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1.
《Applied Geochemistry》1998,13(6):751-765
The origin of 3 types of point defects (A-, A′- and B-centers) in kaolinite, due to natural irradiation and detected by electron paramagnetic resonance spectroscopy (EPR), has been demonstrated by artificial irradiation. The potential use of tracing the dynamics of the transfer of radionuclides through A-centers (i.e. the most stable centers) was qualitatively tested on different low-temperature alteration systems, some associated with U-concentrations. This paper proposes a quantitative approach to the reconstruction of the past migration of radionuclides by dosimetry of A-centers. With this aim in mind, the efficiency of α- and γ-radiations to produce A-centers was determined by experimental irradiation. Parameters extracted from A-center growth curves, together with their relationship with a parameter describing the degree of order of kaolinite, permitted (i) a definition to be made of the dose range in which a given kaolinite could be used as a dosimeter and (ii) the quantitative derivation of U-concentration from the cumulative dose (paleodose) of kaolinites. This was achieved by a formalism that accounted for the contribution of natural radiosources to the production of A-centers. The formalism was applied to the Nopal I U-deposit (Chihuhua, Mexico), considered as a natural analogue of a high level nuclear waste repository. Irrespective of the scenario considered, in terms of kaolinite age and of degree of isotopic disequilibrium in the system, A-center dosimetry permitted the determination of past occurrences of U which were several orders of magnitude higher than the present-day measured U-concentrations. Furthermore, this approach also provided evidence for several previous episodes of U-migration. EPR spectroscopy is thus a unique tool for the quantitative, indirect assessment of past radionuclide migration in the geosphere and kaolinite is a reliable in-situ dosimeter.  相似文献   

2.
The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that ∼80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the α-radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates.  相似文献   

3.
Radiation effects on kaolinite were investigated using He+ ions of 1.5 MeV at radiation doses up to 4.3 × 108 Gy, which are comparable to the doses expected for clay barriers in high-level nuclear waste repositories. The concentration of paramagnetic radiation-induced defects in kaolinite reaches 2 × 1016 spins/mg (400 at. ppm), as determined by electron paramagnetic resonance spectroscopy. The broadening of X-ray diffraction patterns and transmission infrared (IR) absorption bands is mostly related to the structural strain induced by radiation-induced point defects. The broadening of IR absorption spectra is analyzed using an autocorrelation approach and is related to a change in the distribution of vibrational frequencies due to crystal heterogeneities. We theoretically analyze how the effective dielectric properties of kaolinite samples depend on macroscopic parameters and how irradiation can modify some of them. Irradiation leads to an increase in the electronic polarizability of kaolinite particles, related to the accumulation of radiation-induced electronic point defects.  相似文献   

4.
U-deposit hosted in hydrothermally altered tuffs in Mexico, together with weathering profiles from Cameroon were studied as natural analogues of radionuclide release and migration. Using petrological and spectroscopic methods (infrared and electron paramagnetic resonance), we have distinguished successive secondary mineral parageneses and the behaviour of radionuclides.

In the U-deposit, the mineral parageneses show that uranium migration is mainly controlled by the redox potential and silica activity of the altering solutions. The high silica content of the solutions is caused by the intense alteration of volcanic rocks. Two types of secondary clay mineral parageneses are evidenced: a kaolinization, intense where uranium is accumulated in the welded tuffs, and a smectitization mainly developed in the underlying weakly welded tuffs.

Several types of kaolinite have been defined according to their genesis (fillings in fissures and feldspar pseudomorphs), their location relative to a breccia pipe where uranium has accumulated (core and rim of the pipe; surrounding rhyolitic tuffs), and particle morphology, structural order and substitutional Fecontent. It is shown that the variations of the concentration of paramagnetic defect centres, always more than ten times as important than those measured in weathering kaolinites, are only correlated to the location of the kaolinites. The highest values correspond to the breccia pipe kaolinites, e.g. kaolinites located in the uranium accumulation zones. Moreover, one or two main defects centres are detected depending on the intimate association of kaolinites with uranium-bearing minerals. Besides, in weathering kaolinites from U-depleted laterites, defect centre concentrations are correlated to the total Fe203 content in bulk samples. This means that the defect centre acts as a memory of the travel of uranium when this element was sorbed onto iron gels in the first stage of weathering.

It is concluded that paramagnetic defect centres in kaolinites might allow an efficient fingerprint of successive irradiations in the natural analogues under study and could be an useful tool to control radionuclides migration through kaolinite-containing clayey materials such as those used for waste repository.

A better understanding of radiation efficiency as well as accurate dose-ratekaolinite-containing clayey materials such as those used for waste reposit estimation are needed for a quantitative tracing of the migration ofA better understanding of radiation efficiency as well as accurate dose-ratekaolin radionuclide elements. With this aim, a simulation has been undertaken withestimation are needed for a quantitative tracing of the migration of various radiations sources. We have determined for each irradiation the parameters of the paramagnetic centres created in order to understand the way they are forming. The knowledge of the parameters governing the formation and the stability of the radiation centres in kaolinites allow to use this mineral as a natural dosimeter.  相似文献   


5.
This study presents the first unequivocal identification of natural radiation-induced defects in illites. Middle Proterozoic illites related to unconformity-type uranium deposits of Canada and Australia were studied using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. The saturation behaviour of EPR spectra as a function of power demonstrates that native defects of illites are different from those known in other clays as kaolinite, dickite or smectite. Q-band spectra indicate the presence of several––at least two––native defects. The EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g ∥ = 2.032 and g ⊥ = 1.993. The corresponding defect is named as Ai center. The study of oriented specimen confirms the strong anisotropy, and shows that the main defect has its g ∥ component perpendicular to the (ab) plane of illite. These defects in illite correspond to electron holes located on oxygen atoms of the structure and likely associated to Si, according to the lack of hyperfine structure. The Ai center in illite has similar EPR parameters to the A center in kaolinite and dickite. The isochronal annealing data suggest that illite can be used as a dosimeter in the geosphere. However, the determination of half-life and activation energy of the Ai center requires additional work.  相似文献   

6.
煤系地层中高岭石的形态-成因类型   总被引：3，自引：0，他引：3  
大量的透射电子显微镜(TEM)观察表明:煤系地层中的主要粘士矿物——高岭石有独特的形貌特征。这些特征反映物源、沉积环境以及成岩后生作用的特点。本文将矿物形态与成煤各阶段的地质环境相联系,把煤系地层中高岭石划分为碎屑状高岭石、胶凝状高岭石、玻屑状高岭石、鳞片状高岭石四种形态-成因类型。它们相应的成因类型为沉积成因、胶体成因、火山灰蚀变成因、成岩变质成因。认识和研究高岭石矿物的形态与成因,在煤田地质学领域中有理论和实用意义。  相似文献   

7.
Electron spin resonance and infrared spectroscopic studies of lignite and ball clay from South Devon, and of extracts obtained from them by solvent fractionation, revealed similarities between corresponding organic components associated with both materials. All fractions exhibited a free radical resonance at g = 2.0037, which occurred with greatest intensity in the humic acids. Additional ESR features due to Fe3+, Mn2+ and VO2+ complexes were observed. Ferric ions give rise to resonances at g = 4.2 which have not been previously reported in the case of natural carbonaceous materials. It is shown that the paramagnetic species associated with the ball clay and lignite extracts do not significantly contribute to the observed ESR spectra of kaolinites, the latter being attributable to substituted Fe3+ ions and defect centres within the kaolinite lattice.  相似文献   

8.
Summary ?Sheet silicates of the serpentine–kaolin-group (serpentine, kaolinite, dickite, nacrite, halloysite), the talc–pyrophyllite-group (talc, pyrophyllite), the smectite-group (montmorillonite), and illite (as a mineral of the mica-group) were investigated to obtain information concerning their cathodoluminescence behaviour. The study included analyses by cathodoluminescence (CL microscopy and spectroscopy), electron paramagnetic resonance (EPR), X-Ray diffraction (XRD), scanning electron microscopy (SEM) and trace element analysis. In general, all dioctahedral clay minerals exhibit a visible CL. Kaolinite, dickite, nacrite and pyrophyllite have a characteristic deep blue CL, whereas halloysite emission is in the greenish-blue region. On the contrary, the trioctahedral minerals (serpentine, talc) and illite do not show visible CL. The characteristic blue CL is caused by an intense emission band around 400 nm (double peak with two maxima at 375 and 410 nm). EPR measurements indicate that this blue emission can be related to radiation induced defect centres (RID), which occur as electron holes trapped on apical oxygens (Si–O centre) or located at the Al–O–Al group (Al substituting Si in the tetrahedron). Additional CL emission bands were detected at 580 nm in halloysite and kaolinite, and between 700 and 800 nm in kaolinite, dickite, nacrite and pyrophyllite. Time-resolved spectral CL measurements show typical luminescence kinetics for the different clay minerals, which enable differentiation between the various dioctahedral minerals (e.g. kaolinite and dickite), even in thin section. Received December 3, 2001; revised version accepted February 27, 2002  相似文献   

9.
The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.  相似文献   

10.
夏  陈开惠  姬素荣 《地质科学》1979,14(4):322-329
江西省景德镇市高岭村是世界闻名的高岭土产地,高岭石矿物的定名即渊源于此。搞清该区高岭土的成因和矿物组成,对我国粘土矿物学的研究具有一定意义。1978年我们对高岭村地区的高岭土矿物作了进一步的分析研究,获得了一些新认识,报导出来供有关方面讨论。  相似文献   

11.
Interactions of iron (Fe) with the nitrogen (N) cycle have emerged and contain elements of abiotic and biological reactions. One such abiotic reaction which has received little study is the reactivity of NO2 ? and Fe(II) associated with a major clay mineral, kaolinite. The main objective of this study was to evaluate the reactivity of NO2 ? with Fe(II) added to kaolinite under anoxic conditions. Stirred batch reactivity experiments were carried out with 10 g L?1 kaolinite spiked with 25 and 100 µM Fe(II) at pH 6.45 in an anaerobic chamber. Approximately 500 µM NO2 ? was added to initiate the reaction with Fe(II)-loaded kaolinite. The rate of nitrite removal from solution was 2.4-fold slower in the high Fe(II) treatment when compared with the low Fe(II) treatment. A large portion of the NO2 ? removed from solution was confirmed to be reduced to N2O(g) in the Fe(II)-kaolinite slurries. However, NO2 ? reduction was also noticed in the presence of kaolinite-alone and to somewhat lesser extent in the presence of dithionite-citrate-bicarbonate (DCB)-treated kaolinite. Chemical extractions coupled with infrared spectroscopy suggest that Fe(III) oxide mineral impurities and structural Fe(III) in kaolinite may participate in NO2 ? removal from solution. Furthermore, a magnetite mineral was identified based on X-ray diffraction analysis of untreated kaolinite and DCB-treated kaolinite. Our findings reveal a novel pathway of NO2 ? transformation in the environment in the presence of Fe(II) associated (sorbed and impurity) with kaolinite.  相似文献   

12.
Previous research by our group (e.g., [Chem. Geol. 132 (1996) 25; Geochim. Cosmochim. Acta 64 (2000) 1363]) has shown that an aerobic Pseudomonas mendocina bacterium enhances Fe(hydr)oxide dissolution in order to obtain Fe under Fe-limited conditions. The P. mendocina is incapable of utilizing Fe as a terminal electron acceptor and requires several orders of magnitude lower Fe concentrations than do dissimilatory Fe reducing bacteria. The research reported here compared the effects of the P. mendocina on dissolution of well and poorly ordered Clay Minerals Society Source Clay kaolinites KGa-1b and KGa-2, respectively, under Fe-limited conditions. KGa-1b and KGa-2 contain 0.04 and 0.94 bulk wt.% Fe, respectively, and their surface Fe/Si atomic RATIOS=0.008 and 0.012. Following strong cleaning of the kaolinites in 5.8 M HCl at 85 °C, the surface Fe/Si atomic ratios decreased to 0.004 and 0.008, respectively. Both kaolinites also developed a Si-enriched surface precipitate upon strong cleaning.

Because the P. mendocina take up Fe, we could not measure Fe release from the kaolinite directly, but rather had to monitor it indirectly by comparing microbial populations sizes under Fe-limited growth conditions. We found that microbial growth on uncleaned, weakly cleaned, and strongly cleaned kaolinites increased with the amount of Fe readily available to organic ligands as estimated by dissolution in 0.001 M oxalate (pH 3). This suggests that it is the amount of readily accessible Fe that controls Fe acquisition and hence microbial growth. The trend is based on only a relatively small range of kaolinite Fe contents, and the research thus needs to be expanded to include kaolinites with a broader range of bulk and surface Fe concentrations.

Significant enhancement of Al release was observed in the presence of the bacteria, along with generally some enhancement of Si release. This enhancement of kaolinite dissolution could be related to an observed pH increase from 7–8 to 9 in the presence of the bacteria and/or to production of Al chelating agents. The P. mendocina produce a variety of organic exudates, including siderophores [Chem. Geol. 132 (1996) 25; Geomicrobiology (2001b)], and further studies into the effects of the siderophores on Al complexation and on kaolinite dissolution are ongoing.  相似文献   


13.
The Swedish Nuclear Fuel and Waste Management Company has recently submitted an application for a license to construct a final repository for spent nuclear fuel, at approximately 500?m depth in crystalline bedrock. Migration pathways through the geosphere barrier are geometrically complex, with segments in fractured rock, deformation zones, backfilled tunnels, and near-surface soils. Several simplifications of these complex migration pathways were used in the assessments of repository performance that supported the license application. Specifically, in the geosphere transport calculations, radionuclide transport in soils and tunnels was neglected, and deformation zones were assumed to have transport characteristics of fractured rock. The effects of these simplifications on the projected performance of the geosphere barrier system are addressed. Geosphere performance is shown to be sensitive to how transport characteristics of deformation zones are conceptualized and incorporated into the model. Incorporation of advective groundwater travel time within backfilled tunnels reduces radiological dose from non-sorbing radionuclides such as I-129, while sorption in near-surface soils reduces radiological doses from sorbing radionuclides such as Ra-226. These results help quantify the degree to which geosphere performance was pessimistically assessed, and provide some guidance on how future studies to reduce uncertainty in geosphere performance may be focused.  相似文献   

14.
运用X射线衍射和多重峰分离程序解析高岭石的结构缺陷   总被引：4，自引：0，他引：4  
姚林波  高振敏 《矿物学报》1996,16(2):132-140
本文尝试运用Ｘ射线衍射与多重峰分离方法研究高岭石的结构缺陷，重新评价了Ｈｉｎｃｋｌｅｙ（１９６３）结晶指数（ＨＩ），运用０２，１１区域对（１１１）和（１１１）峰的分峰结果，定义了新的结晶度指数：ＣＩ＝（Ｉ１１１＋Ｉ１１１）／Ｉ１１０，它完全以衍射峰的强度进行计算，较灵敏地反映出高岭石真实的结构信息，探讨了高岭石结晶度与高峰石对称性，有序度以及与高岭石Ｐｂ／３滑移缺陷的关系，结果证明高岭石结晶度越高  相似文献   

15.
大青山巨厚煤层夹矸中高岭石的显微特征及其成因意义   总被引：1，自引：0，他引：1  
本文采用多种电子显微技术,研究了内蒙古大青山煤田晚古生代巨厚煤层平矸－隐晶质高岭石泥岩。透射电镜下,晶体程序不一、形态复杂多变的玻屑状高岭石和盘旋状埃洛石;扫描电镜下,泥粒状高岭石和蠕虫状高岭石及及二者所构成的斑状结构等许多具成因意义的微有（０．１～１０－μｍ）显微特征,充分证明该夹矸层为火山灰蚀变成因（ｔｏｎｓｔｅｉｎ）。  相似文献   

16.
Natural radiation-induced defects were identified in specimens of sudoite (Al–Mg di-trioctahedral chlorite) related to unconformity-type uranium deposits at the base of the Athabasca Group (Saskatchewan, Canada), using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. X-band spectra indicate the presence of a main native defect, named the As-center, whose EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g // = 2,051 and g ⊥ = 2,005, and a secondary defect with apparent component g = 2,025. The study of oriented specimens shows that the main defect has its g // component perpendicular to the (ab) plane of sudoite. The As-center corresponds to an electron hole located on oxygen atoms of the structure and is likely associated with Si, according to the lack of hyperfine structure. The As-center in sudoite has EPR parameters similar to the A-center in kaolinite and dickite, and the Ai-center in illite. The saturation behavior of EPR spectra as a function of power demonstrates that native defects of sudoite are different from those known in other clays, such as kaolinite, dickite or smectite, but are similar to those of illite. The isochronal annealing data suggest that the main defect in sudoite is stable to more than 300°C. The corresponding defects characterized in sudoite may have the potential for tracing past radionuclide migration around unconformity-type uranium deposits.  相似文献   

17.
张慧  周安朝 《沉积学报》2000,18(4):515-520
借助于X光衍射仪、透射电子显微镜(TEM)和扫描电子显微镜(SEM)等手段,较详细地揭示了内蒙大青山晚古生代(C2-P1)煤系中降落火山灰的蚀变特征,以此来论述沉积环境对火山灰蚀变作用的影响。泥炭沼泽环境中,降落火山灰以原地直接高岭石化为主,残留较多的火山玻屑和晶屑形态,并具斑状结构;非泥炭沼泽环境中,降落火山灰以异地凝胶化-多种粘土矿物化为主,微层理、微层面等沉积岩特征明显。火山灰降落的环境不同,其蚀变作用和蚀变产物亦不同。本文对加深煤系火山事件沉积的认识和研究煤系高岭岩的成因机理颇有意义。  相似文献   

18.
 The kinetics and mechanism of hydrothermal formation of zeolite A from natural kaolinites have been studied using as starting materials two international kaolinite standards (KGa-1 and KGa-2 from Georgia, USA) exhibiting a different degree of stacking disorder. Precursors utilized for the synthesis were prepared by heating the kaolinites at 800 °C. Metakaolinite was also prepared from KGa-1 by thermal activation at 600 °C. The hydrothermal syntheses were accomplished by heating the samples in NaOH solutions at temperatures between 70 and 110 °C. The kinetic experiments were performed by time-resolved synchrotron powder diffraction in isothermal mode using a transmission geometry and an Image Plate detector. The results of the kinetic analysis are interpreted in the light of the structural state of the starting kaolinite, and of the temperature of activation of the precursor material. For kaolinite activated at high temperature the nucleation and crystallization of zeolite A is essentially independent of the defect density of the original kaolinite, and the thermal history of the precursor seems to be the main controlling parameter. The formation process of zeolite A from metakaolinite materials obtained at lower activation temperatures shows significantly faster reaction rates and lower apparent activation energies. This is again interpreted in the light of the short range inhomogeneities present in metakaolinite. As the reaction proceeds metastable zeolite A transforms into hydroxy-sodalite. Received April 18, 1996 / Revised, accepted September 27, 1996  相似文献   

19.
Determination of the mineral age based on the creation of radiation structure defects depends on the thermal stability of these defects. Accumulation of radiation defects in solids in cooling systems taking into account their time dependent first-order as well as second-order annihilation is analyzed theoretically. Second-order kinetics in comparison with first-order leads to increased duration of a transitional period and to a lower accumulation rate. The contribution of the transitional period is determined by the cooling rate of the geological system and by the production rate of structure defects. Resulting formulas are applied for dating rock-forming quartz from the Eldzhurtinsky granite massif (Great Caucasus, Russia) with the help of electron paramagnetic resonance. It is shown that the EPR age calculated with the formulas derived in this study in comparison with the results of the conventional additive dose technique is closer to the results of 40Ar/39Ar dating. Due to annihilation of radiation defects even at low temperature the EPR dating method can be applied to a limited time range. Received: 18 March 1997 / Revised, accepted: 28 July 1997  相似文献   

20.
高岭石的结晶有序度及其对纸张涂布粘浓度的影响   总被引：5，自引：0，他引：5  
周国平  林毓川 《矿物学报》1991,11(3):267-273
粘浓度是纸张涂布的一项重要指标,测定的变化范围从46.4—72％。用Hinckley方法测定了高岭石的结晶度指数,从0.57变化至1.40。当粘浓度>67％时,结晶度指数>1。粘浓度<67％,结晶度指数<1,有序高岭石有利于粘浓度提高。随着结构无序化,晶粒变小,晶片变薄,比表面增加,晶体边缘和角易破损,使粘浓度降低。结构无序化破坏了颗粒的均匀性,使颗粒长/厚比增加,粘浓度降低。高岭石八面体中Fe~(3+)的存在一定程度上影响了有序度。  相似文献   

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