Results 1 - 10 of 2004
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[en] The MTT (Mean Transit Time) of a catchment without significant surface flow is normally taken to be the sum of mean residence time in the unsaturated zone and mean residence time in the saturated zone. However, the Chalk is a multi-porosity limestone aquifer, with a microporous matrix. This means that the movement of water through the Chalk can occur in complex ways, making the prediction of MTT far from straightforward. Although the Chalk is a regionally-important aquifer, no study of catchment MTT has yet been published. The present study is based on the catchment of the River Lambourn in Berkshire, UK, with an area of 235 km2. Interfluve areas rarely rise above 200 m asl (above sea level), whereas river elevation at the foot of the catchment is ∼50 m asl. Mean annual precipitation is 731 mm. The thickness of the Chalk unsaturated zone reaches a maximum of over 100 m at the water divide at the top of the escarpment on the northern flank of the catchment.
[en] In this study, thermoluminescence (TL) properties of the calcite extracted from natural sand which is used in making roasted chickpeas were investigated. And also the effects of different thermal treatments on thermoluminescence glow curve were observed. Two distinct TL peaks were observed at ∼130 °C and ∼230 °C. The annealing of sample, especially at 900 °C, causes a huge enhancement in sensitization of TL. Linearity in dose response is observed for the values up to 0.6 kGy and above 0.6 kGy linearity is not preserved and dose response becomes sublinear. The best reproducibility is obtained when the samples are annealed between 400°C and 600 °C. - Highlights: • The natural sand sample used in making roasted chickpea shows thermoluminescence properties. • Annealing at 900 °C for about 15 min gives best TL output. • A good sensitization of about 70 factor was observed in annealed samples when they were compared with no annealed samples. • At doses lower than 0.6 kGy, dose response is linear and sublinear at doses higher than 0.6 kGy. • The best reproducibility is obtained when the samples are annealed between 400 °C and 600 °C
[en] In an effort to evaluate the reliability of pole-figure measurements, a sample of experimentally deformed polycrystalline calcite was circulated among nine neutron diffraction facilities. This report compares results of the project both in terms of experimental pole figures and three-dimensional orientation distributions. In general textures agree very closely. For pole figures with strong diffraction intensities, standard deviations from the mean are 0.04-0.06 m.r.d. (multiples of a random distribution) with a spread of maxima values of 0.18 m.r.d. The spread is considerably larger for pole figures with weak diffraction intensities (0.37 m.r.d. for 110) and so are standard deviations (0.20 m.r.d.). For weak diffraction peaks one-dimensional position-sensitive detectors have an advantage over single-tube detectors. Two-dimensional position-sensitive detectors combined with time of flight (TOF) offer new possibilities but data processing needs to be improved. Greater care must be devoted to angle conventions: Four of nine pole figures were inverted. This study can provide a basis for quantitative texture analysis and estimation of experimental uncertainties. (orig.)
[en] The influence of milling on the transformations of the calcium carbonate phase has been studied using a laboratory centrifuge mill as well as a high energy planetary mill for grinding. The second one was fitted with two motors for freely selecting the translation and rotation movements of the grinding jars and therefore to vary the relative contribution of the shear and impact forces. The transformation phase of the calcite and aragonite has been observed although it is not thermodynamically aided. This transformation is helped by the application of shear forces more than by the energy released by the impact of the balls against the grinding material. A mechanism is proposed that explains this behavior (CW)
[en] Complete text of publication follows. Magnetic susceptibility and its anisotropy (AMS) are sensitive indicators for trace amounts of paramagnetic impurities and second-phase minerals in rocks that display diamagnetic bulk susceptibility. To illustrate this relationship a set of highly strained calcite mylonites has been collected from the overturned limb (shear zone) of the Morcles Nappe (Helvetic Alps, southwestern Switzerland). The sample set consists of white and gray fine-grained calcite mylonites (Upper and Lower Urgonian) with less than 10 vol% of secondary phase content and varying amounts of paramagnetic Fe and Mn impurities. The AMS of these specimens are studied using a combination of low- and high-field magnetic susceptibility measurements. In addition, high-field AMS measurements are performed at 77 K. The use of these methods illustrates a strong relationship between the magnetic susceptibility, the development of crystallographic preferred orientation (CPO) of calcite and the Fe plus Mn impurity content. The bulk magnetic susceptibility and AMS varies systematically according to the amount of Fe and Mn. At room-temperature the AMS results from a combination of the diamagnetic and paramagnetic sub-fabrics, whereas at 77 K the paramagnetic sub-fabric is dominant. The k1 and k3 axes invert positions when comparing the AMS at room temperature and 77 K. The degree of anisotropy is shown to be related to bulk susceptibility, which in itself is directly dependent on the amount of Fe incorporated into the calcite's lattice, and the strength of the calcite's CPO. Our results indicate that AMS can be used as a sensitive tool for studying the relationship between trace element chemistry and deformation.
[en] This paper discusses the mineralogy and genesis of the Gogi uranium deposit, located in the Bhima Basin, near village Gogi, Gulbarga district, Karnataka India. The mineralization is closely associated with the Gogi-Kurlagere fault, occurring as veins and veinlets, in basement granitoids and the overlying Shahabad limestones. The deposit is of complex mineralogy and is unique. The main radioactive phases identified are pitchblende, coffinite, U adsorbed on organic matter, clay, sericite and chlorite. It also occurs as labile uranium along grain boundaries and as U-Ti and U-Fe-Si complexes. Mineralization is polyphase, hydrothermal (epithermal), epigenetic, fault/structurally controlled vein type. (author)
[en] The principal mechanisms of the nasturan replacement by calcite -intrametasomatism, frontal metasomatism, dispersive metasomatism, and transformative metasomatism - are discussed in terms of G.L. Pospelov's (1973) concept. The main chemical condition required by the process is an oxidized environment, in which the tetravalent uranium of pitchblende or transitional reduced phases (coffinite) oxidizes to yield readily soluble uranyl compounds. The latter are replaced by calcite
[en] Samples of brazilian calcite, exposed to gamma radiation in laboratory and heated at constant rate of 2.70 C/s, showed three glow peaks at 150, 250 and 350 0C in their thermoluminescent emission curves. The analysis of these peaks, using different models, indicated that they follow a second order kinetics; it has been obtained, for the activation energy, 1.3, 1.5 and 1.7 eV, and, for the pre-exponential factors, 8.1 x 10 14, 6.8 x10 13 and 2.4 x 1012 s-1. Although the total thermoluminescent emission has stayed constant, the relative height of glow peaks has changed with the temperature of annealing in the range of 400 to 700 0C. Exposed samples were also illuminated with ultraviolet light and the resultant curves showed partial or total bleaching or some glow peaks and the growth of peaks at lower temperatures. Samples of virgin calcite, submited to increasing exposures of gamma rays, showed a corresponding enhancement of the optical absorption bands in the range of 25000 to 47000 cm-1 A subsequent illumination of these samples with ultraviolet light produced a decrease of the optical absorption bands at the same range. (author)
[pt]A cruva de emissao termoluminescente da calcita brasileira exposta a radiacao gama em laboratorio, e aquecida a partir da temperatura ambiente com uma taxa linear de 2,70C/s, apresentou tres picos as temperaturas de 150, 250 e 3500C. Uma analise desses picos utilizando diferentes modelos mostrou que eles obedecem a uma cinetica de segunda ordem e os valores obtidos para as energias de ativacao foram 1,3; 1,5; 1,7 eV e para os fatores pre-exponenciais 8,1x1014s-1, 6,8x1013 s-1, 2,4x1012 s-1. A altura relativa desses picos mudou com a temperatura de tratamento termico no intervalo de 400 a 7000C, porem a emissao termoluminescente total permaneceu constante. Foi verificado que a radiacao ultravioleta tanto preenche quanto esvazia armadilhas com taxas que dependem do grau de preenchimento de cada pico. Medidas de absorcao optica de amostras de calcita virgem irradiadas com radiacao gama com exposicoes crescentes apresentaram um correspondente crrescimento da abosrbancia nas bandas localizadas no intervalo entre 25000 e 47000 cm-1. Uma posterior iluminacao com radiacao ultravioleta produziu sobre essas mesmas bandas uma diminuicao que se acentua com a exposicao. (autor)