Texture effect on polarized EXAFS amplitude     

A. Manceau  M. L. Schlegel 《Physics and Chemistry of Minerals》2001,28(1):52-56

 Application of polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy to thin films of fine-grained minerals is emerging as a powerful method to investigate the in-plane and out-of-plane local structure of phyllosilicates. Mineral platelets have no preferential orientation in the plane of the film, and their c* axes are oriented essentially along the film normal. The angular dependence of the EXAFS amplitude critically depends on the orientation distribution of c* axes due to mosaic spread. The effect of film texture on EXAFS amplitude has been calculated as a function of the mosaic spread, the orientation of the electric field vector, and the crystallographic orientation of the atomic pair. Calculations show that the reduction in amplitude for partially ordered films is more important when the electric field vector is perpendicular to the film plane. For phyllosilicates, no significant deviation from single crystal dichroism occurs when the mosaic spread is less than ±20–25° half-width at half-maximum (HWHM) for parallel measurement, and ±15–20° HWHM for normal measurement. Graphs are given for correcting EXAFS-derived coordination numbers for texture effects. Received: 3 February 2000 / Accepted: 26 July 2000  相似文献   

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy     

Rainer Dähn  André M ScheideggerAlain Manceau  Michel L Schlegel  Bart BaeyensMichael H Bradbury  Magali Morales 《Geochimica et cosmochimica acta》2002,66(13):2335-2347

Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO₃)₂), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 Å in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide- or a Ni-phyllosilicate-like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 Å in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 Å in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.  相似文献   

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces     

Michel L. Schlegel  Kathryn L. Nagy  Paul FenterNeil C. Sturchio 《Geochimica et cosmochimica acta》2002,66(17):3037-3054

The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (< 0.4 Å for terminal oxygens and < 0.2 Å for silicon and oxygen atoms fully coordinated to structural tetrahedra) is limited to a depth of 14 Å. Electron density profiles for all measured interfaces are consistent with a single layer of adsorbed water, with no evidence for additional organization of water molecules into distinct layers extending into the bulk solution. Similar interfacial structures were observed for natural and annealed surfaces of identical crystallographic orientation, indicating that extensive reconstruction of the silica network at the quartz surface did not occur under the annealing conditions.  相似文献   

A review of atmospheric gravity waves and travelling ionospheric disturbances: 1982-1995     

K. Hocke  K. Schlegel 《Annales Geophysicae》1996,14(9):917-940

Recent investigations of atmospheric gravity waves (AGW) and travelling ionospheric disturbances (TID) in the Earth’s thermosphere and ionosphere are reviewed. In the past decade, the generation of gravity waves at high latitudes and their subsequent propagation to low latitudes have been studied by several global model simulations and coordinated observation campaigns such as the Worldwide Atmospheric Gravity-wave Study (WAGS), the results are presented in the first part of the review. The second part describes the progress towards understanding the AGW/TID characteristics. It points to the AGW/TID relationship which has been recently revealed with the aid of model-data comparisons and by the application of new inversion techniques. We describe the morphology and climatology of gravity waves and their ionospheric manifestations, TIDs, from numerous new observations.  相似文献   

Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones (Meuse/Haute–Marne,France)     

M. Descostes  V. Blin  F. Bazer-Bachi  P. Meier  B. Grenut  J. Radwan  M.L. Schlegel  S. Buschaert  D. Coelho  E. Tevissen 《Applied Geochemistry》2008

  相似文献   

Case studies of the propagation characteristics of auroral TIDS with EISCAT CP2 data using maximum entropy cross-spectral analysis     

S. Y. Ma  K. Schlegel  J. S. Xu 《Annales Geophysicae》1998,16(2):161-167

In this paper case studies of propagation characteristics of two TIDs are presented which are induced by atmospheric gravity waves in the auroral F-region on a magnetic quiet day. By means of maximum entropy cross-spectral analysis of EISCAT CP2 data, apparent full wave-number vectors of the TIDs are obtained as a function of height. The analysis results show that the two events considered can be classified as moderately large-scale TID and medium-scale TID, respectively. One exhibits a dominant period of about 72 min, a mean horizontal phase speed of about 180 m/s (corresponding to a horizontal wavelength of about 780 km) directed south-eastwards and a vertical phase speed of 55 m/s for a height of about 300 km. The other example shows a dominant period of 44 min, a mean horizontal phase velocity of about 160 m/s (corresponding to a horizontal wavelength of about 420 km) directed southwestwards, and a vertical phase velocity of about 50 m/s at 250 km altitude.Max-Planck-Institut für Aeronomie, Germany. On leave from Department of Space Physics, Wuhan University, China  相似文献   

Mapping and monitoring of urban landuse of Hisar Town,Haryana using remote sensing techniques     

AK Jain  RS Hooda  J Nath  ML Manchanda 《Journal of the Indian Society of Remote Sensing》1991,19(2):125-134

Aerial photographs for the year 1965 and 1975 and Landsat TM (FCC) 1986 data on 1:50,000 scale were used to map and monitor the pattern of growth and development of Hisar Town during the last 20 years. The different landuse categories identified are residential, commercial, industrial, recreational, vacant lands etc. The original shape of the town was nucleated one, but presently it is star-shaped, which is due to extension of the urban centre along the radiating roads. It has mainly elongated itself along the Delhi and Rajgarh roads. The presence of Government Livestock Farm (GLF), Equine Breeding Farm and HAU Farm towards North-west of the town has completely checked its growth on these sides. The total residential-cum-commercial area has nearly doubled during 1965–1975 and trebled by 1986. The change in net area under different types of urban land-use categories, described above has also been calculated. Degraded scrub lands and sandy wasteland on the fringes of the town have been identified where further development can be planned to avoid encroachment on good agricultural lands. Recommendations have been given for future urban planning of the town.  相似文献   

Characteristics of very large aspect angle E-region coherent echoes at 933 MHz     

B. J. Jackel  D. R. Moorcroft  K. Schlegel 《Annales Geophysicae》1997,15(1):54-62

The EISCAT UHF radar system was used to study the characteristics of E-region coherent backscatter at very large magnetic aspect angles (5–11°). Data taken using 60 s pulses during elevation scans through horizontally uniform backscatter permitted the use of inversion techniques to determine height profiles of the scattering layer. The layer was always singly peaked, with a mean height of 104 km, and mean thickness (full width at half maximum) of 10 km, both independent of aspect angle. Aspect sensitivities were also estimated, with the Sodankylä-Tromsø link observing 5 dB/degree at aspect angles near 5°, decreasing to 3 dB/degree at 10° aspect angle. Observed coherent phase velocities from all three stations were found to be roughly consistent with LOS measurements of a common E-region phase velocity vector. The E-region phase velocity had the same orientation as the F-region ion drift velocity, but was approximately 50% smaller in magnitude. Spectra were narrow with skewness of about –1 (for negative velocities), increasing slightly with aspect angle.  相似文献   

X-Ray Detection of SN 1994W in NGC 4041?     

Schlegel EM 《The Astrophysical journal》1999,527(2):L85-L88

Optical spectra of SN 1994W in NGC 4041 revealed the presence of a dense (Ne>108 cm-3) circumstellar shell. An observation with the ROSAT High-Resolution Imager detected a source, with a luminosity of approximately 8x1039 ergs s-1, coincident with the position of SN 1994W to within 1&farcs;4. The positional coincidence plus the optical evidence for a dense circumstellar shell support the identification of the X-ray source as SN 1994W.  相似文献   

Glass-iron-clay interactions in a radioactive waste geological disposal: An integrated laboratory-scale experiment     

Guillaume de Combarieu  Michel L. Schlegel  Delphine NeffEddy Foy  Delphine VantelonPhilippe Barboux  Stéphane Gin 《Applied Geochemistry》2011,26(1):65-79

Glass-iron-clay setups were reacted at 90 °C for 6-18 months to investigate the coupled interactions between glass alteration, Fe corrosion and clay transformation. The reacted interfaces were probed at the microscopic level using complementary characterization methods (scanning electron microscopy coupled with energy-dispersive X-ray analysis, micro-Raman spectroscopy, micro X-ray diffraction, micro X-ray fluorescence spectroscopy, and micro X-ray absorption near-edge structure spectroscopy). The 10-μm thick Fe foil was fully corroded within 10 months, exposing glass to the pore solution. Iron corrosion led to the formation of a layer containing mostly magnetite, siderite and Fe-rich phyllosilicates with one tetrahedral and one octahedral sheet (TO) or two tetrahedral and one octahedral (TOT) sheet per layer. The clay in contact with this corrosion layer was enriched in siderite (FeCO₃). Glass alteration resulted in the formation of a gel layer whose thickness increased with reaction time (from 20 μm after 6 months to 80 μm after 18 months) and a thin layer of secondary precipitates that concentrated lanthanides, P, and Mo. Assuming conservative behavior of Zr, the Si molar concentration in the gel is about 57% that in the glass. Glass dissolution remained at a rate close to the initial dissolution rate r₀. The data are consistent with glass dissolution sustained by the uptake of dissolved Si and charge-compensating cations on secondary (corrosion) products, thus maintaining the gel porosity open and facilitating the leaching of easily soluble elements.  相似文献