THE FATE OF SMECTITE IN KOH SOLUTIONS

International audienceThe aim of the present study was to investigate the detailed evolution of the SAz-1 smectite in 1 M KOH at 80°C at a solid/liquid ratio of 1/80. AFM observations indicated no change in crystal size or shape. XRD measurements at 40% relative humidity revealed changes in expandability of the smectite. The 001 reflection profile of smectite was modelled using the trial-and-error approach of Sakharov et al., 1999b. The results indicate that with increasing run time the number of non expandable layers with zero or one water layer increases and that the coherent scattering domain size of the smectite decreases. Infrared spectroscopy of the reacted smectite suggests that there is no change from the initial clay products. The dehydroxylation temperature showed a slight decrease from 619° to 605°C. STA measurements demonstrated that the cis-vacant character of the octahedral sheet remained nearly unchanged throughout the experiment. Determination of the average layer charge showed a continuous increase from 0.32 to 0.42 eq/Si (Si/Al)4O10 whereas the layer charge distribution indicated the appearance of high charged smectite layers with a charge of ~ 0.6 eq/Si (Si/Al)4O10 and the disappearance of the low charged layers. XPS and SEM measurements indicate an increase of the aluminium in the smectite samples. Isotope data support the theory of a internal diffusion mechanism by gradual changes in δ 18O values. From these data it appears that KOH solutions provoke a mineralogical change in the 2:1 layer of the smectite minerals which increases the layer charge by increasing the Al content. This mineralogical change does not involve dissolution/crystallization processes and then must show solid state transformation of the clays at 80°C

Mineralogy and geochemistry of the sedimentary kaolin deposits from Sinai, Egypt : Implications for control by the source rocks

Abstract-Mineralogical and geochemical variations among the Carboniferous and Cretaceous sedimentary kaolin deposits from Sinai provided an opportunity to examine the effect of the source area on compositions of the deposits. The Carboniferous kaolin deposits are mineralogically and geochemically heterogeneous. The Khaboba and Hasbar deposits consist of kaolinite, quartz, anatase, illite, chlorite, zircon, and leucoxene. The shale-normalized rare earth element (REE) patterns of the Khaboba deposit showed a slight LREE over HREE enrichment ((La/Yb) =1.19 1.51) with a MREE depletion (Gd/ Gd* =0.51 0.75), while the Hasbar kaolin had a MREE enrichment. The Abu Natash kaolin deposit consisted of kaolinite, anatase, and a little quartz with larger TiO, Cr, and V and smaller Zr and Nb contents compared to other Carboniferous deposits. The shale-normalized REE patterns of the Abu Natash deposit exhibited a positive Eu anomaly (Eu/Eu* =1.28 1.40) and a MREE enrichment (Gd/Gd* = 1.41 2.05). The Cretaceous deposits were relatively homogeneous in terms of mineralogical composition and geochemistry and are composed of kaolinite, quartz, anatase, rutile, zircon, and leucoxene. The Cretaceous kaolin deposits showed mostly flat shale-normalized REE patterns with a variable LREE depletion. The presence of illite and chlorite, the absence of rutile, large Zr and Nb contents, and the REE patterns suggested a component of weathered low-grade metasediments as a source for the Carboniferous deposits in the Khaboba and Hasbar areas, while the large Ti, Cr, and V, and small quartz contents indicated mafic source rocks for the Abu Natash deposit. The abundance of high-Cr rutile and the absence of illite and chlorite, and large Zr, Ti, Cr, and V contents suggested a mixture of medium- to high-grade metamafic and granitic rocks as source rocks for the Cretaceous kaolin deposits. The occurrence of alkaline rocks in the source of the deposits studied was identified by high-Nb contents and the presence of bastnaesite. The mineralogical and geochemical heterogeneity and lesser maturity of the Carboniferous deposits suggested local sources for each deposit and their deposition in basins close to the sources. The mineralogical and geochemical homogeneity and maturity of the Cretaceous deposits, on the other hand, indicated common sources for all deposits and their deposition in relatively remote basins

Clay mineral and geochemical composition of Cenozoic paleosol in the Eastern Alps (Austria)

Red clays from Cenozoic palaeosols of the Eastern Alps record periods of stagnating uplift and decrease of relief. Tropical to sub-tropical weathering of a crystalline substratum formed dominant or abundant kaolinite, reflecting Paleogene and Early Miocene conditions, respectively. Abundant illite and chlorite, but a lack of kaolinite in red clays on the plateaus of the Northern Calcareous Alps reflects feldspar-poor compositons of the Cenozoic siliciclastic cover. The presence of high Ba/Sr and Rb/Sr ratios and vermiculite in these red clays indicates high precipitation and temperate weathering conditions, respectively, during the Late Miocene and Early Pilocene on the uplifting plateaus of the Northern Calcareous Alps

Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58–2.50 Ga-old Transvaal Supergroup carbonate platform, South Africa

The Neoarchean-Paleoproterozoic Transvaal Supergroup in South Africa contains the well-preserved stromatolitic Campbellrand-Malmani carbonate platform, which was deposited in shallow seawater shortly before the 2.40–2.32 Ga Great Oxidation Event (GOE). This platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for (isotope-) geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the GOE. Mo isotopes in sedimentary archives are widely used as a proxy for redox-changes in modern and ancient environments and recent evidence suggests that oxy-molybdate (MoO42−) is directly transferred from ocean water to inorganic carbonates with negligible fractionation, thus reflecting oceanic Mo isotope signatures. In this study we analyzed major and trace element compositions as well as Mo isotopic compositions of carbonate and mudstone samples from the KMF-5 drill core. Geochemical indicators, such as Fe and Mn concentrations and Fe-to-Mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform. Heavy δ30Si values of silicified carbonates between 0.53 and 2.35‰ point to Si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint. This assessment is supported by the frequent observation of rip-up structures of silica (chert) layers within the entire sedimentary succession. The δ98Mo values of whole rock samples throughout the Malmani-Campbellrand platform range between −0.82 and +1.40‰, similar to values reported for deeper slope carbonates from the Griqualand West area, but variations are independent from lithology or depositional water depth. These large variations in δ98Mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere-ocean system at that time, in agreement with earlier Mo isotopic studies on Campbellrand carbonates and shales. A similar range in δ98Mo values for carbonates between +0.40 and +0.87‰, however, was also found on the hand specimen scale, indicating the danger of a sample bias on the Mo isotopic stratigraphy of this carbonate platform. Results of previously unpublished adsorption experiments of Mo on CaCO3 clearly indicate that the Mo inventory of Malmani-Campbellrand carbonates was not only influenced by primary adsorption from seawater, but to a much larger degree by secondary processes during early diagenesis, which also affected the Mo isotopic composition of the samples on a local scale. Our results indicate that Mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification, and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through Earth's history. Nevertheless, we interpret our heavy Mo isotopic signatures from carbonates as a minimum value for Neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic Mo reservoir at that time

Modelling of geochemical and isotopic changes in a column experiment for degradation of TCE by zero-valent iron

Zero-valent iron (ZVI) permeable-reactive barriers have become an increasingly used remediation option for the in situ removal of various organic and inorganic chemicals from contaminated groundwater. In the present study a process-based numerical model for the transport and reactions of chlorinated hydrocarbon in the presence of ZVI has been developed and applied to analyse a comprehensive data set from laboratory-scale flow-through experiments. The model formulation includes a reaction network for the individual sequential and/or parallel transformation of chlorinated hydrocarbons by ZVI, for the resulting geochemical changes such as mineral precipitation, and for the carbon isotope fractionation that occurs during each of the transformation reactions of the organic compounds. The isotopic fractionation was modelled by formulating separate reaction networks for lighter (12C) and heavier (13C) isotopes. The simulation of a column experiment involving the parallel degradation of TCE by hydrogenolysis and β-elimination can conclusively reproduce the observed concentration profiles of all collected organic and inorganic data as well as the observed carbon isotope ratios of TCE and its daughter products. © 2007 Elsevier B.V. All rights reserved

Archaean high-K granitoids produced by remelting of earlier Tonalite-Trondhjemite-Granodiorite (TTG) in the Sangmelima region of the Ntem Complex of the Congo craton, southern Cameroon

IF=1.219info:eu-repo/semantics/publishe

Stable water isotope patterns in a climate change hotspot: The isotope hydrology framework of Corsica (western Mediterranean)

International audienceThe Mediterranean is regarded as a region of intense climate change. To better understand future climate change, this area has been the target of several palaeoclimate studies which also studied stable isotope proxies that are directly linked to the stable isotope composition of water, such as tree rings, tooth enamel or speleothems. For such work, it is also essential to establish an isotope hydrology framework of the region of interest. Surface waters from streams and lakes as well as groundwater from springs on the island of Corsica were sampled between 2003 and 2009 for their oxygen and hydrogen isotope compositions. Isotope values from lake waters were enriched in heavier isotopes and define a local evaporation line (LEL). On the other hand, stream and spring waters reflect the isotope composition of local precipitation in the catchment. The intersection of the LEL and the linear fit of the spring and stream waters reflect the mean isotope composition of the annual precipitation (δP) with values of-8.6(±0.2) ‰ for δ18O and-58(±2) ‰ for δ2H. This value is also a good indicator of the average isotope composition of the local groundwater in the island. Surface water samples reflect the altitude isotope effect with a value of-0.17(±0.02) ‰ per 100 m elevation for oxygen isotopes. At Vizzavona Pass in central Corsica, water samples from two catchments within a lateral distance of only a few hundred metres showed unexpected but systematic differences in their stable isotope composition. At this specific location, the direction of exposure seems to be an important factor. The differences were likely caused by isotopic enrichment during recharge in warm weather conditions in south-exposed valley flanks compared to the opposite, north-exposed valley flanks.</p

TTG magmatism in the Congo craton; a view from major and trace element geochemistry, Rb-Sr and Sm-Nd systematics: case of the Sangmelima region, Ntem complex, southern Cameroon

IF=1.219info:eu-repo/semantics/publishe