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排序方式: 共有260条查询结果,搜索用时 15 毫秒
91.
Amélie Bordage Etienne Balan Johan P. R. de Villiers Robert Cromarty Amélie Juhin Claire Carvallo Georges Calas P. V. Sunder Raju Pieter Glatzel 《Physics and Chemistry of Minerals》2011,38(6):449-458
The oxidation state of vanadium in natural and synthetic Fe–Ti oxides is determined using high-energy resolution fluorescence-detected
X-ray absorption spectroscopy (HERFD-XAS). Eleven natural magnetite-bearing samples from a borehole of the Main Magnetite
Layer of the Bushveld Complex (South Africa), five synthetic Fe oxide samples, and three natural hematite-bearing samples
from Dharwar supergroup (India) are investigated. V K edge spectra were recorded on the ID26 beamline at the European Synchrotron Radiation Facility (Grenoble, France), and the
pre-edge features were used to determine the local environment and oxidation state of vanadium. In the case of the magnetite
samples (natural and synthetic), we show that vanadium is incorporated in the octahedral site of the spinel structure under
two oxidation states: +III and +IV. The variations of the pre-edge area are interpreted as various proportions in V3+ and V4+ (between 9.5 and 16.3% of V4+), V3+ being the main oxidation state. In particular, the variations of the V4+/V3+ ratio along the profile of the Main Magnetite Layer seem to follow the crystallization sequence of the layer. In the case
of the hematite samples from India, the pre-edge features indicate that vanadium is substituted to Fe and mainly incorporated
as V4+ (between 40 and 72% of V4+). We also demonstrate the potentiality of HERFD-XAS for mineralogical studies, since it can filter out the unwanted fluorescence
and give better resolved spectra than conventional XAS. 相似文献
92.
Karim Benzerara Jennyfer Miot Guillaume Morin Georges Ona-Nguema Feriel Skouri-Panet Céline Férard 《Comptes Rendus Geoscience》2011,343(2-3):160-167
Microorganisms can mediate the formation of minerals by a process called biomineralization. This process offers an efficient way to sequester inorganic pollutants within relatively stable solid phases. Here we review some of the main mechanisms involved in the mediation of mineral precipitation by microorganisms. This includes supersaturation caused by metabolic activity, the triggering of nucleation by production of more or less specific organic molecules, and the impact of mineral growth. While these processes have been widely studied in the laboratory, assessment of their importance in the environment is more difficult. We illustrate this difficulty using a case study on an As-contaminated acid mine drainage located in the South of France (Carnoulès, Gard). In particular, we explore the potential relationships that might exist between microbial diversity and mineral precipitation. The present review, far from being exhaustive, highlights some recent advances in the field of biomineralogy and provides non-specialists an introduction to some of the main approaches and some questions that remain unanswered. 相似文献
93.
Emmanuel Fritsch Etienne Balan Nadia Régina Do Nascimento Thierry Allard Marion Bardy Guilherme Bueno Sylvie Derenne Adolpho José Melfi Georges Calas 《Comptes Rendus Geoscience》2011,343(2-3):188-198
This article reports major results from collaborative research between France and Brazil on soil and water systems, carried out in the Upper Amazon Basin. It reveals the weathering processes acting in the partly inundated, low elevation plateaus of the Basin, mostly covered by evergreen forest. Our findings are based on geochemical data and mineral spectroscopy that probe the crystal chemistry of Fe and Al in mineral phases (mainly kaolinite, Al- and Fe-(hydr)oxides) of tropical soils (laterites). These techniques reveal crystal alterations in mineral populations of different ages and changes of metal speciation associated with mineral or organic phases. These results provide an integrated model of soil formation and changes (from laterites to podzols) in distinct hydrological compartments of the Amazon landscapes and under altered water regimes. 相似文献
94.
95.
Tomographers map mesoscale ocean structure by inverting acoustic travel-time measurements through networks of underwater paths. To know where to deploy sensors and how to interpret their measurements, one must first understand the "forward problem," that is, how the sound channel and mesoscale features refract sound in three dimensions, and how such refraction alters the pulse-arrival sequence. We use a Hamiltonian ray-tracing program called HARPO to compute the refraction by continuous three-dimensional ocean models and to display the results in ways that add insight about refractive effects. We first simulate propagation in a simple range-independent sound channel, showing how pulse-arrival sequence depends on channel parameters and sensor placement. Next, we add linear range dependence and show that it is hard to extract range information from pulse measurements at one range. Finally, we add a simple model of a mesoscale eddy including its currents and show that deflection and splitting of the sound channel significantly alter the pulse-arrival sequence. Two diagrams that have not been widely used before are useful ways to display the arrival-time and ray-focusing perturbations caused by changes in ocean structure: they are plots of range versus launch angle and range versus travel time. Examples of azimuthal deflection, three-dimensional eigenrays, and reciprocal propagation through eddy currents are shown, and simplified methods for estimating the travel time of three-dimensional eigenrays are evaluated. 相似文献
96.
Georges Meynet 《Astrophysics and Space Science》2002,279(1-2):183-186
Stellar models with axial rotation and very low metallicity Z, typical of the early star generations have been computed. These
models account for the effects of rotation in the same way as previous grids which successfully reproduce the He- and N-enrichments
observed at the surface of massive stars, and also the high numbers of red supergiants observed in low Z galaxies (Meynet and Maeder, 2000; Maeder and Meynet, 2001). At very low metallicity (Z = 0.00001, i.e. one two thousandth of the solar metallicity),these models naturally lead to the production of primary nitrogen.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
97.
Georges Balmino 《Celestial Mechanics and Dynamical Astronomy》1994,60(3):331-364
The spherical harmonic coefficients of the gravitational potential of an homogeneous body are analytically derived from the harmonics describing its shape. General formulas are given as well as detailed expressions up to the fifth order of the topography harmonics. The volume, surface and inertia tensor of the body are obtained as by-products. The case of a triaxial ellipsoid is given as example and used for numerical checking. Another numerical scheme for verification is provided. The application to Phobos is made and the convergence of the expressions for the harmonics is numerically established.
Résumé Les harmoniques du champ de gravitation d'un corps homogène de forme donnée sont calculés analytiquement à partir des harmoniques du développement en série du rayon vecteur exprimant la forme de la surface du corps. Outre la formule générale, des expressions détaillées, au cinquième ordre des harmoniques du rayon vecteur, sont données sous une forme bien adaptée à la programmation. Le volume, la surface et le tenseur d'inertie du corps sont calculés analytiquement `a partir des formules générales. Le cas de l'ellipsoide triaxial est pris comme test des formules établies. Un autre test numérique est fourni dans le cas le plus général. Ceci est appliqué à Phobos, et la convergence des expressions fournissant les harmoniques est numériquement démontrée.相似文献
98.
99.
Colin Frank Wilson Eric Chassefière Emmanuel Hinglais Kevin H. Baines Tibor S. Balint Jean-Jacques Berthelier Jacques Blamont Georges Durry Csaba S. Ferencz Robert E. Grimm Takeshi Imamura Jean-Luc Josset Fran?ois Leblanc Sebastien Lebonnois Johannes J. Leitner Sanjay S. Limaye Bernard Marty Ernesto Palomba Sergei V. Pogrebenko Scot C. R. Rafkin Dean L. Talboys Rainer Wieler Liudmila V. Zasova Cyrill Szopa 《Experimental Astronomy》2012,33(2-3):305-335
The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55?km, where temperatures and pressures are benign (~25°C and ~0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond. 相似文献
100.
Georges G. Cravins 《The Professional geographer》2013,65(1):138-141
The spatial correlation, or colocation, of two or more variables is a fundamental issue in geographical analysis but has received much less attention than the spatial correlation of values within a single variable, or autocorrelation. A recent paper by Leslie and Kronenfeld (2011) contributes to spatial correlation analysis in its development of a colocation statistic for categorical data that is interpreted in the same way as a location quotient, a frequently used measure in human geography and other branches of regional analysis. Geographically weighted colocation measures for categorical data are further developed in this article by generalizing Leslie and Kronenfeld's global measure as well as specifying a local counterpart for each global statistic using two different types of spatial filters: fixed and adaptive. These geographically weighted colocation quotients are applied to the spatial distribution of housing types to demonstrate their utility and interpretation. 相似文献