Mixed-layer Illite Smectite Research Articles

Evaluation of egyptian glauconitic clay

Samples of Egyptian glauconitic material from El Gidida area were investigated. The characterization study included the mineralogical and chemical composition, crystal morphology, surface area and porosity. The effect of heat treatment and acid activation on the surface area, pore volume and microporosity of glauconitic clay was also investigated. Data obtained by using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), X-ray Fluorescence (XRF), and Infrared (IR) spectroscopy, reveal that the studied glauconitic material is di-octahedral with an iron-rich illite-smectite mixed layer composed of a range of expandable layers (<10 to 40%), in addition to variable percentages of kaolin and quartz. As the relative amounts of kaolin and quartz increased, illite-smectite mixed layers decreased. Low K 2 O content in glauconitic material is related to the composition of the parental glaucony mineral phase as well as to destabilization of glauconite. Morphologically, glauconite pellets consist of spheroidal aggregates of very fine particles, arranged parallel to each other, and separated by debris. However, many of the platelets are bent and curly, generating numerous mesopores. The surface area and pore volume ranged between 80-147 m 2 /g and 0.164-0.341 cm 3 /g, respectively. Details of the above characterization studies are given in this paper.

Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia

Clay minerals associated with the unconformity-type uranium deposits of the East Alligator Rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the Kombolgie unconformity. Alteration develops a similar pattern at regional and smaller scales, with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity (e.g., faults, fractures, breccias). These discontinuities were mechanically active during the hydrothermal activity. The hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the Kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks. The time-space alteration sequence established in this study can be summarized as follows: (1) early crystallization of illite ± illite-smectite (I-S) mixed layer minerals + LREE-Sr–bearing aluminum phosphate-sulfates (APS) in sandstones above the unconformity, illite ± I-S mixed layer minerals + sudoite + APS on both sides of the unconformity and in fault gouges, and illite ± I-S mixed layer minerals + chlorite ± apatite deeper in the basement; and (2) late precipitation of chlorite in the open fracture network mainly centered below the unconformity. The main stage of uranium deposition was associated with chloritization in the basement rocks. Smaller particle size, lath-shaped or flaky morphology, common interstratification with expandable layers, lower crystallinity, and Mg-rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and/or chlorite. Hydrothermal illite can be distinguished from diagenetic and metamorphic illite-phengite by its poor crystallinity along the c-axis, interstratification with minor amounts of smectite layers, and a 1M polytype in which trans-vacant octahedral sites predominate (1 Mt). Chemically, the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher Mg content. The hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size, spherulitic habit, poor crystallinity along the c-axis, and common association with corrensite and/or chlorite-corrensite mixed layers. Most of the hydrothermal chlorite is Mg rich and has a structural formula close to that of clinochlore (XFe <0.20). Near the uranium deposits, the spherules of hydrothermal chlorite are chemically zoned (from XFe <0.10 to XFe <0.40), which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization. The space-time alteration sequence established for clay minerals and the observed transition of APS to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub-basinal brines. In turn, these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron-bearing phyllosilicates (Fe chlorite and biotite) and iron sulfides of metamorphic origin. The ore deposits are surrounded by zones in which the degree of fluid-rock interaction was very high and chloritization is most abundant. The octahedral Mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks, tens to hundreds of meters below. The crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30° to 50°C lower than those expected at the peak diagenesis of the Kombolgie sandstones near the unconformity.

Alteration of smectites induced by hydrolytic exchange

AbstractHydrolytic exchange was performed experimentally on four smectitic clays to evaluate the extent of clay alteration induced by this process and the associated ‘auto-transformation’ of H+ clays. Clay samples were Na-saturated and submitted to 10, 50 and 100 wetting-drying (WD) cycles and characterized after treatment using X-ray diffraction (XRD), infrared spectroscopy (FTIR) and cation exchange capacity analysis. Evidence for hydrolytic exchange was given by increasing amounts of exchangeable Mg2+ and precipitation of Na soluble salts for samples subjected to 100 WD cycles. Results indicated a decrease in the interlayer charge after 10 WD cycles but no further decrease was observed after 50 and 100 WD cycles. For one sample, XRD data indicated a decrease in the proportion of the smectite phase and a relative increase in the concentration of illite-smectite mixed layers also present in the sample. The results suggested that the reaction induces first a decrease in the layer charge and then a partial dissolution of some smectite layers.

Alteration of the Callovo–Oxfordian clay from Meuse-Haute Marne underground laboratory (France) by alkaline solution. I. A XRD and CEC study

The reaction of the clay fraction of the Callovo–Oxfordian hard shale formation hosting the French underground laboratory site, with high pH NaOH, KOH and Ca(OH) 2 solutions has been investigated through closed system experiments at 60, 90 and 120 °C over 6, 24 and 168 h. The mineralogical composition of the run samples has been determined using X-ray diffraction (XRD) of randomly oriented powders showing the formation of different species of zeolites (analcime, chabazite, phillipsite) and Ca silicates (tobermorite, katoite). The phyllosilicates were studied using XRD of oriented preparations and cation exchange capacity measurements. Detrital or diagenetic mica and chlorite in the <2 μm fraction remain unchanged. On the contrary, the smectite and random illite–smectite mixed layer minerals are strongly reactive. The expandable layers of montmorillonite type are selectively dissolved while beidellitic ones survive or are transitionally formed.

Glauconitic clay of El Gidida, Egypt: evaluation and surface modification

Three samples of glauconitic material from El Gidida, Egypt were evaluated in terms of the mineralogical and chemical composition, crystal morphology, surface area and porosity. The effect of heat treatment and acid activation on the surface area, pore volume and microporosity of glauconitic clay was investigated. Data obtained by using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), X-ray Florescence (XRF), and Infrared (IR) spectroscopy, reveal that the glauconitic material of El Gidida area is di-octahedral with an iron-rich illite-smectite mixed layer composed of a range of expandable layers (<10–40%), in addition to variable percentages of kaolin and quartz. As the relative amounts of kaolin and quartz increased, illite-smectite mixed layers decreased. Low K 2O content in glauconitic material is related to the composition of the parental glaucony mineral phase as well as to destabilization of glauconite. Morphologically, glauconite pellets consist of spheroidal aggregates of very fine particles, arranged parallel to each other, and separated by debris. However, many of the platelets are bent and curly, generating numerous mesopores. The surface area and pore volume ranged between 147–80 m 2/g and 0.341–0.164 cm 3/g, respectively. Details of the above characterization studies are given in this paper.

Glauconitic clay of El Gidida, Egypt: evaluation and surface modification

Three samples of glauconitic material from El Gidida, Egypt were evaluated in terms of the mineralogical and chemical composition, crystal morphology, surface area and porosity. The effect of heat treatment and acid activation on the surface area, pore volume and microporosity of glauconitic clay was investigated. Data obtained by using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), X-ray Florescence (XRF), and Infrared (IR) spectroscopy, reveal that the glauconitic material of El Gidida area is di-octahedral with an iron-rich illite-smectite mixed layer composed of a range of expandable layers (<10–40%), in addition to variable percentages of kaolin and quartz. As the relative amounts of kaolin and quartz increased, illite-smectite mixed layers decreased. Low K2O content in glauconitic material is related to the composition of the parental glaucony mineral phase as well as to destabilization of glauconite. Morphologically, glauconite pellets consist of spheroidal aggregates of very fine particles, arranged parallel to each other, and separated by debris. However, many of the platelets are bent and curly, generating numerous mesopores. The surface area and pore volume ranged between 147–80 m2/g and 0.341–0.164 cm3/g, respectively. Details of the above characterization studies are given in this paper.

Rb–Sr systematics of Holocene pelitic sediments and their bearing on whole-rock dating

In many cases, when dealing with argillaceous fine-grained sedimentary rocks, the isotopic ages obtained from Rb–Sr whole-rock isochron calculations are of geological significance, despite the fact that the initial conditions of Sr isotopic homogenization are not fulfilled. To explain this, a mechanical mixing during deposition has been suggested, leading to an aggregate of mixed material with fairly uniform 87Sr/86Sr values, whatever Rb/Sr ratio is found in the analysed samples. This investigation of the behaviour of Rb and Sr during sedimentation and early diagenesis, involves study of more than 60 samples of fine-grained recent sediments from selected coastal localities of Brazil. The results indicate that pelitic samples from some recent to present-day transitional or shallow marine environments, such as the Jacarepaguá tidal flat and the Amazonas River mouth, where halmirolysis could have occurred, may produce nearly horizontal best-fit lines in a Rb–Sr isochron diagram. Moreover, the initial isotopic 87Sr/86Sr ratios appear to be well above 0.710. In open marine environments, such as the Campos Basin, where sampling was spread over more than 100 metres, exchanges between the argillaceous sediments and seawater potentially happen at low temperature, inducing a reduction of the 87Sr/86Sr isotopic composition of the sediments to nearly 0.710. An almost horizontal best-fit line is produced for the entire stratigraphic section, independent of the sedimentation age. We believe that in similar environments such ‘zero age isochrons’ are maintained for some time, testifying to exchanges between the sediment and the interstitial fluids, before the onset of burial diagenesis. The above described studies on fine-grained and recent argillaceous sediments seem to confirm the production of horizontal best-fit lines in isochron diagrams, justifying the application of the Rb–Sr whole-rock method to pelitic sedimentary rocks. The nature of the material is critical. The pelitic sediments to be analysed should contain only very limited amounts of coarse clastic material (especially feldspars and mica fragments) and consist mainly of fine clay minerals in which smectites, illite and mixed layers illite–smectite largely predominate.

Authigenic Minerals and Diagenesis in The Clastic Sediments of The Ghar Formation South and Southeast Iraq.

Study of clay minerals (Kaolinite,Iillite and Illite-Smectite mixed layers) in the clastic sediments of the Ghar Formation (Upper Oligocene-Lower Miocene) is the main indicator for the variation of the pore waters responsible for the eodiagenetic processes affecting on these sediments. Variation of these clay minerals are used also to distinguish the sedimentary environments of the Ghar Formation as transitional environment from fluvial into marine. The genetic relations of these clay minerals and their semi quantitative proportions by XRD and SEM are used also to distinguish the authigenic minerals and their paragenetic sequence. ــــــــــــــــــــــــــــــــــــــــــــــــــــــ

Crystallization Conditions of Fundamental Particles from Mixed-Layer Illite-Smectite of Bentonites Based on Isotopic Data (K-Ar, Rb-Sr and δ18O)

AbstractRb-Sr and oxygen isotope studies, in addition to K-Ar isotopic determinations published previously, are reported on diagenetic and hydrothermal fundamental particles (particle thickness of 0.03 to 0.05 nm and particle ab size of 0.02–0.05 µm) of the East-Slovak Basin. The combined data set allows us to ascertain the crystallization conditions of the illite material from two bentonite units collected at two basinal sites located ~20 km apart, and characterized by prolonged diagenetic conditions induced by progressive burial. A bentonite rock characterized by a short hydrothermal event from the Zempleni mountains to the SW of the East-Slovak basin is also studied.For the two first sites, the δ18O values increase in one case and decrease in the other, when the size of the diagenetic fundamental particles from bentonite samples increases. These variations suggest that temperature increased in one and decreased in the second of the two samples collected in the basin, while the particles were growing. In the case of the hydrothermal bentonite, the δ18O values of the different size-fractions consisting of fundamental particles remain about constant, suggesting constant temperature and fluid chemistry.The Rb-Sr dates of the fundamental particles of the three bentonite rocks were systematically higher than the corresponding K-Ar ages. The 87Sr/86Sr ratios, which are initially involved in the particle nucleation, appeared higher than that of contemporaneous sea-water. In all cases, the initial 87Sr/ 86Sr ratio decreases when particle size increases, which implies supply of external Sr into the bentonite units. This external Sr seems to have had an 87Sr/86Sr ratio close or identical to that of the contemporaneous sea water. This means that Sr, probably of sea-water origin, progressively diffused from host shales into the bentonite units, during burial diagenesis. In turn it favors the suggestion made previously about diffusion of K from shales into the bentonite layers during illitization of the smectite from these units.

Rosia Poieni copper deposit, Apuseni Mountains, Romania: advanced argillic overprint of a porphyry system

The Rosia Poieni deposit is the largest porphyry copper deposit in the Apuseni Mountains, Romania. Hydrothermal alteration and mineralization are related to the Middle Miocene emplacement of a subvolcanic body, the Fundoaia microdiorite. Zonation of the alteration associated with the porphyry copper deposit is recognized from the deep and central part of the porphyritic intrusion towards shallower and outer portions. Four alteration types have been distinguished: potassic, phyllic, advanced argillic, and propylitic. Potassic alteration affects mainly the Fundoaia subvolcanic body. The andesitic host rocks are altered only in the immediate contact zone with the Fundoaia intrusion. Mg-biotite and K-feldspar are the main alteration minerals of the potassic assemblage, accompanied by ubiquitous quartz; chlorite, and anhydrite are also present. Magnetite, pyrite, chalcopyrite and minor bornite, are associated with this alteration. Phyllic alteration has overprinted the margin of the potassic zone, and formed peripheral to it. It is characterized by the replacement of almost all early minerals by abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite, and kaolinite. Pyrite is abundant and represents the main sulfide in this alteration zone. Advanced argillic alteration affects the upper part of the volcanic structure. The mineral assemblage comprises alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite, and enargite (luzonite). Alunite forms well-developed crystals. Veins with enargite (luzonite) and pyrite in a gangue of quartz, pyrophyllite and diaspore, are present within and around the subvolcanic intrusion. This alteration type is partially controlled by fractures. A zonal distribution of alteration minerals is observed from the centre of fractures outwards with: (1) vuggy quartz; (2) quartz + alunite; (3) quartz + kaolinite ± alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; (4) illite + illite-smectite mixed-layer minerals ± kaolinite ± alunite, and e) chlorite + albite + epidote. Propylitic alteration is present distal to all other alteration types and consists of chlorite, epidote, albite, and carbonates. Mineral parageneses, mineral stability fields, and alteration mineral geothermometers indicate that the different alteration assemblages are the result of changes in both fluid composition and temperature of the system. The alteration minerals reflect cooling of the hydrothermal system from >400 °C (biotite), to 300–200 °C (chlorite and illite in veinlets) and to lower temperatures of kaolinite, illite-smectite mixed layers, and smectite crystallization. Hydrothermal alteration started with an extensive potassic zone in the central part of the system that passed laterally to the propylitic zone. It was followed by phyllic overprint of the early-altered rocks. Nearly barren advanced argillic alteration subsequently superimposed the upper levels of the porphyry copper alteration zones. The close spatial association between porphyry mineralization and advanced argillic alteration suggests that they are genetically part of the same magmatic-hydrothermal system that includes a porphyry intrusion at depth and an epithermal environment of the advanced argillic type near the surface.

Decolorization of Vegetable Oils: A Study of the Mechanism of Adsorption of β-Carotene by an Acid-Activated Bentonite from Cyprus

AbstractThe mechanism of decolorization of crude maize and sunflower oils was studied by means of adsorption of β-carotene by a low-grade bentonite, containing mixed-layered illite-smectite. Decolorization depends on temperature and the time required for equilibrium decreases with increasing temperature. The study of the kinetics of adsorption showed that decolorization of maize oil is a first-order process which occurs in two steps: a first fast step with higher activation energy (25.6 kJ mol−1), indicating the influence of a chemical interaction between the pigment and the clay surface, followed by a second slow step with low activation energy (12.3 kJ mol−1), characteristic of physical adsorption on the previously adsorbed molecules. Decolorization of sunflower oil is also a first-order process, described by a single mechanism with intermediate activation energy (19.0 kJ mol−1). Adsorption isotherms of decolorization of maize oil follow the Freundlich equation, indicating the existence of heterogeneous adsorption sites on the solid's surface. Heterogeneity is attributed both to different active centers on the smectite surface (Brönsted and Lewis centers) and to the different phases present in bentonite, such as illitic layers and clinoptilolite, which also have active centers on their surfaces.

Geothermometry, geochronology, and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza Island, Italy, was hydrothermally altered, producing four distinct alteration zones based on X-ray diffraction mineralogy and field textures: (1) nonpervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone, characterized by smectite and disordered opal-CT. The other three zones exhibit more complete alteration of the hyaloclastite. The propylitic zone is characterized by mixed-layer illite–smectite (I-S) with 10 to 85% I, mordenite, opal-C, and authigenic K-feldspar (akspar). The silicic zone is characterized by I-S with ≥90% I, pure illite, quartz, akspar, and occasional albite. The sericitic zone consists primarily of I-S with ≥66% I, pure illite, quartz, and minor akspar and pyrite. K/Ar dates of I-S indicate hydrothermal alteration occurred at 3.38 ± 0.08 Ma. Oxygen isotope compositions of I-S systematically decrease from zones 1 to 4. In the argillic zone, smectite has δ 18O values of 21.7 to 22.0‰ and I-S from the propylitic, silicic, and sericitic zones ranges from 14.5 to 16.3‰, 12.5 to 14.0‰, and 8.6 to 11.9‰, respectively. δ 18O values for quartz from the silicic and sericitic zones range from 12.6 to 15.9‰. By use of isotope fractionation equations and data from authigenic quartz-hosted primary fluid inclusions, alteration temperatures ranged from 50 to 65°C for the argillic zone, 85 to 125°C for the propylitic zone, 110 to 210°C for the silicic zone, and 145 to 225°C for the sericitic zone. Fluid inclusion data and calculated δ 18O water values indicate that hydrothermal fluids were seawater dominated. Mass-transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species, especially Si. Systematic gains in Mg occur in all alteration zones as a result of I-S clay mineral formation, and systematic losses of Na, Ca, and K occur in most zones. With the exception of Ca, calculations of mass transfer associated with hydrothermal alteration on Ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater. The anomalous Ca loss at Ponza may be due to hydrothermal formation of anhydrite and later low-temperature dissolution. On the basis of Mg enrichments derived from circulating seawater, we estimate the following minimum water/rock ratios: 9, 3, 6, and 9 for the argillic, propylitic, silicic, and sericitic zones, respectively. Hydrothermal fluid pH for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids.

Late Quaternary clay mineral record in Central Lake Baikal (Academician Ridge, Siberia)

We investigated the mineralogical composition of two cores recovered on the Academician Ridge (Central Lake Baikal, Siberia). Sedimentological features show that the cores are unaffected by turbidity currents. However, hemipelagic deposition is not continuous, but intermittently disturbed by syn- or post-sediment reworking (e.g., bioturbation, slumps, faulting). Such modes of deposition are consistent with the complex uplift history of the ridge. Bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial/interglacial stages, and diluted by diatom-rich intervals related to warmer interglacial stages. The core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve SPECMAP. The ∼8-m cores partly recover the last four interglacial/glacial cycles, i.e., since oxygen isotope stage 8. We test the use of clay minerals as a proxy for paleoclimatic reconstruction. The clays are more weathered during the diatom-rich intervals in agreement with warmer climate conditions. However, the mean clay composition does not change significantly through glacial/interglacial stages. This observation implies that, in the Academician Ridge sediments, a simple smectite/illite ratio (S/I) does not alone provide a reliable indicator of climatic variation. It reflects the complex clay assemblages, especially the smectite group, delivered to Central Lake Baikal. Smectites include primarily illite–smectite mixed layers, made of a mixture of montmorillonite and beidellite. According to their behavior after cation saturation, the illite–smectite mixed layers are primarily transformed smectites, with some neoformed smectites intermittently observed. In addition, Al-smectites occur in minor proportions. We conclude that the S/I ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites.

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks, Bowen Basin, Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite–smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic ( R≥3) illite–smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic ( R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu) sn ratios show negative correlations with the illite content in illite–smectite and positive correlations with the δ 18O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu) sn ratios and illite content in illite–smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/rock ratio conditions. Increasing HREE enrichment with δ 18O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin. Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins.

The chemical character of fluids forming diagenetic illite in the Southern Appalachian Basin

Diagenetic illite (mixed layer illite-smectite, I-S) from Ordovician and Devonian potassium bentonite (K-bentonite) and shales was studied in the proximal southern Appalachian Basin (Alabama through Virginia) to understand in more detail the timing of illite formation as well as attempt to describe the chemistry of the fluids that formed illite during the Alleghanian Orogeny through combined K/Rb and oxygen isotope analyses. The clay fraction of the K-bentonites is composed predominantly of illite-smectite (I-S) and chlorite. Most I-S exhibited Kalkberg order (IISI) with >85% illite layers. The K-Ar dates of I-S from Ordovician K-bentonites and Devonian K-bentonites mostly range from 260-310 Ma and indicate I-S formed during the Alleghanian Orogeny. Internally concordant dates and similar percentages of illite layers of I-S are measured from several samples collected within a thick (1 m) Ordovician K-bentonite at Fort Payne, Alabama.

The composition of the subsurface Neogene shales of the Surma group from the Sylhet Trough, Bengal Basin, Bangladesh

Nineteen shale samples from six wells penetrating the Neogene Surma Group in the Sylhet Trough in northeastern Bangladesh were analyzed by X-ray diffraction, X-ray fluorescence spectrometry and direct current plasma optical emission spectrometry. Mineralogically, the subsurface Neogene shales of the Surma Group comprise predominantly quartz, plagioclase, illite, chlorite, kaolinite, with lower amounts of K-feldspar, dolomite and smectite, and minor to trace amounts of calcite and siderite. Significant variation in the content of K-feldspar, smectite and illite–smectite mixed layer clays with depth has been observed. K-feldspar has not been noted below the depth of about 3200 m, and smectite has not been observed deeper than 3000 m. It is inferred that illite–chlorite, quartz and feldspar are predominantly detrital in origin, reflecting a fast-rising source terrain with no intense weathering activity. Geochemically, the mean major element composition of the shales is similar to that of the average shale with the exception of the CaO content, which is lower here. The low CaO content could be due to lower carbonate and total organic content. The samples are enriched in Sc and Cu and are depleted in Be, Sr and La. Other trace elements have values close to the average shale. The geochemical characteristics suggest that the Neogene shales of the Surma Group were derived from an active margin setting related to a recycled source.

Mineralogy and chemistry of bentonite (?) deposits at Minjingu, Lake Manyara, North Tanzania

Olive green clays likely to be bentonitic in composition have been mineralogically and chemically studied. They occur in association with other lacustrine sediments at Lake Manyara. Radiocarbon dates from four diatom horizons indicate ages ranging from 12 Ka to 135 Ka suggesting a Mid-Holocene age. Middle Pleistocene age have been assigned to the ridged oncolites of Lake Manyara. The olive green coloured clays in the Manyara basin are known to occur in association with other lake beds including phosphorite deposits, stromatolites, bioturbated silty clays, partly silicified marls, conglomerates and olive green coloured opal beds. The results presented herein are from the olive green coloured clays. The olive green clays (bentonite?) are a result of devitrification or alteration of volcanic ashes and/or pyroclasts. The green clays occur in different forms as they are separated from each other by other lacustrine sediments. The alteration might have taken place in slightly different environments in terms of salinity and alkalinity. One of the top layer is friable and shows conchoidal fractures when dry. The other beds below in the lacustrine sequence are cemented with calcite and some dolomite as well as zeolites. The lowermost layer in the sequence is friable and shows cracks filled with coarse crystalline calcite. Mineralogically the bentonite is composed of the clay minerals illite, illite-smectite mixed layer clays, and chlorite. Other authigenic minerals include various zeolites (analcime, clinoptilolite, erionite and some traces of mordenite), opal, and fluorapatite. The clays have magnesium contents varying from 3.01% to 7.43%. The calcium contents vary widely due to presence or absence of one of the two minerals calcite or apatite. Trace elements like Ba, Ce, Sr, Zr are equally attributed to the presence of calcite and apatite. The formation of the illite-smectite mixed layer clays in an alternating manner with other lake sediments depicts different episodes of volcanic eruptions in the area. The mineralogical composition of smectites, zeolites, and opal in the green clays suggests a deposition of pyroclasts and volcanic ashes in a closed lake system with fluctuating levels. Due to evaporation alkalinity and salinity levels were fluctuating. The clays might have been bentonite which have undergone illitisation, a phenomena noted in other neighbouring rift basins.

Clay mineralogy of the Late Cretaceous and early Tertiary successions of the Cauvery Basin (southeastern India): implications for sediment source and palaeoclimates at the K/T boundary

Clay mineral analyses were carried out on late Maastrichtian–early Palaeocene (Cretaceous–Tertiary) sedimentary rocks in the Ariyalur area of the Cauvery Basin (southeastern India). These investigations were aimed at determining the sedimentary provenance and palaeoclimate of the area in the latest Cretaceous and earliest Tertiary. The late Cretaceous deposits show significant palaeogeographic variation in clay mineral contents across the study area. Mixed-layer illite-smectites dominate the clay mineral content of the uppermost Maastrichtian of all areas, with elevated smectite, illite, palygorskite, sepiolite, kaolinite and chlorite occurring at intervals through the succession. This combination and widespread variation in the clay mineral suite suggests a lithologically highly variable source terrain and lack of sediment mixing during transport. Mixed-layer clays occur with both kaolinite and palygorskite, interpreted in terms of a seasonal (warm and wet, warm and dry) source area. The uppermost Maastrichtian of the Niniyur section displays an upward increase in illite content, suggesting that the adjacent continental area became strongly influenced by physical weathering just prior to the K/T boundary. The return of mixed-layer clays, smectite, illite and kaolinite during the early Tertiary suggests that chemical weathering in a seasonal climate once again controlled the formation of clay minerals. These results imply that the volcanism associated with the Deccan Traps had no sudden effect on the Cretaceous or Tertiary sedimentation and clay mineralogy of this area.

Clay diagenesis and low-grade metamorphism of Tithonian and Berriasian sediments in the Cameros Basin (Spain)

AbstractThe clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The lowest-grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 65 85% illite layers) + discrete illite (IC = 0.5 0.65Δ°2θ). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic grade from the northwest part of the basin to the southeast. This trend does not follow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbión and Enciso Groups), which exhibit a higher metamorphic grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.

Electron Microscopy Study of Volcanic Tuff Alteration to Illite-Smectite Under Hydrothermal Conditions

AbstractExperimental alteration of volcanic tuff from Almeria, southeastern Spain, was performed in solutions with different Na/K ratios (0.01, 1, 10, and 100), different total salt concentrations (0.01, 0.1, 0.2, 0.33, and 1 M), and in deionized water, at 60, 80, 120, and 160°C, for periods of 60, 90, 180, and 360 d. Two particle size fractions of volcanic tuff were used: 10–200 and 20–60 μm. Alteration products were examined by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), laser-particle size analysis, scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM-EDS), image computer analysis, and transmission electron microscopy with microanalysis (TEM-AEM). XRD detected neoformed phases only in the products from experiments of 180–360 d at high temperatures (120–160°C), and with Na/K ratios above unity and in deionized water. The synthesized phase is a random mixed-layer illite-smectite (I-S) with 75% smectite. The quantity of newly formed I-S, determined by FTIR, ranged between 3–30%. There was no apparent change in grain size and shape of the grains after the experiments as compared to before.SEM-EDS and TEM-AEM revealed the following alteration sequence: 1) intense etching on glass-grain surfaces; 2) formation of hemispherical morphologies on grain surfaces; 3) precipitation of very thin, individual flakes of illite-smectite on glass-grain surfaces; 4) development of I-S at the edges of glass grains; and 5) development of I-S honeycomb structures either covering large areas of the glass grains or resulting from the complete alteration of glass grains. A direct transformation of glass to I-S seems to be the major reaction mechanism, although there also is evidence of glass dissolution and subsequent I-S precipitation.