Mixed-layer Phases Research Articles

Features of the mineralogical composition of solod and solod meadow-chestnut soil, the case of soils of the liman depression at Dzhanybek research station of the Institute of Forest Science RAS

The structure of the soil cover at the territory of the clayey semidesert of the Northern Caspian Sea is represented by triparted soil complex and peculiar soils like solod. Solods are formed in relief elements that are constant for this territory – mesodepressions, which are referred to limans. Two soil profiles were studied in the bottom of the liman – solod, and on its peripheral part – solodized meadow-chestnut soil. The formation of the solod profile occurs because of periodic moistening with fresh water and systematic flooding during the snowmelt period. The second soil is in the zone of transit of surface water to the liman. The soil profiles have the same type of eluvialilluvial distribution of the clay fraction, while differentiation is more distinct in solod. Mineralogical analysis established the same type of qualitative composition of clay and clastogenic minerals in both soils. Clay minerals are represented by illite, smectite, mixed-layer illite-smectite, accompanied by kaolinite and chlorite. Clastogenic minerals are represented by quartz, mica, K-feldspar, plagioclase, and chlorite. Quartz and feldspars are accumulated in the solodized horizons of the soil profiles; the content of quartz and feldspars decreases in the textural horizons, while the content of mica and chlorite increases. The distribution of the smectite phase in the solod profile reveals a difference in the ratio of illite-smectite packets, structures with a low content of smectite packets predominate in the solod part of the profile, and the proportion with a swelling packet in mixed-layer minerals increases in textural horizons. The solodized meadow-chestnut soil is distinguished by a smaller thickness of the solodized horizon, a higher content of the smectite component, and a calm character of the profile differentiation of clay and clastogenic minerals. In both soils, labile minerals with signs of superdispersion are recorded in the solodized and textural horizons. The superdispersion of labile minerals has different causes. In solod, it is associated with the action of low-mineralized water and the hydrolysis of soil minerals during the wet seasons of the year, as well as with the intense regime of intrasoil weathering in situ, and in the solodized meadow-chestnut soil, with the passage of the stage of solonetz soils. Differences in the smectite component were also revealed: in the solodized horizons of solod, it is represented by individual smectite and the mixed-layer phase of illitesmectite; and in the meadow-chestnut soil – by mixed-layer illite-smectite.

Strong magnetic response regulated by layer-stacking perturbations in an Aurivillius-phase oxide

An excellent platform to investigate the role of layer-perovskite stacking in tuning magnetic properties was constructed in Aurivillius-type Bi11Fe2.9Co0.1Ti6O33 by A-site non-magnetic Sm-doping. Nanoscale layer-stacking evolution was clearly observed from the close-order intergrowth of the 4- and 5-layer slabs to the low-average-n mixed-layer phase, as increasing the doping-level. Such layer-stacking perturbations induced a coexistence between disorder mixed-layers and local order-layers, giving rise to strong ferromagnetic-antiferromagnetic interactions at lower temperatures reflecting in larger magnetic coercive fields (2Ec ∼ 0.8 T) and higher remanent magnetization. Based on X-ray absorption spectra, the anomalous magnetic responses at 50–150 K may be ascribed to simultaneous strengthening of iron 3d-state vacancies and metal–oxygen orbital hybridization, generating the large local ferromagnetic-state into intrinsic antiferromagnetic-state, thus achieving a strong coupling.

Crystal chemistry of globular layered silicates of the Troitsko-Bainovskoye fre-clay deposit (Middle Urals)

Research subject. The composition and crystal chemistry of glauconite from the open deposits (K2cn-cp) of the Poldnevskaya area of the Troitsko-Bainovskoye deposit of fire clays.Materials and methods. An analysis of the mineralogical and crystallochemical properties of glauconite was carried out using a set of modern analytical methods (“Geonauka” Centre for Collective Use, IG FRC Komi SC UB RAS): X-ray diffractometry, IR-spectroscopy, scanning electron microscopy, modeling of diffraction profiles.Results and conclusions. The crystal-chemical features of glauconite were determined. Two main glauconite varieties (green and light green) in the rocks of the Zaikovskaya suite were distinguished and their differences and degree of maturity have been established. It was found that dark green globules are close to glauconite and represent a more “mature” variety, while the light green ones are a disordered mixed-layer mineral of the mica (glauconite) – smectite series and are a product of incomplete replacement of the original smectite by glauconite. Phase heterogeneity of glauconite globules was revealed. It has been established that the rocks of the Troitsko-Bainovskoe deposit are exposed to intensive transformation under the influence of acidic surface and technogenic waters, which result in the destruction of the initial minerals and the appearance of secondary mineralization. Clay minerals of the glauconite-bearing rocks are represented by disordered mixed-layer phases of predominantly illite/smectite type.

Soil formation and mineralogical changes on basaltic lava vs scoria along a hydroclimatic gradient on Santa Cruz Island, Galápagos

Climate and parent material type are important soil forming factors. On Santa Cruz Island, Galápagos, the climate shows a pronounced altitudinal zonation from arid lowlands to humid highlands, and the volcanic parent materials, although chemically rather uniform (basaltic), show pronounced differences in porosity, including dense lava flows and very vesicular scoria deposits. In this study, the effect of these two driving factors on pedogenesis was investigated. Our results show that pH and effective cation exchange capacity (CECeff) decreased, but potential cation exchange capacity (CECpot), phosphate retention, and Alo + ½Feo as well as Alp/Alo increased with increasing elevation regardless of parent material type. Conversely, topsoil bulk density (BD) and organic carbon (OC) content did not show a clear altitudinal pattern on lava but decreased (BD) or increased (OC) with increasing elevation on scoria. Clay mineralogy changed from minor (2:1-type) mixed layer phases in the low-elevation arid zone to vermiculitic (lava) or kaolinitic mineralogy (scoria) at mid-elevation (humid), whereas short-range-order (SRO) minerals (allophane and ferrihydrite) dominated in the colloidal fraction of both lava- and scoria-derived soils under very humid conditions at high elevation. The sequence of soil types developed on lava was, with increasing elevation, Cambisol → Phaeozem → Andosol, while on scoria it was Regosol → Umbrisol → Andosol. Our results show that climate is the overriding factor controlling the major mineralogical and pedogenic changes in the studied environment; however, the higher porosity of the scoria parent materials considerably accelerated weathering and pedogenic processes compared to the denser lavas. Our study highlights the importance of the parent material’s physical nature in modulating rates of chemical weathering and pedogenesis.

Chemistry-dependent Raman spectral features of glauconite and nontronite: Implications for mineral identification and provenance analysis

Abstract This study provides a comprehensive Raman spectral characterization of nontronite and glauconitenontronite mixed-layer phases from seafloor hydrothermal fields. These 2:1 phyllosilicates, which show isomorphous cation exchange between Mg2++Fe2+ and Fe3++Al3+ in the dioctahedral sheets, exhibit three diagnostic Raman peaks in the low wavenumber region (v1 ~241–257 cm−1; v2 ~600–606 cm−1; v3 ~690 cm−1), and one peak at ~3548–3570 cm−1 (v4). With increasing (Mg2++Fe2+)oct, the presumed stretching band of octahedral OH-O bonds (v1) is displaced to a higher wavenumber, whereas the stretching band of tetrahedral Si-O-Si bonds (v2) is shifted to a lower wavenumber. Peak v4, which relates to O-H bonds of hydroxyls linked to octahedral cations, shows a downshift with increasing (Mg2++Fe2+)oct. The band v4 can be mathematically fitted by three bands, two of which strongly correlate with the cation occupancy in the octahedral sheets; i.e., vibrations of hydroxyls linked to triva-lent cations (Fe3+ and Al3+) are mainly represented by a band at ~3560–3573 cm−1, whereas divalent cations (Mg2+ and Fe2+) mainly contribute to a band at ~3538–3540 cm−1. This result is consistent with theoretical considerations for dioctahedral phyllosilicates, which predict for the incorporation of Mg2+ and Fe2+ a weakening/lengthening of O-H bonds in the OH groups, accounting for a downshift of the O-H vibrations. Hence, this is one of the first studies that trace how even subtle chemical modifications in phyllosilicates influence Raman spectral features. The reported findings have implications for mineral identification and provenance analysis, such as during surface exploration on Mars, where compositionally diverse phyllosilicates occur.

Rock alteration at the post-Variscan nonconformity: implications for Carboniferous–Permian surface weathering versus burial diagenesis and paleoclimate evaluation

Abstract. A nonconformity refers to a hiatal surface located between metamorphic or igneous rocks and overlying sedimentary or volcanic rocks. These surfaces are key features with respect to understanding the relations among climate, lithosphere and tectonic movements during ancient times. In this study, the petrological, mineralogical and geochemical characteristics of Variscan basement rock as well as its overlying Permian volcano-sedimentary succession from a drill core in the Sprendlinger Horst, Germany, are analyzed by means of polarization microscopy, and environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and inductively coupled plasma mass spectrometry analyses. In the gabbroic diorite of the basement, the intensity of micro- and macro-fractures increases towards the top, indicating an intense physical weathering. The overlying Permian volcanic rock is a basaltic andesite that shows less intense physical weathering compared with the gabbroic diorite. In both segments, secondary minerals are dominated by illite and a mixed-layer phase of illite and smectite (I–S). The corrected chemical index of alteration (CIA) and the plagioclase index of alteration (PIA) indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ values for both basaltic andesite and gabbroic diorite indicate an abnormal enrichment of K, Rb and Cs that cannot be observed in the overlying Permian sedimentary rocks. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. The overall order of element depletion in both basaltic andesite and gabbroic diorite during the weathering process is as follows: large-ion lithophile elements (LILEs) > rare earth elements (REEs) > high-field-strength elements (HFSEs). Concerning the REEs, heavy rare earth elements (HREEs) are less depleted than light rare earth elements (LREEs). Our study shows that features of supergene physical and chemical paleo-weathering are well conserved at the post-Variscan nonconformity despite hypogene alteration. Both can be distinguished by characteristic minerals and geochemical indices. Based on these results, a new workflow to eliminate distractions for paleoclimate evaluation and evolution is developed.

Diagenesis of the Malmian Mikulov Formation source rock, Vienna Basin: Focus on matrix and pores

Diagenetic processes and pore development in the matrix of the 1000 m thick main source rock for oil and gas in the Vienna Basin, the autochthonous Malmian mudstones of the Mikulov Formation have been studied. Core samples from wells over a true vertical depth range of 1400 m–8551 m were available. The bulk samples contain quartz, minor amounts of plagioclase, pyrite and a large but variable proportion of calcite; the clay mineral content ranges from 14 to 47%. The clay fraction contains a prominent illite-smectite (I–S) mixed-layer phase, illite, chlorite and kaolinite. The quantities of I–S and kaolinite decrease with depth, whereas illite and chlorite increase with depth. Diagenesis has involved a gradual transformation of smectite to illite through mixed-layer I–S intermediates. The ordering of the mixed layer I–S changes with increasing depth from R0 to R1 and R3. The R1 transformation of the mixed-layer I–S occurs at approximately 3000 m and vitrinite reflectance values of 0.4%–0.6%. Based on petrographic evidence, the cations resulting from the illitization of smectite were the source of a variety of late diagenetic mineral cements, such as Fe and Mg for chlorite formation and for ferroan dolomite precipitation. Illitization also provided Si for local quartz cementation. During diagenesis nanometer to micrometer size pores developed because of specific mineral frameworks and dissolution processes. Organic matter pores developed in deeper, thermally mature samples. Phyllosilicate framework pores between brittle grains are commonly observed. Pores caused by partial dissolution of carbonate grains also occur. In places diagenetic cements, such as quartz overgrowths or carbonate cements keep pores propped open. The connectivity of the pores cannot be established unequivocally from SEM photomicrographs, but they likely contribute to the creation and preservation of effective porosity and gas storage capacity of these rocks.

Особенности коры выветривания кимберлитов северо-востока Анголы

Bведение: Приведены результаты комплексного изучения геологического строения и вещественного состава коры выветривания кимберлитов северо-востока Анголы. Важной задачей прикладного характера является оценка степени обогащения коры выветривания глубинными минералами для вопросов прогнозирования и поисков новых месторождений алмазов как коренного, так и рассыпного типов. Методика: В качестве эталонного объекта выбрана кимберлитовая трубка Катока, характеризующаяся не только незначительным эрозионным срезом и сохранением кратерной постройки, а и широким петрографическим спектром пород и их интенсивным постмагматическим и гипергенным изменением. Для изучения геологического строения и вещественного состава кор выветривания слабо эродированных кимберлитовых трубок Анголы была собрана коллекция штуфных образцов и протолочных проб из верхних горизонтов трубки Катока и ряда других тел этого поля, а также вмещающих кимберлиты пород. Результаты и обсуждение: В кимберлитовой трубке Катока выделяется два генетических типа пород: собственно кимберлиты с их производными и комплекс вулканогенно-осадочных образований, заполняющих верхние части кратера диатремы. В целом, профиль коры выветривания разделён на две части: нижнюю и верхнюю. Нижняя часть (зона дезинтеграции) представлена слабо изменёнными автолитовыми кимберлитовыми брекчиями, трещиноватыми и частично обохрёнными. Верхняя, наиболее гипергенно изменённая зона, в свою очередь, разделена на карбонатную и бескарбонатную. Заключение: Отмечено два процесса изменения кимберлитовых пород региона. Первый – поэтапное превращение оливина в высокотемпературный, затем и низкотемпературный серпентин. Второй –деградация серпентина с одновременным возникновением серии разбухающих минералов, начиная с сапонита через ряд промежуточно метастабильных смешанослойных образований, из которых впервые идентифицированная упорядоченная разновидность лизардит-сапонита является относительно наиболее устойчивой по сравнению с другими упорядоченными смешанослойными фазами триоктаэдрического типа, что подчёркивает специфические условия формирования коры выветривания кимберлитов африканского региона.

Mineralogical comparative analysis of the clay fraction of solonetz and chestnut soil without apparent microrelief in the north-western part of the Caspian lowland

A comparative analysis of the mineralogical composition of the clay fraction (< 1 μ m) of chestnut soil and solonetz has been performed in the article. Soils were studied in the oldest part of the Caspian lowland in north-west where the microrelief is not quite apparent (microdepressions no more than 5–10 cm depth), but with a contrast soil cover. The content of silt fraction in chestnut soil varies from 25.6 to 33.9%, in the solonetz – from 11.7 to 51.6%. In all horizons of the compared soils, except for the upper surface horizons (SEL, AU1), mixed-layered minerals dominate (39–52% in the fraction) over illite (27–37% in the fraction). In the surface horizons illite (48%) dominates over mixed-layer minerals (35%). The compared soils have some similarities in their crystal-chemical state: the imperfection of the kaolinite structure and the superdispersed state of the mixed-layer phase in the upper horizons, as well as the appearance of individual smectite and chlorite packets in the mixed-layer phase in the lower horizons (BC, C). The degree of perfection of the kaolinite structure both in the solonetz and in the meadow-chestnut soil increases towards the bottom of the profiles. The obtained data allow suggesting that in the meadow-chestnut soil the clay phase retained traces of the solonetzic stage of soil formation.

A multidisciplinary approach to investigate weathering processes affecting gneissic rocks (Calabria, southern Italy)

The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.

Micron-scale distribution of metals in Cambrian metalliferous shales, South China: Insights into local biologically driven redox disequilibrium

Early Cambrian black shales of the Niutitang Formation are found across the Yangtze Platform in a 1600-km belt extending across south China. A thin organic matter-rich layer in the lowermost part of this formation contains exceptional concentrations in Mo, Ni, Se, Re, Os, As, Hg, Sb, Ag, Au, Pt, Pd, and Ag. Due to their extreme metal enrichment, these black shales provide a rare opportunity to study the interface between semi-metals, metals and biogenic material. We report the first detailed μm-scale investigation of metal distributions and associations in samples from two sites, Zunyi and Sancha, located hundreds of km apart, using a combination of analytical techniques including X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM), Synchrotron-based XRF mapping, electron probe micro-analyser (EPMA), particle induced x-ray emission (PIXE) probe and x-ray absorption near-edge structures (XANES) imaging. Strong μm-scale variations in metal and semi-metal distributions were highlighted by Synchrotron-based XRF mapping of samples from both sites. At both locations, U is present within phosphorite nodules. Arsenic, Mo and Se are particularly abundant in the organic-rich matrix, showing strong associations with organic matter. Nickel, however, shows different distributions between the two sites. It is mainly present in association with the organic matter at the Sancha site while it is found in abundance in millerite at the Zunyi site, suggesting slightly different local conditions at Zunyi favouring Ni-S associations over Ni-organic matter. At both sites, biogenic structures were re-mineralised with laminations dominated by different metals, indicating a likely control of organic matter over metal distributions. In addition, the XANES imaging highlighted different redox states of As over μm-scale areas. While As−1 appears to be mainly present in pyrite, As+3 was mainly detected in association within the carbon and MoS2 mixed layer phase (MoSC). Overall, the present results emphasize the complexity of metal associations in this mineralised layer, the significant role of organic matter in the accumulation and precipitation of metals and semi-metals in these metalliferous shales and highlight how biogenic activity can induce μm-scale variations in redox conditions in sediments.

Phase transitions in mixed-layer Aurivillius phases

The Aurivillius phases Bi7Ti4NbO21, CaBi8Ti7O27, and SrBi8Ti7O27 were synthesized by solid-phase reactions. The results of high-temperature X-ray diffraction and thermal analyses of certain Aurivillius phases are presented. Principal regularities of thermal expansion of the synthesized compounds are established. A correlation between the Curie temperature and the structure of the compounds was found.

Clay mineral diagenesis in Cretaceous clastic reservoirs from West African passive margins (the South Gabon Basin) and its impact on regional geology and basin evolution history

The Gabon coastal region located along the western African margin hosts several sedimentary basins that developed prior, during and after the opening of the South Atlantic Ocean. A range of diagenetic processes controlled the distribution of clay paragenesis shown to be of high importance for the understanding of the basin's burial history and geotectonic development. Materials acquired for this study stem from the siliciclastic fluvio-lacustrine-deltaic, petroleum bearing, Early Cretaceous Dentale and Gamba formations cored by two respective wells at depths of ~1500m. Sampled materials were analysed by X-ray diffraction, automated electron microscopy, and inductively coupled plasma mass spectrometry in order to reconstruct eogenetic and mesogenetic variations directing the formation of clay assemblages in the basin. The clay contents in both cores consist of authigenic mixed-layer minerals like illite-smectite, chlorite-smectite and berthierine-chlorite, and some minor detrital illite/mica and chlorite. I-Sm and C-Sm phase chemistry implied that the original dioctahedral (montmorillonite to beidellite) and trioctahedral (saponite) smectite precursors formed out of acid volcanic feedstock during eogenesis. Different magmatic fractionation degrees, from rhyodacite to trachyandesite, reflected in the uniform REE curves of volcanic glass conform to an active geotectonic development of the Cretaceous margins of Africa. Mesogenesis led to the decrease of smectite and formation of mixed-layered phases; I-Sm composition showed maximal burial depths of sediments to be ~1000m and ~500m deeper than today for Gamba and Dentale sediments, respectively. Besides, temperature, a major role in the formation of mixed-layer minerals had the porosity of sediments and geochemistry of smectitic precursors. Thus, Fe-Mg smectite showed higher thermal stability and lower rates of transformations in non-expanding clays during burial compared to Al-rich smectite. Different burial histories of sediments from the two cores can be attributed to rift-related normal faulting and subsequent differential denudation.

Weathering of the Ethiopian volcanic province: A new weathering index to characterize and compare soils

Soil formation occurs through numerous physical and chemical weathering processes acting to alter the parent rock on the Earth’s surface. Samples of surface soils were collected over a range of elevations (2000–3600 m) from profiles directly overlying basaltic to more felsic parent rocks, over a region in NW Ethiopia. The soils were investigated to determine their chemical composition and X-ray diffraction was used to identify and quantify individual mineral phases. The data set was analyzed using non-parametric statistics (Spearman’s Rank and Mann-Whitney U tests) to compare the soils forming over the two parent rocks. Principal component analysis (PCA) was used to identify the mineral alteration assemblage and formation during pedogenesis. The extent of alteration was quantified using several chemical weathering indices (Chemical Index of Alteration = CIA; Chemical Index of Weathering = CIW), including an index calculated by multivariate analyses of the soil chemical composition data (weathering “ W ” index). Further to this we devised and tested a new weathering index ( W min) using multivariate analysis of the soil mineralogy, to estimate the extent of weathering and physico-chemical proprieties of the parent rock from which the soil formed. The soils present a fair to advanced stage of alteration, with abundant iron (Fe) oxides (up to 40 wt%) and phyllosilicates (up to 57 wt%), including kaolinite-smectite (K-S) mixed-layer phases. The K-S was composed of either 30–50% kaolinite or 94–98% kaolinite layers. Discrete kaolinite was also present. The bimodal K-S mineralogical composition is likely due to two precursor phases: feldspar for the kaolinite-rich K-S and volcanic glass for the smectite-rich K-S. K-S with intermediate composition (50–94% kaolinite) was rare, due to its instability. Statistical analysis showed significant differences between the chemical compositions of the soils developed on the two different parent volcanic compositions. The soils overlying the more felsic parent rocks were less altered than those overlying the flood basalt. When comparing the weathering indices calculated in this study, we conclude that while the CIA and CIW may be more readily determined, the W and W min indices can elucidate information on the composition of the original rock from which they formed. The W index is more sensitive to certain variables when compared with the newly derived mineralogical W min index; however the W min index takes into account mineral phases within the sample, which provides a more detailed interpretation of weathering rates than chemistry alone. In addition the W min index correlated with meteorological variables, such as elevation (and consequently temperature and precipitation), known to influence the degree of pedogenesis. The W min index can be used to enhance our understanding of the processes that occur during weathering processes to supplement information gained from traditional chemical weathering indices.

Changes in the structures of mixed-layer illite-smectite during flooding of terrigenous oil reservoirs

A difference-spectrum method is proposed for the qualitative assessment of changes of illite-smectite structures accompanying the flooding of oil reservoirs. The method permits one to get an open system and reduce the application of procedures based on Markov’s chain formalism. A computer simulation is made to obtain spectra by subtracting the spectrum of an ethylene glycol-saturated sample from the spectrum of an air-dried preparation throughout the entire range of concentrations of illite and smectite components with a short-range order factor R = 0 or R = 3.It has been established that only in the presence of filtration are the maximum and minimum of the spectra in the range of 12.5-9.4 A complicated by a number of local extrema, whose position is specified by the structure of intermediate phases. The flooding process first involves mixed-layer phases with R = 0, leading to a partial segregation of the structures into phase with one and two networks of interlayer H2O. When the secondary mica particles break, phases with R = 3 appear along the boundaries of nanoblocks, first only with 1 H2O and then only with 2 H2O in labile interspaces. Their coexistence with the phases R = 0 in the sample proves the existence of percolation effects due to two-phase filtration in the porous medium. The fully flooded reservoir is always dominated by a mechanical mixture of illite-smectite phases of different nature with R = 0 and with different ratios of components. Transformation of mica that can drastically reduce oil production begins long before the appearance of flooding zones, which are revealed by standard logging methods.

Transformation of Kaolinite into Smectite and Iron-Illite in Brazilian Mangrove Soils

This study investigated soil clay mineralogy of mangrove forests along the Brazilian coast in eight regions of different environmental characteristics, with a focus on the crystallochemical features and genesis of 2:1 phyllosilicates. Samples from two different depths (0–30 and 60–90 cm) and two clay size fractions (<2 and <0.2 μm) were studied. The analytical tools used were X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy–energy dispersive spectroscopy (TEM-EDS). A complex assemblage was found in both fractions, including high proportions of kaolinite, illite (Fe-illite/glauconite), and smectite (Fe-beidellite, Fe-montmorillonite), minor amounts of gibbsite and quartz, and traces of K-feldspar, halloysite, and amphibole crystals (TEM-EDS data). Evidence of illite–smectite and possibly kaolinite–smectite mixed-layer phases was found in the <0.2-μm fraction. An authigenic transformation process likely taking place, from kaolinite to Fe-illite/glauconite, through transitory kaolinite–smectite and illite–smectite phases. The reaction is triggered by high Fe activity in solution generated by dissolution of Fe oxides and pyrite present in the sediment, at approximately neutral pH and high salt content in the water. Smectite illitization takes place by substitution of Al3+ for Si4+ in the tetrahedral sheet, of Fe and Mg2+ for Al3+ in the octahedral sheet, and progressive Fe reduction, all of which produces an increase in the layer charge and K uptake. The sequential nature of the transformation (kaolinite–smectite–illite) suggests a solid-state transformation process. Two unusual kaolinite-rich mangroves were found, indicating low reactivity or high deposition rate of continental sediments of soils derived from the Barreiras Group, dominated by kaolinite.

Crystal-chemistry of interstratified Mg/Fe-clay minerals from seafloor hydrothermal sites

Seafloor hydrothermal sites generate abundant Mg- and Fe-rich clays. These clays are structurally and compositionally interesting because these environments are characterized by large, dynamic temperature and chemical gradients in their deposition environment, which promote the formation of chemically and structurally complex clays, including interstratified phases. The system is also interesting as a proxy for the study of the large Mg- and Fe-rich phyllosilicate deposits on Mars, which are broadly characterized as smectitic clay of hydrothermal, volcanic or sedimentary origin. Thirty submarine samples and four terrestrial ones, for comparison, were studied by means of X-ray diffraction (XRD), thermogravimetry (TG), mid-IR and Mössbauer spectroscopies and chemical analysis. The samples include nontronite and the mixed-layer phases glauconite–nontronite, talc–nontronite and talc–saponite. Some of the talc–saponite samples have Fe contents well above those typical for these Mg-rich, trioctahedral phases (up to 1.69 Fe per O10[OH]2, in the tetrahedral and octahedral sheets). Tetrahedral Fe ranges from 0 to 0.66atoms per O10[OH]2 across the samples. As found in previous studies of similar specimens, Fe promotes the retention of molecular water that is released upon heating above 200C, and is mainly emplaced in non-expandable layers (talc and glauconite layers). In talc–nontronite and talc–saponite octahedral Fe (both di- and trivalent) appears to be bound to this trapped molecular water, whereas in glauconite–nontronite the bond appears to be with tetrahedral Fe. Samples typically show more than one dehydroxylation event in the TG analysis. The weight loss at each dehydroxylation event is broadly consistent with the proportion of individual layers as determined by means of XRD, but there is no good correlation between both. By contrast, the weight loss at each dehydroxylation event correlates with the chemistry of the layers, where certain cations promote chemical domains in the octahedral sheet (e.g., trioctahedral, nontronite-like, and montmorillonite-like) that dehydroxylate at the several temperatures. The correlations found for talc–nontronite and glauconite–nontronite samples suggest that the distribution of cations in the octahedral sheets of most, but not all, samples is intermediate between total dispersion and total segregation, perhaps random. The talc–nontronite samples have talc layers with cation-deficient octahedral sheets. The above results are best explained by layers of polar character, where the octahedral sheets are composed of domains with different chemical characteristics corresponding to the two types of layers in the interstratified clay. The existence of molecular water (bands at 1635 and 3300–3500cm−1, or ~6.1 and 2.9–3.1μm, respectively) bound in non-expanding layers is relevant to the mineral characterization of Martian Fe-rich clays with spectroscopic methods, where molecular water is so far assumed to be linked to smectite. The variety of mixed-layer clays in which Fe is found in the submarine samples suggests that mixed-layering may be important in Martian clays, which are spectroscopically variable but generally Fe-rich.

Diagenesis and very low grade metamorphism in a 7,012 m-deep well Hongcan 1, eastern Tibetan plateau

This study uses clay mineral assemblages, illite “crystallinity” (IC), chlorite “crystallinity” (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7 km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8 m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500 m, and illite/smectite mixed-layer phases extending to a depth of 3,000 m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000 m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2θ (upper and lower boundaries, respectively) corresponding to Kübler’s IC limits at 0.25 and 0.42°2θ. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60 % 2M 1 abundance compared to the 1M type at the surface, to 100 % 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite–dolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118–348 °C is estimated for the well with a pressure facies of low to intermediate type.

Phase transition and electrical properties of gallium- and indium-doped Bi10Ti3W3O30

Polycrystalline samples of gallium- and indium-doped Bi10Ti3W3O30 (mixed-layer Aurivillius phase with the Ti4+ and W6+ distributed at random over the perovskite-like slabs) have been prepared by solid-state reactions, and their polymorphism and electrical properties have been studied. Doping with both In3+ and Ga3+ yields limited solid solutions and shifts the ferroelectric phase transition to lower temperatures. The heterovalent substitutions of In3+ and Ga3+ for Ti4+ and W6+ increase the oxygen vacancy concentration and, accordingly, the conductivity of the material relative to the undoped compound.

Interstratified vermiculite–mica in the gneiss–metapelite–serpentinite rocks at Hafafit area, Southern Eastern Desert, Egypt: From metasomatism to weathering

The Hafafit vermiculite in the Southern Eastern Desert of Egypt at the contact of the metapelite and serpentinite rocks with the pegmatites and gneisses of the Hafafit uplift is the only known deposit in the Arabian–Nubian Shield (ANS) rocks of the Eastern Desert (ED). It is distinctively interstratified with mica. The mineralogy and mineral chemistry of this vermiculite at four sites (HV1, HV2, HV3 and HV4) were studied to better understand its origin, which might refers to a specific geologic setting retained to Hafafit area. The vermiculite at Hafafit forms with phlogopite, actinolite–tremolite, asbestos–anthophyllite–talc and talc zones that are arranged from pegmatite and gneisses to the metapelite and serpentinite rocks. These zones were probably formed by metasomatism that related to the intrusion of the granitoid rocks and the connected pegmatites in the upper Pan-African. The XRD and EMPA studies of the interstratified vermiculite-mica concluded that vermiculitization took place through a layer-by-layer transformation of original micas. This formed, in decreasing abundance, mixed-layer phases of biotite/vermiculite (hydrobiotite), phlogopite/vermiculite (hydrophlogopite) and chlorite/vermiculite (corrensite) and discrete phases of vermiculite, chlorite and smectite. A model is suggested, in which chemical weathering by the moving downward meteoric water led to replacement of the interlayer K, in biotite from gneiss and in phlogopite from metasomatic zones, by H 2O molecules, Fe 2+ was oxidized and (OH) − replaced O 2− forming hydrobiotite and hydrophlogopite. By more K remove, Fe was replaced by Mg with the introduction of more layers of H 2O molecules leading to formation of the vermiculite. Weathering formed corrensite mixed-layer and chlorite expandable minerals on the expense of chlorite. Formation of the incomplete smectite-like layers and Al-hydroxy interlayers (13.97 Ǻ) took place at the expense of vermiculite, replacing the Mg interlayer cations (12.63 Ǻ). Weathering took place mostly by low-pH solutions and in warm environment and the most extensive degree of weathering was at the HV4 site, in which the lode of vermiculite is the biggest. We propose that vermiculitization at Hafafit occurred due to a specific integration between the geologic setting (including rock type and tectonics) of the area and weathering processes producing the only vermiculite deposit in the ANS rocks of the ED of Egypt.