Proportions Of Illite Research Articles

Origin and distribution of clay minerals in semi-arid Sahelian soils: case of Kori Ouallam watershed, south-western Niger

AbstractThis study examines the origin and distribution of clay minerals of the pedological horizons of Kori Ouallam watershed (south-western Niger). It is based on field sampling campaigns and a series of laboratory analyses. A total of 49 samples were analysed, 28 from surface horizons (0–10 cm depth) and 21 from pedological profiles (0–1 m depth). The samples were analysed by X-ray diffraction on bulk and clay (<2 μm) fractions, X-ray fluorescence spectrometry, laser granulometry, organic matter and calcium carbonate content, macroscopic observations (binocular loupe) and scanning electron microscopy equipped with an energy-dispersive spectrometry system. The pedological horizons are characterized by low organic matter contents (<1%) and no calcium carbonate. The particle-size distribution shows net textural differentiation, with a predominance of sandy loam to sandy clay loam textures in the upper horizons and clay loam to clay in the deep horizons. The main major oxides were SiO2 (46.3–89.0%), Al2O3 (5.0–24.2%) and Fe2O3 (1.0–27.9%). Kaolinite (64–98%) is the predominant clay mineral at all horizons, associated with low to moderate proportions of illite (1–34%) and traces of chlorite. Kaolinite is essentially inherited from the parent rock, whereas illite results from chemical alteration by bisialitization of the primary minerals initially rich in potassium feldspar contained in the parent rock. However, soil texture and organic matter vary independently with clay mineralogy. An extended study of all of the pedological facies that make up south-western Niger, combined with supplementary analyses, would further improve our understanding of clay mineralogy in the Sahelian zone.

Lithofacies assemblage and effects on diagenesis in lacustrine tight sandstone reservoirs: Samples from Upper Triassic Yanchang Formation, Ordos Basin, China

Lacustrine basins are rich in unconventional oil and gas resources and the heterogeneity of matrix reservoir quality seriously affects its development. Previous studies have recognized that diagenesis is the key factor affecting the heterogeneity of matrix reservoir quality, but diagenesis controlled by lithofacies assemblages is poorly studied. In this study, tight sandstones in the Triassic (Ladinian-Carnian) Chang 7 Member of Ordos Basin were taken as examples. Lithofacies, lithofacies assemblages, diagenesis evolution and distribution were investigated by casting thin section analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion and carbon-oxygen isotope. The results indicate that eight lithofacies assemblages in Chang 7 exhibit variations in lithofacies, sand-mud ratio and grain size rhythm, controlling the distribution and intensity of diagenesis. Variations in average proportion of lithofacies assemblages are linked to lake level cycles. Compared to cementation, the mechanical compaction has a higher adverse effect on primary porosity of tight sandstones in Chang 7. Siltstone lithofacies show stronger compaction and cementation (mainly clay cementation), whereas fine sandstone lithofacies exhibit stronger dissolution, especially FSpc (fine sandstone with parallel or cross-bedding). Modest proportions of illite (<4%) and chlorite (<1.5%) in fine sandstones tend to form clay coatings, resisting compaction and preserving more porosity. Conversely, abundant pore-filling illite (>4%) and chlorite (>1.5%) result in a noticeable decline in porosity. Particle size and sorting affect reservoir quality by controlling compaction strength. The origin of carbonate cement in sandstones is closely related to mud-rich lithofacies within lithofacies assemblages. Carbonate cementation preferentially occurs in fine sandstones within 0.4 m from the sand-mud interface and the thickness of adjacent mud-rich lithofacies is greater than 0.1 m. Interfaces between fine sandstones serve as fluid migration pathways and bidirectional erosion, facilitated by high porosity and feldspar content. In contrast, the clay-rich matrix in siltstones acts as a barrier hindering fluid migration, creating unidirectional erosion in fine sandstones near the interfaces between siltstones and fine sandstones. Ultimately, five evolution processes linking diagenesis with lithofacies assemblages are summarized, highlighting the differential reservoir quality. This study contributes to understanding the mechanism behind diagenetic variations in tight sandstones and the reservoir evaluation. Consequently, it offers geological reference for the development of unconventional oil and gas resources in comparable lacustrine basins.

Climatic and weathering conditions in southern high latitudes during the Turonian-Santonian interval: New insights from IODP Site U1512 (Bight Basin, Southern Australia)

A detailed study, based on a coupled mineralogical and geochemical approach, was done on IODP Site U1512 sediments (Bight Basin, Southern Australia). The mineral assemblages of the clay size fraction (<2 μm), obtained by X-ray diffraction, have been compared to inorganic (major and trace elements) geochemical signals, obtained from both whole rock and clay size fraction, and Sr/Nd isotopes, obtained from clay size fraction to determine climatic and weathering conditions during the Turonian to Santonian interval (∼94–84 Ma, Late Cretaceous) in the southern high latitude band. This study reveals that the mineral assemblages of clay fraction consist of R0-type illite/smectite mixed-layers (considered as smectites), illites, and kaolinites associated with opal-CT and clinoptilolites. Electron microscopy observations have demonstrated a negligible impact of burial diagenesis on clay assemblages while authigenesis contributions of clay minerals are limited and associated with minor volcanic contributions. Clay assemblages would be thus dominantly primary in origin. Based on Sr and Nd isotopic values, clay minerals are interpreted as the resulting from the weathering of rocks and pedogenic blankets of Eastern Australian Highlands and from the remobilization of older sediments from nearby uplifted basins. The chemical compositions of both whole rock and clay size fraction show a relative stability during the studied interval with two exceptions: during the Middle/Late Turonian times for Si, P and Ba and at the Turonian-Coniacian boundary for Ti, K and Zr, associated respectively to an increase of biogenic fraction and to an enhanced input of terrigenous material. The clay assemblages, dominated by smectites, show weak variations along the borehole reflecting stable climatic conditions during the 10-Myr Turonian-Santonian interval. The dominance of smectites, coupled to the absence of chlorite and the low proportions of illite, which are both considered as primary clay minerals, indicates a dominant warm/temperate and sub-humid climate during the studied interval. However, changes in weathering conditions occurred during the Turonian-Santonian interval in southern high latitude. The Early Turonian, characterized by high Chemical Index of Alteration values (>75), is followed by an interval of lower chemical weathering with enhanced biogenic silica during the Middle/Late Turonian. The slight increase of kaolinites, recorded during Coniacian and coeval with increase of Ti/Al and Zr/Al ratios, probably reflects an increase in coarser terrigenous inputs associated with a sea-level fall.

Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation

Elucidating the mechanisms of heavy metal (HM) adsorption on clay minerals is key to solving HM pollution in soil. In this study, the adsorption of four HM atoms (As, Cd, Cr, and Hg) on the illite(001) surface was investigated using density functional theory calculations. Different adsorption configurations were investigated and the electronic properties (i.e., adsorption energy (Ead) and electron transfer) were analyzed. The Ead values of the four HM atoms on the illite(001) surface were found to be As > Cr > Cd > Hg. The Ead values for the most stable adsorption configurations of As, Cr, Cd, and Hg were −1.8554, −0.7982, −0.3358, and −0.2678 eV, respectively. The As atoms show effective chemisorption at all six adsorption sites, while Cd, Cr, and Hg atoms mainly exhibited physisorption. The hollow and top (O) sites were more favorable than the top (K) sites for the adsorption of HM atoms. The Gibbs free energy results show that the illite(001) surface was energetically favorable for the adsorption of As and Cr atoms under the influence of 298 K and 1 atm. After adsorption, there was a redistribution of positions and reconfiguration of the chemical bonding of the surface atoms, with a non-negligible influence around the upper surface atoms. Bader charge analysis shows electrons were transferred from the surface to the HM atoms, and a strong correlation between the valence electron variations and the adsorption energy was observed. HM atoms had a high electronic state overlap with the surface O atoms near the Fermi energy level, indicating that the surface O atoms, though not the topmost atoms around the surface, significantly influence HM adsorption. The above results show illite(001) preferentially adsorbed As among all four investigated HM atoms, indicating that soils containing a high proportion of illite might be more prone to As pollution.

A preliminary mineralogical and physicochemical characterization of the Neogene clays from the Timgad Basin (Massif of Aur\xe8s, NE Algeria): potential use in the manufacturing of bricks and ceramic industry

This study aims at the mineralogical and physicochemical characterization of the clay formations of the Miocene age from the Timgad Basin (Massif of Aurès, NE Algeria), in order to evaluate their possible valorization, notably for their potential use in the manufacturing of bricks and ceramic industry. For this purpose, four samples were taken from a clay-dominated formation that outcrops 5 km east of the Timgad city. Each sample was collected, prepared, and analyzed by the appropriate analytical methods of characterization such as X-ray diffraction, X-ray fluorescence, and scanning electron microscopy. Moreover, other complementary analyses are performed in this study such as laser granulometry and geotechnical tests. The results of the different tests revealed that the clay formations of the Timgad Basin are non-refractory clays and characterized by medium plasticity. These clays are constituted of more than 50% of fine fraction, mainly represented by kaolinite in association with non-negligible proportions of illite, chlorite, mixed-layer clay minerals, and traces of smectite. Besides, this clayey assemblage is accompanied by some proportion of quartz and calcite, as well as traces of hematite, feldspar, and gypsum. In the light of these results and in combination with the particle size distribution, as well as the results of geotechnical tests, it is concluded that the Neogene clays of the Timgad Basin present high limits of Atterberg. Consequently, their use in the field of manufacturing of bricks and terra cotta ‘Terre Cuite’ products is subordinated to a preliminary treatment with addition of a degreasing agent, in the form of coarse sand, in order to improve their plasticity.Article HighlightsMineralogical and physicochemical properties of the Neogene clays from the Timgad Basin have been investigated.The studied clays have ideal mineralogical composition and physicochemical properties thus making them applicable in the manufacturing of bricks and ceramic industry.Addition of coarse sand to the Neogene clays will greatly improve the plasticity of these raw materials.

Controls on composition and diagenesis of wave- and river-dominated deltas: impacts on reservoir properties. An example from the La Anita Formation (Argentina)

Wave-, tidal-, and fluvial-dominated deltas are important hydrocarbon reservoirs worldwide, and it is well known that depositional facies types, detrital composition and diagenetic products are key aspects when evaluating potential reservoir properties. Moreover, detrital composition and diagenetic products are controlled by external forces such as tectonism, climate, and depositional conditions.The Upper Cretaceous La Anita Formation (Austral-Magallanes Basin, Argentina) is a deltaic succession that shows a clear vertical variation in the relative role of wave, tidal and fluvial processes. The formation was accumulated during the foreland stage of the basin under relatively warm climate conditions. A detailed compositional and diagenetic analysis for these deposits is provided to evaluate the role of the controlling factors on the detrital and authigenic composition, and the impact on reservoir properties.Samples were analyzed by conventional petrography, X-ray diffraction and scanning electron microscopy to obtain a full spectrum of its detrital, authigenic and diagenetic composition. The analyzed sandstones were classified as lithic arenites which come from the fold-thrust belt region and from the magmatic arc located toward the westward. The clay mineral composition is characterized by variable proportions of illite, smectite, kaolinite and mixed-layer illite-smectite with the sporadic presence of chlorite. Stratigraphic variations in the abundance of authigenic kaolinite and smectite indicate a change in the climatic conditions during the depositions of the unit evolving from tropical to more temperate conditions. Diagenetic processes affecting the porosity are considered as controlled mainly by the lithological nature of the deposits and depositional facies type. Sandstone porosity is mainly primary in origin, moderate to good in abundance and show micropore to mesopore as the main pore-sizes.This work enhances that the diagenetic products are heavily controlled by the dominant depositional process. The presence of diagenetic products as quartz overgrowth and calcite and hematite cement reduces pore spaces and affects the reservoir properties. The development of these types of cement is conditioned by the relative dominance of wave and river processes. Under wave-dominated conditions, quartz tends to develop thick overgrowths. Conversely, under river-dominated conditions quartz overgrowths are thinner and calcite and hematite cementation patches are developed.

Prediction of water retention properties of Syrian clayey soils

Studies on clayey soils developed in temperate areas have shown that their water retention properties are related to both the clay content and the specific pore volume of the clay, the latter being related to the hydric history of the soil, that is to the drying/wetting cycles. Our objective was to discuss the validity of these results for clayey soils developed in semi-arid areas. Samples were collected in soils located in Syria. Physico-chemical properties were determined. Water content was measured at field capacity and for water potentials ranging from −10 to −15,000 hPa. X-ray diffraction analyses were performed on the clay fraction to identify the clay. Results showed that the clays have both a high cation exchange capacity (0.707–0.891 mmol+ g−1 of clay) and a high external specific surface area (112 and 178 m2 g−1 of clay). These values are consistent with the X-ray diffraction results which showed the presence of a high proportion of smectite in most horizons and secondarily of varying proportions of illite and chlorite; kaolinite, while present, was not abundant. Results also showed that the amount of water retained by the clay according to the water potential was closely related to the specific pore volume of the clay at field capacity. Regression equations established by using both the data published earlier and those of this study enabled us to predict the water retention properties of clayey soils for a larger range of clay mineralogy and climatic environments including semi-arid environments than previously discussed in the literature.

High-Temperature Mineral Formation after Firing Clay Materials Associated with Mined Coal in Teruel (Spain)

The production of porcelain stoneware has experienced a considerable increase. Therefore, it was necessary to undertake an investigation that would allow knowing the mineralogical evolution that porcelain stoneware undergoes during the firing process, as well as establishing the influence of the formation of mullite and other mineral or vitreous phases and their quantification. The firing transformations of mine spoils associated with mined coal in the Utrillas-Escucha-Estercuel and Ariño-Andorra areas are studied in this paper. The mineralogical composition of the bulk mine spoils is kaolinite, illite, chlorite, and smectites (in traces), with quartz and feldspar, and minor hematite, calcite, and dolomite. The main objective is to understand the generation of high-temperature mineral phases after firing, and their quantification. The formation of mullite and other high-temperature phases are studied from samples that include variable proportions of illite. Samples with a high content of illite generate mullite at 995 °C. Cristobalite was not detected as a high-temperature phase. Mullite is the most abundant mineral. The hercynite content is higher at low temperatures (995 °C), and hematite content is higher at 1150 °C. The vitreous phase represents about 50% of fired bodies. Despite observing a porous microstructure, the non-porous areas are well sintered.

Threshold Pore Pressure Gradients in Water-Bearing Tight Sandstone Gas Reservoirs

Tight gas reservoirs commonly occur in clastic formations having a complex pore structure and a high water saturation, which results in a threshold pressure gradient (TPG) for gas seepage. The micropore characteristics of a tight sandstone gas reservoir (Tuha oilfield, Xinjiang, China) were studied, based on X-ray diffraction, scanning electron microscopy and high pressure mercury testing. The TPG of gas in cores of the tight gas reservoir was investigated under various water saturation conditions, paying special attention to core permeability and water saturation impact on the TPG. A mathematical TPG model applied a multiple linear regression method to evaluate the influence of core permeability and water saturation. The results show that the tight sandstone gas reservoir has a high content of clay minerals, and especially a large proportion of illite–smectite mixed layers. The pore diameter is distributed below 1 micron, comprising mesopores and micropores. With a decrease of reservoir permeability, the number of micropores increases sharply. Saturated water tight cores show an obvious non-linear seepage characteristic, and the TPG of gas increases with a decrease of core permeability or an increase of water saturation. The TPG model has a high prediction accuracy and shows that permeability has a greater impact on TPG at high water saturation, while water saturation has a greater impact on TPG at low permeability.

Concentrations and modes of occurrence of some potentially valuable and toxic elements in the No. 5 coal from the Yanzishan Mine, Datong Coalfield, Shanxi Province, China

In this study, the mineralogical compositions, concentrations and occurrence modes of the elements in the No.5 coal from the Yanzishan Mine were investigated, using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). The No. 5 coal is characterized by high ash yield content (mean 32.30%) and low sulfur content (mean 0.47%). The minerals are mainly composed of kaolinite, quartz and pyrite, with a small proportion of illite, apatite, siderite, sphalerite and Ti-oxide. Compared with the Chinese coals, the No. 5 coal has much higher percentages of SiO2 (mean 16.22%) and Al2O3 (mean 14.57%) than other major oxides. Compared with the world hard coals and Chinese coals, the No. 5 coal is enriched in some potentially valuable (Al2O3, Li, Ge, Zr, and Hf) and toxic (Be, As, Se, Cd, and Pb) elements. Lithium and Hf are mainly associated with clay minerals; Be, Se, and Zr mainly have both inorganically and organically affinities; As mainly occurs as a sulfide form in pyrite; Cd is incorporated in clay minerals; and Pb has an affinity for both clay minerals and organic sulfur. All the coal samples are characterized by weakly positive or no pronounced Eu anomalies, indicating that the rare earth elements and yttrium (REY) likewise originated from granite of the Yinshan Oldland together with injection of the low temperature hydrothermal fluids.

Illite/smectite diagenesis and thermal evolution of Lower Cretaceous–Paleogene successions in the Dolna Kamchiya Depression, Eastern Bulgaria

The diagenesis of mixed-layer illite/smectite (I/S) minerals in Lower Cretaceous–Paleogene successions from the Dolna Kamchiya Depression was studied, using X-ray diffraction analysis of the clay fraction (&lt;2 μm) from core samples. The proportion of illite in I/S and degree of ordering increase with depth irrespective of the geological age, indicating that highly expandable I/S compositions were progressively illitized during the burial evolution. Lowest smectite values are recorded in the Lower Cretaceous deposits, whereas in the Paleogene sections are documented great regional variations in the I/S mineralogy, caused by differential basin subsidence. The transition from randomly interstratified (R0) to R1-ordered I/S occurs between depths of 2200 m and 2400 m, and crosscuts the major stratigraphic boundaries. The variable patterns of I/S depth profiles resulted from the combined influence of temperature, burial history, sedimentation and subsidence rates, and basin geodynamic types on diagenesis. The rapid increase in illite content in the I/S clays across the main unconformities reflects the great thickness of eroded sediments during uplift and denudation events and/or elevated heat flow. The reconstructed palaeogeothermal gradient for the Eocene after corrections for decompaction and erosion would correspond to the values for foreland basin settings reported in the literature. Application of the I/S geothermometer to the Lower Cretaceous–Paleogene rocks demonstrates a general trend of increased organic maturity toward southeast in relatively isochronous deposits. The new results appear to be the first record for the effect of a multiphase basin evolution on the smectite illitization in sedimentary successions from the Dolna Kamchiya Depression.

Mineral and textural transformations in aluminium-rich clays during ceramic firing

The aim of this study has been to analyse the mineralogical and textural transformations of a set of aluminium-rich shales of interest for refractory and ceramic uses, fired from 800°C to 1300°C. To that end, raw and fired samples were analysed by X-ray diffraction, transmitted light microscopy, field emission scanning electron microscopy, and transmission electron microscopy. Raw samples comprise variable proportions of illite, pyrophyllite, orthoclase, quartz, kaolinite, mixed-layer I-Sm, and organic matter. At temperatures below 800°C, kaolinite, mixed-layer I-Sm, and organic matter are destabilized, indicating that they are the least stable phases in the firing process. Illite, pyrophyllite, and orthoclase remain until 1000°C and show a broader stability field during firing than in natural environments. Quartz persists throughout the entire firing process, although it is partly replaced by vitreous phase. Hematite crystallizes at 900°C. Vitrification begins at 1000°C, marking the first significant textural change. From 1000°C mullite starts to crystallize from the Si- and Al-rich vitreous phase. The mullite composition is not stoichiometric and probably as temperature increases Si is partially replaced by Al, Fe and Ti in the structure. Nevertheless, with the increase of the firing temperature, the mullite composition is closer to the theoretical composition and also to that of natural mullites. Furthermore its crystal thickness increases with temperature up to 70nm.

Geochemistry of Lower Devonian terrigenous sedimentary rocks from the Belgian Ardenne: Source proxy and paleogeographic reconstruction

Major and trace element (Rb, Sr, Ba, Zr, Ni, V, Zn, Cr, Y, Ce) compositions are studied in Lower Devonian terrigenous sediments from the Dinant Synclinorium and Ardenne Anticline (Belgian Ardenne). Five cross sections encompassing 148 samples in 19 formations have been studied. In the conventional [Al2O3+Fe2O3-K2O-Na2O]–[K2O]–[FeO+MgO] triangle, the rock compositions plot on the illite–chlorite tie line. This suggests that (1) the mineral association has reached equilibrium in the pressure and temperature range of the postdepositional evolution of the sediments; (2) the chemical system was open and K was a mobile component. Although the original composition of the sediment was modified, the AAFM=Al2O3/Al2O3+FeOt+MgO parameter that reflects the illite-chlorite proportions can be used as a proxy of the source composition. A principal component analysis of the major and trace element compositions permits to define the behaviour of various elements currently used as proxies of various processes. Noteworthy it is shown that Ti, V and part of Cr are correlated with the illite proportion, and part of Cr, together with Zr, with resistate minerals such as quartz, chromite and zircon. The evolution with stratigraphic age in the 5 cross sections of the AAFM proxy reveals that there is a major change in the northern part of the Ardenne Anticline at the limit between the Mirwart Formation and younger formations, corresponding approximately to the Lochkovian to Pragian transition. There is a clear shift from an aluminium-rich source to a more conventional chlorite-rich source. In the paleogeographic evolution of this region the only possible source of the aluminium-rich sediment is the Rocroi Massif. Our results confirm sedimentological and palynological evidence that a “Rocroi Island” existed in the Lochkovian Sea. A decrease in Zr concentrations with distance to the Rocroi Massif also supports this hypothesis. The stratigraphic evolution of several parameters shows local variations that can be attributed to variable streams and depositional conditions in an alluvial deltaic or shallow sea environment.

Evaluating the paleoclimatic significance of clay mineral records from a late Pleistocene loess-paleosol section of the Ili Basin, Central Asia

AbstractIn this study, we present clay mineral records from a late Pleistocene loess-paleosol sequence in the Ili Basin, Central Asia, and assess their significance for paleoclimatic reconstruction. The results show that the clay minerals are mainly illite (average 60%) and chlorite (28%), with minor kaolinite (9%) and smectite (3%). Illite was of detrital origin with no obvious modification to its crystal structure. Increases in illite content in the loess are ascribed to wind intensity rather than pedogenesis. High proportions of illite in the clay fraction, and of muscovite in the bulk samples of the paleosol units, may lead to an overestimation of the weathering intensity. Kaolinite was likely inherited from the sedimentary rocks, while chlorite might have been inherited from both sedimentary and metamorphic rocks. The paleoclimatic signals of kaolinite and chlorite were unclear, due to reworking by both fluvial and eolian systems. Smectite was more likely formed by the transformation of biotite and illite, and its variation in the loess sequence was also controlled by wind intensity; this was largely due to aggregation and is unlikely to reflect moisture changes. Although the interpretation of paleoclimate evolution may contain some uncertainties, clay mineralogy does provide the possibility of tracing dust provenance.

Origin of Minerals and Critical Metals in an Argillized Tuff from the Huayingshan Coalfield, Southwestern China

This paper reports the minerals in an argillized tuff from the Lvshuidong mine, Huayingshan Coalfield, southwestern China. The clay assemblages of the samples are mainly kaolinite, varying proportions of illite and trace chlorite. Other minerals include various proportions of pyrite, small proportions of carbonates (calcite and ankerite), anatase, rutile, hematite, florencite and rare analcite. The clay mineralogy of the tuff profile changes vertically due to the change in the depositional environment. Although illite is minor in the middle and lower parts of the profile, it is relatively abundant in a few topmost samples where the proportion of illite is comparable to that of kaolinite. This is probably because the original volcanic ash was mainly deposited in a continental environment, but marine water may have percolated to the uppermost layers of the ash bed during early diagenesis, leading to the formation of concretions of concentric rings of kaolinite and illite. The samples of this study are derived from alkali mafic volcanic ash with relatively high concentrations of critical metals, including Nb, Ta, Zr, Hf, rare earth elements and Y.

Low temperature, authigenic illite and carbonates in a mixed dolomite-clastic lagoonal and pedogenic setting, Spanish Central System, Spain

The aim of this study was to further our understanding of the pedogenic and lacustrine modification of clay minerals. Some of these modifications are of special interest because they constitute reverse weathering reactions, rare in surface environments, and because there is not yet an accurate assessment of their global relevance in mineralogical and geochemical cycles. For this study, two sections from the Central System in Spain were selected. Both are sections through the Uppper Cenomanian-Turonian mixed clastic and carbonate succession, containing both calcite and dolomite, in the Sierra de Guadarrama. Mid-Turonian sea level fall resulted in the formation of a coastal plain environment in which extensive pedogenesis occurred around saline lagoons. The mineralogical changes that have occurred as a result of sedimentation in saline lagoons and as a consequence of pedogenesis are described. Textural relationships indicate that the dolomite cement pre-dates the calcite. Silicate minerals are represented by quartz, kaolinite, illite-smectite, illite, minor plagioclase and alkali feldspar, and trace chlorite and palygorskite. There is a positive correlation between the intensity of pedogenesis and the proportion of illite in the clay assemblage in one of the sections, indicating pedogenic illitisation. In this section, the intensity of the illitisation process increases up, reaching a maximum where pedogenesis is most intense in the middle part, and then decreases as marine influence increases towards the top of the Alcorlo Formation and the overlying marine Tranquera Formation. The clay assemblages are consistent with a slow transformation process from kaolinite to illite by way of illite-smectite, taking place under surface conditions. The illitisation process has resulted in a less Fe-rich, more Mg-, and Al-rich illite than the majority of previously documented cases in the near surface. Formation of Al-rich illite is not therefore restricted to the deep subsurface.The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg2+ substitution for Al3+ (or Fe3+) and Fe3+ to Fe2+ reduction. Mg2+ enrichment may have occurred principally in saline lagoons or lakes, while Fe3+ to Fe2+ reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg.

Reconstruction of the Late Cretaceous-Paleocene paleoenvironment (northern Tunisia) from biostratigraphy, geochemistry and clay mineralogy

A multidisciplinary approach was adopted for the reconstitution of the depositional environments based on clay minerals distribution, biostratigraphy and geochemical properties of the Late Cretaceous-Paleocene deposits. Evolution of the mineralogical compositions (i.e. clay minerals contents) was ascertained in two cross sections from the Late Cretaceous and Paleocene belonging to different paleogeographic domains: the Bir M’Cherga (Tunisian Dorsal) and Tajerouine (Northern West of Tunisia) study sites. Chemical analysis of the original clay samples, collected in both sites (i.e. Late Cretaceous and Paleocene clays), showed significant amounts of oxides including calcium, silica, alumina and iron. Three distinct mineralogical zones were identified. Clay mineral assemblages of the Late Cretaceous showed the predominance of kaolinite, subordinated by variable proportions of illite and smectite. In contrast, those of Paleocene clearly showed the predominance of smectite to the detriment of kaolinite and illite. The predominance of kaolinite in the Late Cretaceous clay deposits indicated a calm and shallow depositional environment influenced by hot and wet climate, especially in northeastern Tunisia (Bir M’Cherga study site). Kaolinite typically forms under lacustrine environment where appreciable movement of water would be expected, leading to the development of kaolinite. During Paleocene, the smectite-enriched mineralogical sequence suggested similar climatic conditions with further deepening of the depositional environment in salt lakes. The regional geodynamic context of both sections showed an important Maastrichtian unconformity with a Paleocene major hiatus extending from the East to the West.

Petrological, mineralogical, and geochemical compositions of Early Jurassic coals in the Yining Coalfield, Xinjiang, China

The petrological, geochemical, and mineralogical compositions of 10 coal seams (Early Jurassic) from an exploratory borehole (Hole ZKJ502) in the Jieliangzi mining area of the Yining Coalfield, Xinjiang Uygur Autonomous Region, China, were investigated, using optical microscopy and field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer (FE SEM-EDS), as well as X-ray powder diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The Yining coals are of subbituminous to highly volatile C bituminous in rank (vitrinite reflectance, average 0.49%) and are characterized by low sulfur content (average 0.45%), low ash yield (average 4.10%), high volatile matter (average 38.82%), high proportion of inertinite (average 49.5%), and low concentrations of most trace elements. Relative to the upper continental crust, the rare earth elements and yttrium in the Yining coals are mainly characterized by light and heavy REY enrichment. The minerals in the Yining coals are dominated by quartz, kaolinite, and siderite, followed by pyrite, bassanite, and hexahydrite, along with small proportions of illite, calcite, dolomite, gorceixite, goyazite, ankerite, marcasite, diaspore, florencite, hematite, and millerite. The similarity between the Yining coals and the magmatic rocks from the peripheral mountain area of the basin in terms of the Al2O3/TiO2 ratio, the REY enrichment type, the REY distribution patterns, and the Eu anomalous fractionation suggest that the sediment source region was mainly Carboniferous volcanic rocks (andesite and basalts) on peripheral mountains. The acid conditions and lowered water table during peat accumulation may have caused detrital minerals to be leached, leading to the relatively low ash yield. Another reason for the low ash yield may be the simple coalfield structure and poor groundwater migration channels in the rocks and coal seams in the central zone of the basin, which would limit the minerals carried by the groundwater into coals. A number of authigenic minerals (quartz, kaolinite, siderite, and gorceixite) occur as cell-fillings and elevated concentrations of Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Zr, Nb, Mo, Cs, Hf, W, Pb, Th, and REY occur in Nos. 26 and 27 coals, indicating that there was an injection of a small-scale solution.

Multi-proxy orbital chronology in the aftermath of the Aptian Oceanic Anoxic Event 1a: Palaeoceanographic implications (Serre Chaitieu section, Vocontian Basin, SE France)

In the early Aptian, the Oceanic Anoxic Event (OAE) 1a is well defined by a negative ?13C excursion followed by a positive δ13C excursion, spanning the Deshayesites deshayesi and Dufrenoya furcata ammonite biozones. A cyclostratigraphic approach is performed in the Vocontian Basin, France, to estimate the time required for the carbon cycle recovery following the major disturbance associated to OAE1a and to provide durations of ammonite and foraminifer biozones. The Serre Chaitieu section, which consists of hemipelagic blue-grey marls with occasional marker limestone horizons and encompassing the Deshayesites deshayesi Zone to the end of the Epicheloniceras martini Zone, was used as a reference section in the Vocontian Basin. Using field Spectral Gamma Ray (SGR), 450 measurements were performed throughout the section, and a sample of each measured sediment was collected to further perform calcimetry, clay mineralogy, and magnetic susceptibility (MS) measurements. Detrital clay mineral assemblages consist of illite, illite/smectite mixed-layers (I-S), kaolinite and chlorite. Fluctuations of clay minerals are mainly driven by climate change, progradation/drowning of peri-vocontian platforms and sea-level changes. The proportions of illite and kaolinite covary and fluctuate in opposition with I-S. Cyclic fluctuations of relative proportions of clay minerals are particularly well recorded by the kaolinite/chlorite ratio (K/C). Spectral analyses, using the multi-taper and the amplitude spectrogram methods, were performed on SGR, MS, CaCO3 and K/C signals to detect sedimentary cycles related to an orbital forcing throughout the series. The geochronometer 405-kyr eccentricity cycle well expressed and significant (up to 99% confidence level) is used to provide a robust temporal framework. More than five 405-kyr eccentricity cycles are recognised, providing a total duration of at least 2.49 myr for the whole sedimentary succession. The minimum duration of the D. furcata Zone is assessed at 0.42 myr, and the duration of the E. martini Zone at 1.52 myr. Amplitude spectrograms show a strengthened signal of obliquity during the D. furcata Zone, which is coherent with the global cooling that has been depicted for this interval, and which could have favored the development of lowextension polar ice and thus the lowering of the sea level. Durations of C-isotope zones, worldwide correlated, are also calculated. From these results, the duration of the return to equilibrium in the carbon cycle in the aftermath of OAE1a could be calculated at 1.35 myr.

Origin of reddening in a paleosol buried by lava flows in Fogo island (Cape Verde)

In Fogo island (Cape Verde), a buried paleosol with a reddened topsoil developed on mafic lapilli, intercalated between intrusive carbonatites and an alkaline lava flow, was studied. Significant mineralogical, crystalchemistry and chemical variations were found along the profile. Upwards a decrease of the grain size occurs. Increasing weathering of magnetite to maghemite, and ultimately to hematite is also clearly observed. The Fe3+/Fe2+ ratio in silicate phases increases, along with an increase of illite proportion in the mixed-layer I-Sm. Discrete smectite was detected only in the bottom level related with weak drainage conditions. In the upper level there is a significant gain of Na, Cr, P, Si, and particularly As. The colour variations (towards the paleosurface: dark reddish brown – yellowish brown – red – weak red) are mainly due to the fine fraction of the various levels. Hematite is the main iron phase responsible for the colour and may explain the significant retention of Cr, P, As and LREE in the topsoil. Here hematite is more abundant and occurs as larger-sized particles than in the lower levels. This hematite grain growth is probably due to the thermal effect of the overlying hot volcanic unit. Thus the main alteration processes of the lapilli deposits appear to have been (i) weathering mechanisms in arid environment with the formation of an incipient paleosol, and (ii) thermal metamorphism due to the overlying lava contributing for reddening and cementation of the upper levels. Biological remains are absent due to the paleoenvironmental conditions and/or to the effect of heating.