Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction,thermal expansion and heat capacity     

X. Liu  Y. Wang  R. C. Liebermann  P. D. Maniar  A. Navrotsky 《Physics and Chemistry of Minerals》1991,18(4):224-230

High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO₃ (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10^-5 (K^-1) below Tc and 3.5x 10^-5 (K^-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.A National Science Foundation Science and Technology Center  相似文献   

High-temperature X-ray diffraction study of Co3O4: Transition from normal to disordered spinel     

Xing Liu  Charles T. Prewitt 《Physics and Chemistry of Minerals》1990,17(2):168-172

A high-temperature x-ray powder diffraction structural refinement indicates that the room-temperature normal spinel, Co²⁺[Co ₂ ³⁺ ]O₄, transforms to a disordered spinel at temperatures above 1150±30 K, as shown by the decrease of the oxygen u parameter. It is also shown that this transition between normal and disordered spinel is a high order one, where the degree of disordering increases with increasing temperature up to the highest temperature of this study, 1201 K.  相似文献   

An analysis of the strong zonal circulation within the stratosphere of Venus     

V. Ramanathan  R.D. Cess 《Icarus》1975,25(1):89-103

A dynamical model is presented for the observed strong zonal circulation within the stratosphere of Venus. The model neglects rotational effects and considers a compressible and radiating atmosphere. It is shown that diurnal radiative heating is negligible within the lower stratosphere, a region below 85km, while observational evidence for the strong zonal circulation pertains to the lower stratosphere within which a direct thermal driving for the circulation is absent. The analysis, however, suggests that propagating internal gravity waves generated by diurnal solar heating of the upper stratosphere induce mean zonal velocities within the upper and lower stratosphere.Considering the linearized equations of motion and energy, and following Stern's (1971) analysis for an analogous problem, it is shown that the zonal velocity induced by internal gravity waves is retrograde in direction, a result which is in agreement with observation. The nonlinear equations of motion and energy are then solved by an approximate analytical method to determine the magnitude of the zonal velocity. This velocity increases from zero at the tropopause to about 200 msec^?1 at the 85 km level. The velocity near the uv-cloud level compares favorably with the observed value of 100 msec^?1.  相似文献   

Infrared observations of the surface and atmosphere of Titan     

R.F. Knacke  T. Owen  R.R. Joyce 《Icarus》1975,24(4):460-464

Infrared photometry of Titan, Saturn, and Saturn's Rings at 3.5, 4.9, 17.8, and 18.4 μm is reported. Comparison of the albedo of Titan in the 4.9 μm “window” with the albedo of the rings and with laboratory spectra suggests that frost, possibly water ice, could be a major constituent. If thick clouds are present they must be very dark at 4.9 μm. The 17.8 and 18.4 μm data are not consistent with a clear, dense molecular hydrogen atmosphere.  相似文献   

Possible infrared aurorae on Jupiter     

John Caldwell  A.T. Tokunaga  F.C. Gillett 《Icarus》1980,44(3):667-675

We have observed infrared brightenings near the poles of Jupiter at 8 μm in early 1980. We suspect they are nonthermal in origin. These brightenings are apparently related to the auroral zones which are determined by the magnetic mapping of the magnetotail onto the atmosphere, rather than by the Io flux tube. They were present in both hemispheres in January, present only in the north in February, and probably absent in the south in March. When visible, they were only seen in the hemisphere where the auroral zone was oriented toward the Earth and absent otherwise.  相似文献   

Tectonic patterns on a reoriented planet: Mars     

H.J. Melosh 《Icarus》1980,44(3):745-751

Both geologic and free-air-gravity data suggest that the positive mass anomaly associated with the Tharsis volcanoes may have reoriented Mars' lithosphere by as much as 25°. Since Mars is oblate (with flattening $\text{? ?0.005}$), rotation of the lithosphere over the equatorial bulge by 25° produces membrane stresses of several kilobars, large enough to initiate faulting. These stresses were first evaluated by F.A. Vening-Meinesz (1947, Trans. Amer. Geophys. Union28, 1–61) who treated the lithosphere as a thin elastic shell. The fracture patterns which result from these stresses are determined by the relation between stress and faulting proposed by E.M. Anderson (1951, The Dynamics of Faulting, Oliver & Boyd, Edinburgh). Plots of the magnitude and direction of stresses in a reoriented planet show that near Tharsis the dominant fault type should be north-south- trending normal faults. This normal fault province is centered about 30°N latitude and extends about 45° east and west in longitude. Similar faults should occur at the antipodes, north of Hellas Planitia. The polar regions should be occupied by roughly north-south-trending thrust faults which extend close to the equator south of Tharsis and north of Hellas. The regions between Tharsis and Hellas are subject to compression on a NE-trending axis and extension along a NW axis east of Tharsis (west of Tharsis the directions are NW compression and NE extension), thus predicting a zone of NNW and ENE strike slip faults east of Tharsis (NNE and WNW west of Tharsis). Although these patterns, except for the north-south normal faults north of Tharsis, have not yet been recognized, the discovery of such a tectonic system of the same age as Tharsis would provide strong support for the reorientation idea. Stresses due to reorientation appear to have little to do with Valles Marineris, since the stress normal to the axis of the Valles is predicted to be compressive, whereas geologic evidence suggests extension.  相似文献   

Sources and sinks of sediment to the Amazon margin: The Amapa coast     

M. A. Allison  C. A. Nittrouer  L. E. C. Faria Jr.  O. M. Silveira  A. C. Mendes 《Geo-Marine Letters》1996,16(1):36-40

Modern and Holocene muddy strata were studied along the shoreline adjacent to the Amazon river mouth using sedimentological, radiochemical, physical, and seismic methods. The present paper is a synthesis of the results, collected during the AmasSeds project, that is used to outline a regional shoreline sediment budget. Erosion of relict Amazon muds in southern Amapa supplies 10⁶ tons yr^–1 to the Amazon advective mud stream. Local rivers are sediment-poor (total suspended discharge ~ 1 × 10⁶ tons yr^–1), but form depositional sandflats on the shoreface downdrift of the river mouths. Mudflat accumulation in northern Amapa sequesters 10⁶–10⁷ tons yr^–1 by tidal-flat aggradation, alongshore mudcape accretion, and sediment trapping by mangroves. The processes temporarily store 1.5 × 10⁸ tons of Amazon mud in January–June.  相似文献   

Importance of tropical coastal environments     

Charles A. Nittrouer  Gregg J. Brunskill  Alberto G. Figueiredo 《Geo-Marine Letters》1995,15(3-4):121-126

On a global basis, a very large fraction (>50%) of the mass flux from land to the marine environment enters from tropical rivers. A broad range of processes active in the adjacent coastal ocean determines the fate of this material. The tropical setting causes many of the coastal processes to be fundamentally distinct from those operating in temperate and polar regions. Therefore, their operation cannot necessarily be understood by extrapolation from empirical observations at higher latitudes. Other coastal processes are influenced by the extremely large water, particulate, and solute discharges from tropical rivers. Further complexity is added by the diverse range of geographic settings through which tropical rivers flow and into which they empty. In summary, coastal—ocean processes in the wet tropics are: globally important, fundamentally different than at higher latitudes, and diverse in operation. Many recent advances have been made in the understanding of the tropical coastal ocean, and it is hoped these will continue into the future. Such understanding is important for predicting processes of river—ocean interaction and terrestrial fluxes to the global ocean, which affect and are affected by human populations. It also can be combined with knowledge of temperate and polar settings to provide a comprehensive understanding of the coastal ocean.  相似文献   

Measurement of sediment resuspension rates in Long Island Sound   总被引：1，自引：0，他引：1  

Henry Bokuniewicz  Larry McTiernan  Wayne Davis 《Geo-Marine Letters》1991,11(3-4):159-161

Resuspension rates have been measured on Long Island Sound sediments (New York/Connecticut, USA) under an oscillating grid, using a flume, and in place, using high frequency acoustics, transmissometers, and sediment traps. Despite differences, the results compare favorably. The methods all provide an order-of-magnitude estimate of the resuspension rate and increases under storm conditions can be estimated.  相似文献   

Surfzone currents, by D.R. Basco     

James T. Kirby  Robert A. Dalrymple 《Coastal Engineering》1984,8(4):387-389

  相似文献