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31.
Natural Hazards - The state of Sikkim in India has many steep slopes and has been susceptible to landslides. Since 1968 there have been innumerable losses of lives and properties due to landslides.... 相似文献
32.
Surendra K. Saxena Subrata Ghose A.C. Turnock 《Earth and Planetary Science Letters》1974,21(2):194-200
Four pyroxenes with compositions En48Fs48Wo4, En47·5Fs47·5Wo5, En45Fs45Wo10 and En40Fs40Wo20, synthesized at 1200°C at atmospheric pressure, were heat-treated at 500, 600, 700, and 800°C for various lengths of time. These pyroxenes are variously ordered with respect to Fe2+ and Mg2+ without unmixing. The Fe2+-Mg2+ distribution over the two nonequivalent sites M1 and M2, determined through Mössbauer spectroscopy, is found to be a function of both temperature and concentration of Ca2+ at the M2 site. The preference of Fe2+ for the M2 site increases with decreasing temperature and increasing Ca2+. These data can be used to determine cation equilibration temperatures of lunar and terrestrial pigeonites. The lunar pigeonites usually indicate equilibration temperatures of 700–860°C, except the pigeonite from rock 14053, which may have been subjected to shock heating due to meteoritic impact. 相似文献
33.
L. P. Sharma Nilanchal Patel P. Debnath M. K. Ghose 《Arabian Journal of Geosciences》2012,5(4):789-796
Hilly regions are prone to landslides that cause heavy losses of life and properties every year. A number of researches and
analyses are carried out in the GIS environment to identify landslide vulnerability in the region. The important conditioning
factors identified by the researchers are slope, geological, geomorphologic features, and land use coupled with triggering
factors like rainfall and a few of the anthropogenic activities. Soil forms the uppermost part of the earth crust, and it
is expected that various soil characteristics like depth, surface texture, depth texture, soil erosion, hydraulic conductivity,
stoniness, etc., play significant roles in causing landslide in the area. These factors have been ignored so far by most researchers
while identifying landslide hazard-prone areas. This paper attempts to assess the vulnerability status in parts of East Sikkim,
India, by integrating the influence of the various soil attributes. A composite index called soil stability value was determined
by aggregating the weights assigned to different soil parameters. Finally, based on the soil stability values, the study area
was classified into least vulnerable, moderately vulnerable, and most vulnerable zones of landslide occurrences. Comparison
between the vulnerability zones and the actual landslide occurrences yielded a 90% agreement with the density of landslides
in the most vulnerable zone, demonstrating the efficacy of soil characteristics as potential indicators of landslide events. 相似文献
34.
Subrata Ghose Katsuhiro Tsukimura Dorian M. Hatch 《Physics and Chemistry of Minerals》1989,16(5):483-496
Ilvaite, Ca(Fe2+, Fe3+) Fe2+Si2O7O(OH), a mixed-valence iron silicate shows an insulator-semimetal transition with a band gap of 0.13 eV due to thermally induced charge delocalization between Fe2+ and Fe3+ ions (A sites) in double octahedral chains. The charge delocalization induces a second order crystallographic phase transition on heating from monoclinic (P21/a) to orthorhombic (Pnam) symmetry at 346 K. The unit cell dimensions within the 295–420 K range and the crystal structures at 295, 320, 340, 360, 380 and 400 K have been determined by high temperature single crystal X-ray diffraction. The degree of charge delocalization determined from the sizes of the Fe(Ao) and Fe(Am) octahedra is the primary order parameter, Q which couples linearly with the spontaneous strain component, 13. The order parameter coupling and the associated free energy expression is given. The calculated normal modes of the space group symmetry change are consistent with the experimentally observed atomic displacements, which are parallel and antiparallel to c. Formation of antiphase lamellar twin domains parallel to (001) in the monoclinic phase is predicted to occur as a result of the phase transition. Above Tc (= 346 K), the slow asymptotic decrease of 13 attaining a zero value at 380 K indicates the presence of fluctuating precursor clusters with considerable short-range order above Tc. A peak in the specific heat (Cp) measurements coincides with the onset of longrange order at 380 K, whereas 57Fe Mössbauer measurements indicate the onset of charge localization at a considerbly higher temperature (470 K). The coupling of the d6 electron of the Fe2+ (A) ion with a longitudinal optic phonon with the polarization vector along c
* is the likely mechanism to drive the phase transition. The electronphonon coupling also provides a charge conduction mechanism through electron hopping, whereby the short-bonded Fe2+-Fe3+ pair containing the d6 electron (intermediate polaron) will break up and re-form, thereby propagating the electron one step along the c axis. 相似文献
35.
Subrata Ghose Narayani Choudhury S. L. Chaplot C. Pal Chowdhury S. K. Sharma 《Physics and Chemistry of Minerals》1994,20(7):469-477
Enstatites (Mg2Si2O6) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO6 octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg2+ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, Mg2SiO4 and fayalite, Fe2-SiO4) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)0.6Mg1.4Si2O6 stable at room temperature using the 488 nm or 514.5 nm lines of an Ar+ ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations. 相似文献
36.
M. Ghose 《Environmental Geology》2001,40(11-12):1405-1410
37.
O. Ballet J. M. D. Coey G. Fillion A. Ghose A. Hewat J. R. Regnard 《Physics and Chemistry of Minerals》1989,16(7):672-677
The magnetic properties of two samples of acmite, one natural and the other synthetic, were determined using magnetization and susceptibility measurements, Mössbauer spectroscopy and neutron diffraction. Exchange interactions are quite strongly antiferromagnetic, the paramagnetic Curie temperature being -46 K for a purely ferric synthetic sample, but its Néel temperature is only 8 K. The principal magnetic mode has the periodicity of the crystallographic structure and is made of ferromagnetic chains, coupled antiferromagnetically. Moments are oriented in a direction close to the chain axis, c. The antiferromagnetic exchange between adjacent Fe3+ ions in the same chain is overcome by their coupling to a common Fe3+ neighbour in the next chain. This indicates that the whole (SiO4) group can act as a superexchange ligand in silicates. 相似文献
38.
The crystal structure and site preference of Co2+ in a synthetic Co1.10Mg0.90SiO4 olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co2+ in olivine. The strong preference of Co2+ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg2+ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe2+, Mg2+, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines. 相似文献
39.
40.