed in hydrocarbon-bearing rocks of Miocene to Mississippian age. Using three common applications, the I/S geothermometer is compared to other mineral geothermometers, organic maturation indices, and grades of indigenous hydrocarbons. Good agreement between changes in ordering of I/S and calculated maximum burial temperatures or hydrocarbon maturity suggests that I/S is a reliable semiquantitative geothermometer and an excellent measures of thermal maturity.

Genesis and transformation of minerals in the formation of red (Alfisols) and black (Inceptisols and Vertisols) soils on Deccan basalt in the Western Ghats, India

SUMMARYSix noncalcareous pedons from the basaltic terrain of the Western Ghats in Maharashtra, India, were identified for the present study. Of these, two red‐soil pedons (Typic Haplustalfs) and one black‐soil pedon (Vertic Ustropept) are from the Bhimashankar plateau at an elevation of 1000 m above mean sea level, experiencing a humid (&gt;5000 mm rainfall) tropical climate. The other three pedons of black soils (Typic Chromusterts and Typic Ustropept) are from the semi‐arid zone (500–1000 mm rainfall), at an elevation of 825–893 m.Clay mineralogical investigation indicated that interstratified smectite‐kaolin (Sm/K) is dominant in red soils whereas smectite is dominant in black soils. The Sm/K is formed by the transformation of montmorillonite, the first weathering product of Deccan basalts in a humid tropical climate. We suggest that the interstratification of kaolin with chloritized smectite may also be an important ephemeral stage during the transformation of smectite to kaolinite. The presence of zeolites provided sufficient bases to prevent the complete transformation of Sm/K to kaolinite. The presence of smectites and zeolites made the formation of black soils possible in microdepressions even in a tropical humid climate. The genesis of Sm/K and smectite in red and black soils, respectively, suggests that these soils formed through a progressive landscape reduction process. The presence of both Sm/K and smectite in black soil clays of semi‐arid climate suggests that the smectite of these soils was formed in an earlier humid climate.

Alterations in the Non-Clay-Mineral Fraction of Pelitic Rocks Across the Diagenetic to Low-Grade Metamorphic Transition, Ouachita Mountains, Oklahoma and Arkansas

ABSTRACT The transformation of smectite to illite has been cited by many authors as a source of silica during diagenesis of mudrocks. Illites themselves, however, undergo chemical changes as they recrystallize into micas during high-grade diagenesis/low-grade metamorphism. Average compositions of phyllosilicates from the literature suggest that an equivalent amount of silica is available from transformation of illite to muscovite as from illitization of smectites. The fate of silica released by this process has not been reported, but could be a major contributor to the silt-size quartz population. The quartz and feldspar fraction of pelites from the Stanley Shale (Mississippian) in the Ouachita Mountains of Oklahoma and Arkansas was separated using sodium bisulfate fusions. The mineralogy and the grainsize distribution of this fraction were determined using standard petrographic and X-ray diffraction (XRD) techniques. Bulk rock samples were analyzed using X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) methods. The data obtained were related to illite crystallinity and vitrinite reflectance as reported by Guthrie et al. (1986) and Houseknecht and Matthews (1985). Both the percentage of quartz and the mean grain size of the quartz and feldspar fraction increase with greater illite crystallinity values. The growth in quartz is especially apparent in the finest size fractions. A corresponding decrease in the silica content of the clay-mineral fraction is also observed. Development of quartz polycrystallinity occurs across the same interval. Whole-rock chemical abundances show no statistical correlation with thermal maturity. Relative to titanium, both the major-element and the trace-element concentrations show little variation. Rare-earth element ratios do not correlate with thermal maturity and remain essentially constant. Our results are consistent with reported differences between quartz in schists and their shale precursors, and suggest that release of silica during diagenesis of phyllosilicates continues after the smectite-illite transformation. This silica precipitates as quartz within the pelite, consistent with the suggestion by Blatt (1987) that metapelites are the source of abundant silt-size quartz. The lack of whole-rock chemical variation with thermal maturity implies closed-system behavior across much of the pelite-to-metapelite transition.

Clay sedimentation in the Japan Sea since the Early Miocene: influence of source-rock and hydrothermal activity

X-ray diffraction analyses have been carried out on 128 samples of Miocene to Quaternary sediments from ODP Sites 794, 795 and 797. Some clay fractions of samples from Site 797 have also been studied for rare earth elements and by Nd isotopic analyses. These three sites display similar lithological and clay assemblages (with dominant chlorite, illite and smectite) showing that the sedimentation was homogeneous throughout the whole Japan Sea Basin. Three mineralogical zones are recognized. The first zone (Lower Miocene sandy clay of Sites 794 and 797) is mainly composed of chlorite resulting from hydrothermal transformation of arc-derived smectite, due to sill injections during the initial oceanic spreading stage. The second zone (Lower Miocene to Lower Pliocene siliceous claystone and diatomaceous silty clay) is dominated by arc-derived smectite; the abundance of this mineral decreases upwards while illite and chlorite increase. This trend reflects a change of detrital source, from an eastern arc-derived source ( ϵ Nd t > −3.3; variable LREE enrichment) to a western continental crust source ( ϵ Nd t < −9.4 ; shale-like REE patterns); climatic modifications in the current dynamics are proposed as a cause for this change. The third zone (Upper Pliocene to Recent silty clay with minor diatom oozes) is characterized at Site 797 by increasing amounts of illite and chlorite. This reflects a more and more important western supply which is assumed to be related to tectonic rejuvenations of the Asian margin or climatic modifications affecting the alteration conditions or the current dynamics. At Sites 794 and 795, the more or less sharp supply of chlorite seems to be driven by the incipient subduction zone on the eastern margin of the Japan Sea.

Conversion time from smectite to illite. A preliminary study

During the hydrothermal treatment of a bentonite in the presence of potassium solutions a release of protons, silica and other elements was observed. It is suggested that these products arise as a consequence of the beginning of the transformation of smectite into illite. After the short time of the treatment, 21 days, it is not possible to detect any layer of neoformed mica, but this quantity can be indirectly determined from the stoiquiometric reaction of transformation smectite-to-illite through the number of moles of hydrogen ion released to the solution. Under the assumption that the transformation is a first order reaction, the time needed for the complete destruction of smectite was also calculated. In the range of 100°C smectite resists more than eighty thousand years before being transformed into illite. These results are in agreement with other described in the literature.

Mineralogia y sedimentologia del yacimiento de saponita de Yuncos (Toledo)

Mineralogy and textures of saponite-bearing bentonites and related facies have been studied in quarries located southwest of Yuncos village (Toledo) in the Madrid Neogene Basin. Results allow us to establish three stages related with the evolution of sedimentary environments, where bentonite deposits (middle stage) are at the boundary between lacustrine (lower stage) and distal alluvial facies (upper stage). Lower stage : Regressive mud flat deposits with paleopedogenic features, made up of green clays facies with variable detrital content and hydromorphic mottling at topo Clay mineralogy association: Smectite (tri-dioctahedral)-illite-(kaolinite). Middle stage : Transitional stage of shallow flooding with variable development of paleopedogenic features. The facies association is made up of saponitic brown clays with several textures (massive, fissile-Iaminated, brecciated) and occasional carbonates as calcite ooids or dolomite nodules. Clay mineralogy association: Smectite (trioctahedral)-illite-sepiolite-( kaolinite) . Upper stage : Progradation of distal alluvial fan deposits. The facies association is made up by reddish to brownish mudstones often with bioturbation features and hydromorphic mottling, with sporadic sandy inserts. Clay mineralogy association: Smectite (tri-dioctahedral)- illite-sepiolite-(kaolinite-chlorite). Illite, dioctahedral smectite, kaolinite and chlorite are interpreted as inherited clay minerals, whilst saponite and sepiolite are of authigenic origino Saponite origin is early diagenetic both by dioctahedral smectite transformation and neoformation, being an inhomogeneous genetic process so that the purest bentonite is located far away from distal alluvial progradation where the flooding is longer in time. From a genetic point of view, sepiolite is later than trioctahedral smectite sometimes being in short fibers on saponite sheets, at others in longer fibers as a cement, both cases related to salinity-alkalinity changes in the environment.

Model for the evolution of oxygen isotope ratios in the pore fluids of mudrocks during burial

The determination of oxygen isotopic ratios in authigenic silicates and pore fluids provides vital information on the nature of water-rock interactions in the subsurface and the origin of porosity-occluding phases in reservoir rocks. The δ 18O values of pore fluids generally increase during burial through water-rock interaction. The transformation of detrital smectite to illite is thought to have a major influence on the isotopic evolution of pore fluids, but previous attempts to model this effect have failed to accurately reproduce the observed water-rock isotopic trends with depth. A revised model has been developed which incorporates an empirical rate equation for the illitization reaction. The input variables include the initial mineral and pore water isotopic compositions, time-porosity-depth functions and geothermal histories, and the assumption is made that the isotopic equilibration of the solid phases with pore waters only takes place during reaction. The model is tested against the most complete data set available from the Texas Gulf Coast (USA) and produces a good fit with the available pore fluid and mineral data. The sensitivity of the model is tested for variations in the following parameters: the illite-water fractionation factor; the fractionation of oxygen isotopes between the smectitic and illitic components of the detrital illite-smectite; the oxygen isotope fractionation between pore waters and the interlayer water within the smectite; and the porosity-depth function. The results of the model are shown to be strongly dependent on the porosity-depth function. The best fit between the model and real data is provided by the Dickinson equation for undercompacted shale modified to have a constant porosity below 2600 m to simulate the effects of overpressuring. The model is also sensitive to the degree of oxygen isotopic fractionation between the illite and smectite within the sediment at the time of burial, but not to the extent of fractionation between interlayer water within smectite and the adjacent pore waters. The uncertainty within the illite-water fractionation factor has only a minor effect on the results of the model. Although only incorporating a single mechanism for water-rock interaction, the model provides a tool for testing the interpretations of subsurface pore fluid evolution trends during sediment diagenesis.

Diagenesis of the Central Basque-Cantabrian Basin (Iberian Peninsula) based on illite-smectite distribution

AbstractX-ray diffraction was used to analyse the distribution of clay minerals in the &lt;2 µm fraction of the lutitic and marly facies from the centre of the Basque-Cantabrian Basin (Basque Arc), where the sedimentary section is 2000 to 10,000 m thick. Most of the deposits were laid down during the Cretaceous and Paleogene and were related to the opening and closing of the Bay of Biscay. The most noteworthy variations are in kaolinite, smectite and mixed-layered (R = 0, R = 1 and R ≥ 3) illite-smectite, which can be ascribed both to provenance and to diagenesis. A general diminution in expandability is related to the gradual transformation of smectite to illite from south to north, and with depth. Temperature, residence time and chemical activity during diagenesis are the factors that had greatest influence on the changes in the original mineral assemblages inherited from the source area. On the basis of the clay minerals and the Kübler index, a general scheme is proposed for the diagenetic evolution of the area from the initial stages to anchimetamorphism.

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