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1.
 Four smectites with different total Fe contents (two nontronites, one ferruginous smectite, and one montmorillonite) were reduced to obtain a range of Fe(II)/Fe(III) ratios and their magnetic properties measured with a SQUID (superconducting quantum interference device) as a function of applied magnetic field strength at 5 K and as a function of temperature in a field of 0.1 T. The unaltered nontronite and ferruginous smectite specimens showed antiferromagnetic coupling, whereas the coupling in the reduced samples was ferromagnetic; the paramagnetic Curie temperature increased with increasing Fe(II) content. Data collected after cooling samples in both the presence and absence of an external magnetic field of 0.1 T showed that at low temperatures the reduced (ferromagnetic) nontronite and ferruginous smectite samples exhibit a memory effect of previous magnetic field exposure consistent with superparamagnetic or spin glass behavior. The superparamagnetic/ferromagnetic transition temperature, T f , increased linearly with increasing Fe(II) content for each of the nontronites, but the relationship between T f and Fe(II) content differed for different clays, thus demonstrating that T f is sensitive to isomorphous substitutions in the clay structure. The montmorillonite was paramagnetic in both oxidized and reduced forms. Received: 23 March 1999 / Revised, accepted: 27 August 1999  相似文献   

2.
99Technetium (99Tc) is a fission product of uranium-235 and plutonium-239 and poses a high environmental hazard due to its long half-life (t1/2 = 2.13 × 105 y), abundance in nuclear wastes, and environmental mobility under oxidizing conditions [i.e., Tc(VII)]. Under reducing conditions, Tc(VII) can be reduced to insoluble Tc(IV). Ferrous iron, either in aqueous form (Fe2+) or in mineral form [Fe(II)], has been used to reduce Tc(VII) to Tc(IV). However, the reactivity of Fe(II) from clay minerals, other than nontronite, toward immobilization of Tc(VII) and its role in retention of reduced Tc(IV) has not been investigated. In this study the reactivity of a suite of clay minerals toward Tc(VII) reduction and immobilization was evaluated. The clay minerals chosen for this study included five members in the smectite-illite (S-I) series, (montmorillonite, nontronite, rectorite, mixed layered I-S, and illite), chlorite, and palygorskite. Surface Fe-oxides were removed from these minerals with a modified dithionite-citrate-bicarbonate (DCB) procedure. The total structural Fe content of these clay minerals, after surface Fe-oxide removal, ranged from 0.7% to 30.4% by weight, and the structural Fe(III)/Fe(total) ratio ranged from 45% to 98%. X-ray diffraction (XRD) and Mössbauer spectroscopy results showed that after Fe oxide removal the clay minerals were free of Fe-oxides. Scanning electron microscopy (SEM) revealed that little dissolution occurred during the DCB treatment. Bioreduction experiments were performed in bicarbonate buffer (pH-7) with structural Fe(III) in the clay minerals as the sole electron acceptor, lactate as the sole electron donor, and Shewanella putrefaciens CN32 cells as a mediator. In select tubes, anthraquinone-2,6-disulfate (AQDS) was added as electron shuttle to facilitate electron transfer. In the S-I series, smectite (montmorillonite) was the most reducible (18% and 41% without and with AQDS, respectively) and illite the least (1% for both without and with AQDS). The extent and initial rate of bioreduction were positively correlated with the percent smectite in the S-I series (i.e., layer expandability). Fe(II) in the bioreduced clay minerals subsequently was used to reduce Tc(VII) to Tc(IV) in PIPES buffer. Similar to the trend of bioreduction, in the S-I series, reduced NAu-2 showed the highest reactivity toward Tc(VII), and reduced illite exhibited the least. The initial rate of Tc(VII) reduction, after normalization to clay and Fe(II) concentrations, was positively correlated with the percent smectite in the S-I series. Fe(II) in chlorite and palygorskite was also reactive toward Tc(VII) reduction. These data demonstrate that crystal chemical parameters (layer expandability, Fe and Fe(II) contents, and surface area, etc.) play important roles in controlling the extent and rate of bioreduction and the reactivity toward Tc(VII) reduction. Reduced Tc(IV) resides within clay mineral matrix, and this association could minimize any potential of reoxidation over long term.  相似文献   

3.
Reduction of octahedral Fe in the crystalline structure of smectites influences, possibly controls, surface-sensitive physical and chemical properties. The purpose of this study was to investigate if reduction of structural Fe by Na-dithionite or bacteria affects the chemical environment of constituent cations in montmorillonite, employing solid state multinuclear (29Si and 27Al) magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Reduction of structural Fe resulted in a positive (down field) chemical shift of the main Si Q3 (Q3(0Al)) site which was strongly correlated with Fe(II) content and inferred that distortions in Si-OT (T=Si, Al) bond angles and Si-O bond lengths occur with increasing layer charge. The line width (W) of the 29Si Q3 signal also increased with increasing levels of reduction. No change occurred in the position of the peak maximum for the octahedral Al (27AlVI) signal; however, an increased W was observed for this peak with increasing Fe(II) content. These results are attributed to decreases in Si-O-T bond angles and Si-O bond distances, corresponding to a better fit between the tetrahedral and octahedral sheets brought about by the presence of Fe(II) in the clay structure. The increased 27AlVI signal width (W) may also be due to a lessening of the paramagnetic influence of Fe(III) nuclei and enhancement of 27AlVI signals with different quadrupole coupling constants (QCC). Multinuclear MAS NMR analyses of dithioniteand microbially-reduced montmorillonite indicate that reduction of structural Fe caused reversible changes in the smectite structure, at least as far as this method could discern.  相似文献   

4.
In the context of the potential confinement of high-level radioactive wastes (HLW) within the clay engineered barrier system (EBS) in deep geological formations, the evolution of the retention properties of smectite when interacting with Fe(0) needs to be assessed. If some potential natural analogues describing iron-clay reactivity are easily found, metallic iron-clay interactions are poorly described in studies regarding the Earth. Therefore, experimental investigations are needed. Several parameters influence Fe(0)-clay interactions, such as temperature, the interlayer composition of swelling clays, and the presence of octahedral Fe3+, etc. From a geometrical point of view, it is thought that clay destabilization is mainly controlled by phenomena starting at the edge faces of the particles. In the present work, the rates of the smectite-Fe(0) reaction at 80 °C were assessed by XRD, Mössbauer, and CEC analyses for three smectites. The investigations show marked differences in the degree of stability, which cannot be explained by the crystal-chemistry rules established in previous studies. Therefore, the Fe(0)-smectite interactions were studied in view of textural and energetic surface quantitative analyses. The studied smectites have equivalent nitrogen BET-specific surface areas, equivalent argon edge surface areas and slightly different basal surface areas. This similarity in particle shape indicates that the edge surface area cannot be accounted for when explaining the observed differences in reactivity. However, a correlation is obtained between smectite reactivity and the energetic heterogeneity of its edge faces. This is interpreted in terms of a multiplication of the number of sites on the edge faces, where the electron transfer between Fe(0) and the smectite structure can occur.  相似文献   

5.
Iron-rich smectite is commonly described in the diagenetic fraction of deep-sea sediment, as millimeter to centimeter aggregates dispersed in the sediment, or as a coating on sedimentary particles or nodules. This study examines several factors to elucidate formation mechanisms of a particular iron-rich smectite and its potential transformation to glauconite. The study combines a detailed mineralogical investigation on natural samples and a chemical modeling approach to assess mineralogical reactions and pathways.Transmission electron microscopy (TEM) observations and analytical electron microscopy (TEM-AEM) analyses were conducted on microtomed samples of millimeter- to centimeter-long green grains. These grains are widespread in pelagic calcareous sediment from the Costa Rica margin. They are composed of pyrites that are partially dissolved and are surrounded by amorphous or very poorly crystallized iron-rich particles. Iron-rich montmorillonite grows from an amorphous precursor and its formation requires the input of Si, O, Mg, K, Na and Ca; our results suggest that these inputs are supported by the dissolution of sedimentary phases such as volcanic glasses, siliceous fossils and silicates.Thermodynamic modeling of fluid-sediment interactions was conducted with the geochemical computer code PhreeqC, using mineralogical and pore fluid compositions from sediment samples and calculated estimates for thermodynamic constants of smectites that are not maintained by the computer code. Simulations confirm the possibility that the green grains are the product of pyrite alteration by seawater under oxidizing conditions. The extent of smectite production is controlled by the kinetics of pyrite dissolution and fluid migration. The absence of aluminum in the Costa Rica margin system explains the formation of an iron-rich montmorillonite instead of glauconite, whereas the presence of calcite that buffers the system explains the formation of an iron-rich montmorillonite instead of iron oxides.  相似文献   

6.
The detection of phyllosilicates and sulfates on Mars has revealed a complex aqueous history which suggests distinct geochemical environments separated temporally and spatially. Recent observations by MRO CRISM in Mawrth Vallis have shown that phyllosilicate deposits exhibit a specific stratigraphy, which remains incompletely understood. Moreover, MER Spirit has evidenced association between phyllosilicates, amorphous silica and sulfates. We investigated the hypothesis that these parageneses resulted from the acidic weathering of older phyllosilicate deposits. We exposed nontronite (Fe-rich smectite), montmorillonite (Al-rich smectite) and kaolinite to H2SO4 solutions at pH 0, 2 and 4, and at a temperature of 60 °C. After the acid treatment, a combination of mineralogical techniques was used to assess the degree of alteration of the three phyllosilicate minerals. XRF, XRD and ESEM measurements show that nontronite was the most unstable when acid leached, followed by montmorillonite and then kaolinite. Progressive acidic leaching of nontronite leads to alteration of the phyllosilicate to amorphous silica, along with Fe-sulfate and anatase, and the formation of an acidic Al,Fe-rich solution. Alteration of montmorillonite resulted in the formation of Fe-, Al-, Ca- and Mg-sulfates, and a Al-rich leaching solution. Comparatively, leaching of kaolinite resulted in the formation of Al-sulfates and a Al-rich solution as well, with only slight alteration of the primary mineralogical features. The effects of acid leaching of the phyllosilicates were also observed in NIR reflectance spectra, allowing a comparison with CRISM spectra from Mawrth Vallis. Based on our results, we propose a new model where acid leaching of mixed phyllosilicate deposits leads to kaolinite overlaying montmorillonite, which in turn caps Fe,Mg-smectites. Leaching of cations and subsequent evaporation leads to sulfate deposits, as supported by geochemical modeling, while amorphous silica remains as a residue. Depending on the intensity (pH) and length of exposure of acidic leaching, our model can explain the stratigraphic distribution of phyllosilicates, and the association of sulfates, silica and smectites.  相似文献   

7.
Clay minerals and methanogens are ubiquitous and co-exist in anoxic environments, yet it is unclear whether methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of methanogen Methanosarcina barkeri to reduce structural Fe(III) in iron-rich smectite (nontronite NAu-2) and the relationship between iron reduction and methanogenesis were investigated. Bioreduction experiments were conducted in growth medium using three types of substrate: H2/CO2, methanol, and acetate. Time course methane production and hydrogen consumption were measured by gas chromatography. M. barkeri was able to reduce structural Fe(III) in NAu-2 with H2/CO2 and methanol as substrate, but not with acetate. The extent of bioreduction, as measured by the 1,10-phenanthroline method, was 7-13% with H2/CO2 as substrate, depending on nontronite concentration (5-10 g/L). The extent was higher when methanol was used as a substrate, reaching 25-33%. Methanogenesis was inhibited by Fe(III) reduction in the H2/CO2 culture, but enhanced when methanol was used. High charge smectite and biogenic silica formed as a result of bioreduction. Our results suggest that methanogens may play an important role in biogeochemical cycling of iron in clay minerals and may have important implications for the global methane budget.  相似文献   

8.
The distribution of iron atoms in the octahedral sheet of a series of dioctahedral smectites with varying unit-cell composition and iron content was investigated by Fe K-edge XAS spectroscopy. First-step analysis reveals that the patterns corresponding to backscattering by atoms located between 3 and 4 Å from the absorbing atom are very sensitive to the relative amount of light (Si, Al, Mg) and heavy (Fe) atoms. Detailed modelling of this domain then provides valuable information on the number of iron atoms surrounding octahedral iron. By comparing the number of iron neighbours deduced from EXAFS with that determined from unit-cell composition assuming a statistical distribution, three groups of montmorillonites can be distinguished: (1) clay samples from Wyoming display an ordered distribution of iron atoms; (2) clay samples from Georgia, Milos, China and Washington exhibit a close to random distribution of iron atoms; (3) clay samples from North Africa, Germany, Texas and Arizona display extensive iron clustering. These results complement previously obtained IR results and show that the combination of these two spectroscopic techniques could provide an additional crystal-chemistry-based framework for typological analysis of montmorillonite deposits.  相似文献   

9.
Clay minerals are ubiquitous on epigeosphere, especially in soils and sediments where microbes thrive. The clay-microbe interactions are common in these geological media and greatly contribute to accelerating the mineral transformation process, e.g. the illitization of nontronite (a Fe-rich smectite) catalyzed by microbes under anoxic atmosphere in 2 weeks. However, few has considered montmorillonite, a Fe-poor smectite more typical in natural environments than nontronite. This study therefore focuses on the interaction between montmorillonite and bacteria under conditions relevant to those in natural soils and sediments.  相似文献   

10.
Clay minerals associated with intra-volcanic bole horizons of varied colours and thicknesses contain montmorillonite, halloysite and kaolinite, show distinct microstructures and microaggregates. In kaolinite, Fe3+ ions substitute for Al3+ at octahedral sites. Most of these clays are dioctahedral type, show balance between net layer and interlayer charges. The interstratified illite — smectite (I/S) mixed layers containing variable proportions of montmorillonite. Illite contains sheet-like, well oriented microaggregates. The parallel stacks of chlorite sheets show chlorite/smectite (C/S) mixed layers. Progressive enrichment of Fe and depletion of Al ions with the advancement of kaolinization process is observed. High order of structural and compositional maturity observed in these bole clays, indicate long hiatus between the two volcanic episodes.  相似文献   

11.
Several designs proposed for high-level nuclear waste (HLW) repositories include steel waste canisters surrounded by montmorillonite clay. This work investigates montmorillonite stability in the presence of native Fe, magnetite and aqueous solutions under hydrothermal conditions. Two series of experiments were conducted. In the first, mixtures of Na-montmorillonite, magnetite, native Fe, calcite, and NaCl solutions were reacted at 250 °C, Psat for between 93 and 114 days. In the second series, the starting mixtures included Na-montmorillonite, native Fe and solutions of FeCl2 which were reacted at temperatures of 80, 150, and 250 °C, Psat, for 90-92 days. Experiments were analysed using XRD, FT-IR, TEM, ICP-AES, and ICP-MS. In the first series of experiments, native Fe oxidised to produce magnetite and the starting montmorillonite material was transformed to Fe-rich smectite only when the Fe was added predominantly as Fe metal rather than Fe oxide (magnetite). The Fe-rich smectite was initially Fe(II)-rich, which oxidised to produce an Fe(III)-rich form on exposure to air. The expansion of this material on ethylene glycol solvation was much reduced compared to the montmorillonite starting material. TEM imaging shows that partial loss of tetrahedral sheets occurred during transformation of the montmorillonite, resulting in adjacent layers becoming H-bonded with a 7 Å repeat. The reduced swelling property of the Fe-smectite product may be due predominantly to the structural disruption of smectite layers and the formation of H-bonds. Solute activities corresponded to the approximate stability field calculated for hypothetical Fe(II)-saponite. In the second series of experiments, significant smectite alteration was only observed at 250 °C and the product contained a small proportion of a 7 Å repeat structure, observable by XRD. In these experiments, solute activities coincide with berthierine. The experiments indicate that although bentonite is still a desirable choice of backfill material for HLW repositories, some loss of expandability may result if montmorillonite is altered to Fe-rich smectite at the interface between steel canisters and bentonite.  相似文献   

12.
A quantitative study was performed to understand how Fe(III) site occupancy controls Fe(III) bioreduction in nontronite by Shewanella putrefaciens CN32. NAu-1 and NAu-2 were nontronites and contained Fe(III) in different structural sites with 16 and 23% total iron (w/w), respectively, with almost all iron as Fe(III). Mössbauer spectroscopy showed that Fe(III) was present in the octahedral site in NAu-1 (with a small amount of goethite), but in both the tetrahedral and the octahedral sites in NAu-2. Mössbauer data further showed that the octahedral Fe(III) in NAu-2 existed in at least two environments- trans (M1) and cis (M2) sites. The microbial Fe(III) reduction in NAu-1 and NAu-2 was studied in batch cultures at a nontronite concentration of 5 mg/mL in bicarbonate buffer with lactate as the electron donor. The unreduced and bioreduced nontronites were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and transmission electron microscopy (TEM). In the presence of an electron shuttle, anthraquinone-2,6-disulfonate (AQDS), the extent of bioreduction was 11%-16% for NAu-1 but 28%-32% for NAu-2. The extent of reduction in the absence of AQDS was only 5%-7% for NAu-1 but 14%-18% for NAu-2. The control experiments with heat killed cells and without cells did not show any appreciable reduction (<2%). The extent of reduction in experiments performed with a dialysis membrane to separate cells from clays (without AQDS) was 2%-3% for NAu-1 but 5%-7% for NAu-2, suggesting that cells probably released an electron shuttling compound and/or Fe(III) chelator. The reduction rate was also faster in NAu-2 than that in NAu-1. Mössbauer data of the bioreduced nontronite materials indicated that the Fe(III) reduction in NAu-1 was mostly from the presence of goethite, whereas the reduction in NAu-2 was due to the presence of the tetrahedral and trans-octahedral Fe(III) in the structure. The measured aqueous Fe(II) was negligible. As a result of bioreduction, the average nontronite particle thickness remained nearly the same (from 2.1 to 2.5 nm) for NAu-1, but decreased significantly from 6 to 3.5 nm for NAu-2 with a concomitant change in crystal size distribution. The decrease in crystal size suggests reductive dissolution of nontronite NAu-2, which was supported by aqueous solution chemistry (i.e., aqueous Si). These data suggest that the more extensive Fe(III) bioreduction in NAu-2 was due to the presence of the tetrahedral and the trans-octahedral Fe(III), which was presumed to be more reducible. The biogenic Fe(II) was not associated with biogenic solids or in the aqueous solution. We infer that it may be either adsorbed onto surfaces of nontronite particles/bacteria or in the structure of nontronite. Furthermore, we have demonstrated that natural nontronite clays were capable of supporting cell growth even in medium without added nutrients, possibly due to presence of naturally existing nutrients in the nontronite clays. These results suggest that crystal chemical environment of Fe(III) is an important determinant in controlling the rate and extent of microbial reduction of Fe(III) in nontronite.  相似文献   

13.
The low-temperature synthesis of clay minerals is possible through the aging of freshly prepared hydroxide—silica precipitates. The rapid synthesis of nontronite is only possible at surface temperatures under reducing conditions. Under oxidizing conditions, pure Fe(III)- or pure Al-smectite minerals could not be synthesized at low temperatures. It is only from Fe(II)-containing solutions that nontronite and lembergite, the di-[Fe(III)] and tri-[Fe(II)] octahedral three-layer silicates, are built up in several days at low temperatures. The presence of Fe(II) enables an octahedral layer of the brucite—gibbsite type to be formed. These are necessary for the bidimensional orientation of SiO4 tetrahedrons, leading to clay-mineral formation. The Fe2+ and/or Mg2+ ions are necessary for the formation of the Al3+- and Fe3+-containing three-layer silicate minerals.Under reducing diagenetic conditions, the Fe contents in recent sediments are sufficient to build up Al-rich three-layer minerals under both fresh-water and salt-water conditions.  相似文献   

14.
The smectite-to-chlorite conversion is investigated through long-duration experiments (up to 9 years) conducted at 300 °C. The starting products were the Wyoming bentonite MX80 (79 % smectite), metallic iron and magnetite in contact with a Na–Ca chloride solution. The predominant minerals in the run products were an iron-rich chlorite (chamosite like) and interstratified clays interpreted to be chlorite/smectite and/or corrensite/smectite, accompanied by euhedral crystals of quartz, albite and zeolite. The formation of pure corrensite was not observed in the long-duration experiments. The conversion of smectite into chlorite over time appears to take place in several steps and through several successive mechanisms: a solid-state transformation, significant dissolution of the smectite and direct precipitation from the solution, which is over-saturated with respect to chlorite, allowing the formation of a chamosite-like mineral. The reaction mechanisms are confirmed by X-ray patterns and data obtained on the experimental solutions (pH, contents of Si, Mg, Na and Ca). Because of the availability of some nutrients in the solution, total dissolution of the starting smectite does not lead to 100 % crystallization of chlorite but to a mixture of two dominant clays: chamosite and interstratified chlorite/smectite and/or corrensite/smectite poor in smectite. The role of Fe/(Fe + Mg) in the experimental medium is highlighted by chemical data obtained on newly formed clay particles alongside previously published data. The newly formed iron-rich chlorite has the same composition as that predicted by the geothermometer for diagenetic to low-grade metamorphic conditions, and the quartz + Fe-chlorite + albite experimental assemblage in the 9-year experiment is close to that fixed by water–rock equilibrium.  相似文献   

15.
Experimental batch and miscible-flow cultures were studied in order to determine the mechanistic pathways of microbial Fe(III) respiration in ferruginous smectite clay, NAu-1. The primary purpose was to resolve if alteration of smectite and release of Fe precedes microbial respiration. Alteration of NAu-1, represented by the morphological and mineralogical changes, occurred regardless of the extent of microbial Fe(III) reduction in all of our experimental systems, including those that contained heat-killed bacteria and those in which O2, rather than Fe(III), was the primary terminal electron acceptor. The solid alteration products observed under transmission electron microscopy included poorly crystalline smectite with diffuse electron diffraction signals, discrete grains of Fe-free amorphous aluminosilicate with increased Al/Si ratio, Fe-rich grains, and amorphous Si globules in the immediate vicinity of bacterial cells and extracellular polymeric substances. In reducing systems, Fe was also found as siderite. The small amount of Fe partitioned to the aqueous phase was primarily in the form of dissolved Fe(III) species even in the systems in which Fe(III) was the primary terminal electron acceptor for microbial respiration. From these observations, we conclude that microbial respiration of Fe(III) in our laboratory systems proceeded through the following: (1) alteration of NAu-1 and concurrent release of Fe(III) from the octahedral sheets of NAu-1; and (2) subsequent microbial respiration of Fe(III).  相似文献   

16.
Iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial. Our analysis of published data identifies end‐member compositions and mixtures and allows us to recognize controls of formation of different mineral species. The compositional fields of glaucony, Fe‐illite, Fe–Al smectites are determined in the M+/4Si vs. Fe/Sum of octahedral cations (M+ = interlayer charge). Solid solutions could exist between these phases. The Fe–Al and Fe‐rich clay minerals form two distinct solid solutions. The earliest phases to be formed are Fe–Al smectites or berthierine depending on the sedimentation rate. Reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments. The Fe is incorporated first in pyrite and then in silicates after oxidation. Potassium ions diffuse from the sea‐water–sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe‐illite or glauconite or a mineral assemblage (berthierine–nontronite) according to the available Al ion amounts in the microsystem. Mixed‐layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Despite the common belief of their value as palaeoenvironment indicators, these minerals can form in a variety of environments and over a period of millions of years during sediment burial.  相似文献   

17.
The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes. This bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work, the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic (white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo deposit are described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites, while mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical composition of smectites reflects the different nature of the parent rocks, in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mg contents and a mean structural formulae corresponding to Fe-Mg-rich beidellite and/or to an intermediate smectite member between beidellite and saponite. In addition, chemical composition and textural features of corrensite-like clay minerals in relation to Fe-Mg-rich smectites in the samples have also been studied, suggesting that the former seems to be formed from Fe-Mg-rich smectites. The presence of corrensite in the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration process occurred after bentonite formation, which transformed Fe-Mg-rich smectites into corrensite. This transformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, which produced a temperature increase in the geological media and a supply of Fe-Mg-rich solutions. These physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrón de Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over-packs and the bentonite. These circumstances would favour the transformation of the candidate Al-rich smectites into Fe-Mg-rich smectites and corrensite, as steps prior to formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectites would disappear and the clayey barrier would fail.  相似文献   

18.
Iron-rich clay minerals are abundant in the natural environment and are an important source of iron for microbial metabolism. The objective of this study was to understand the mechanism(s) of enhanced reduction of Fe(III) in iron-rich 2:1 clay minerals under sulfate-reducing conditions. In particular, biogenic reduction of structural Fe(III) in nontronite NAu-2, an Fe-rich smectite-group mineral, was studied using a Desulfovibrio spp. strain G-11 with or without amended sulfate. The microbial production of Fe(II) from NAu-2 is about 10% of total structural Fe(III) (30 mM) when Fe(III) is available as the sole electron acceptor. The measured production of Fe(II), however, can reach 29% of the total structural Fe(III) during sulfate reduction by G-11 when sulfate (50 mM) is concurrently added with NAu-2. In contrast, abiotic production of Fe(II) from the reaction of NAu-2 with Na2S (50 mM) is only ca. 7.5% of the total structural Fe(III). The enhanced reduction of structural Fe(III) by G-11, particularly in the presence of sulfate, is closely related to the growth rate and metabolic activities of the bacteria. Analyses by X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy reveal significant changes in the structure and composition of NAu-2 during its alteration by bacterial sulfate reduction. G-11 can also derive nutrients from NAu-2 to support its growth in the absence of amended minerals and vitamins. Results of this study suggest that sulfate-reducing bacteria may play a more significant role than previously recognized in the cycling of Fe, S, and other elements during alteration of Fe-rich 2:1 clay minerals and other silicate minerals.  相似文献   

19.
Iron-containing smectites are not affected significantly by Mg-containing dilute sulphide solutions. Concentrated solutions reduce most of the Fe in montmorillonites; Fe is partially extracted from nontronite with concomitant disintegration of the structure. It is concluded that direct Mg → Fe exchange in clays seems improbable, but that depletion of Mg in interstitial waters of anoxic sediments may be due to reaction of Mg-containing solutions with partially disintegrated clay.  相似文献   

20.
To assess the dynamics of microbially mediated U-clay redox reactions, we examined the reduction of iron(III)-rich nontronite NAu-2 and uranium(VI) by Shewanella oneidensis MR-1. Bioreduction experiments were conducted with combinations and varied concentrations of MR-1, nontronite, U(VI) and the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Abiotic experiments were conducted to quantify U(VI) sorption to NAu-2, the reduction of U(VI) by chemically-reduced nontronite-Fe(II), and the oxidation of uraninite, U(IV)O2(s), by nontronite-Fe(III). When we incubated S. oneidensis MR-1 at lower concentration (0.5 × 108 cell mL−1) with nontronite (5.0 g L−1) and U(VI) (1.0 mM), little U(VI) reduction occurred compared to nontronite-free incubations, despite the production of abundant Fe(II). The addition of AQDS to U(VI)- and nontronite-containing incubations enhanced both U(VI) and nontronite-Fe(III) reduction. While U(VI) was completely reduced by S. oneidensis MR-1 at higher concentration (1.0 × 108 cell mL−1) in the presence of nontronite, increasing concentrations of nontronite led to progressively slower rates of U(VI) reduction. U(VI) enhanced nontronite-Fe(III) reduction and uraninite was oxidized by nontronite-Fe(III), demonstrating that U served as an effective electron shuttle from S. oneidensis MR-1 to nontronite-Fe(III). The electron-shuttling activity of U can explain the lack or delay of U(VI) reduction observed in the bulk solution. Little U(VI) reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II), suggesting that biologic U(VI) reduction drove U valence cycling in these systems. Under the conditions used in these experiments, we demonstrate that iron-rich smectite may inhibit or delay U(VI) bioreduction.  相似文献   

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