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1.
Summary ?A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal
chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal
free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains
and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae.
The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than
that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated.
The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples.
In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and
decrease the difference between the M2–O3 distances.
Received October 18, 2001; revised version accepted February 15, 2002 相似文献
2.
In this paper closed one-zone models with a birth-rate stellar function depending on gas density and temperature are built up, in order to make a zeroth order approximation of thermo-chemical evolution of galaxies. To get a more realistic picture, the models are allowed first to expand with the universe and then to contract according to three successive time-scales, in order to simulate violent relaxation, rapid contraction, and slow contraction processes.Star formation occurs according to a birth-rate (m, t)
g
n
T
r
m
p (
g
, gas density;T, gas temperature; andm, star mass), which reproduces the usual Schmidt-type law in the limitr0. Further we suppose that synthesized metals and helium are instantaneously mixed when returned to gas component at star death, and that relative metal abundances are time-independent:
(
any element heavier than He,
). An energy fraction of each typical supernova event is assumed to heat interstellar medium, which is also though always to maintain a steady state.Under these hypotheses it becomes possible to use the interstellar cooling function of Dalgarno and McCray (1972), which is related to a special cosmic abundance of elements, to express analytically an interstellar cooling function related to any abundance. At this stage, numerical computations are performed up to about 12 Gy and the results analyzed in order to reproduce some morphological features observed in the galaxy, namely: (i) the extreme scarcity of stars with nearly zero metal content; (ii) the metal content and mass fraction of halo stars; (iii) the metal content and mass fraction of disk stars; and (iv) the ratio of the present-day birth-rate to the past birth-rate averaged over the age of the disk. It is found that a whole class of theoretical models exist, with slope of the mass spectrum between –2.5 and –2.6, which range from a Schmidt's law limitn=1.3 andr=0 to the opposite limitn=0 andr=–1.2, and are able to fit more or less at the same extent the observations mentioned above. On the analysis of further computations, it is also shown that an universal birth-rate stellar function is in contradiction with neither a dynamical hystory starting from the initial expansion of proto-galaxies, nor a morphological sequence where the amount of angular momentum is the only free parameter for any given mass, at least for what concerns spiral types. Finally, possible refinements for future work are briefly outlined. 相似文献
3.
Remote estimation of surficial seafloor properties through the application Angular Range Analysis to multibeam sonar data 总被引:2,自引:2,他引:2
The variation of the backscatter strength with the angle of incidence is an intrinsic property of the seafloor, which can
be used in methods for acoustic seafloor characterization. Although multibeam sonars acquire backscatter over a wide range
of incidence angles, the angular information is normally neglected during standard backscatter processing and mosaicking.
An approach called Angular Range Analysis has been developed to preserve the backscatter angular information, and use it for
remote estimation of seafloor properties. Angular Range Analysis starts with the beam-by-beam time-series of acoustic backscatter
provided by the multibeam sonar and then corrects the backscatter for seafloor slope, beam pattern, time varying and angle
varying gains, and area of insonification. Subsequently a series of parameters are calculated from the stacking of consecutive
time series over a spatial scale that approximates half of the swath width. Based on these calculated parameters and the inversion
of an acoustic backscatter model, we estimate the acoustic impedance and the roughness of the insonified area on the seafloor.
In the process of this inversion, the behavior of the model parameters is constrained by established inter-property relationships.
The approach has been tested using a 300 kHz Simrad EM3000 multibeam sonar in Little Bay, NH. Impedance estimates are compared
to in situ measurements of sound speed. The comparison shows a very good correlation, indicating the potential of this approach for
robust seafloor characterization. 相似文献
4.
5.
6.
A wave model for the Great Barrier Reef 总被引:1,自引:0,他引:1
A new wind wave generation model, WAMGBR, is presented that has been adapted from WAM especially for use in the complex geometry of the Great Barrier Reef. A technique (reef parameterization) has been presented that incorporates sub-grid scale dissipation caused by coral reefs. Three other improvements to WAM have been proposed. An explicit/implicit finite difference scheme has been implemented that allows for more efficient modelling (longer time steps) while maintaining diffusive characteristics that are at least as good as those of WAM. An offset in discrete angles creates more uniform diffusive characteristics. And, a transformed spherical coordinate system allows for more efficient grid sizes and smaller grid dependent refraction. Comparisons between modelling techniques and between model and measured data show that WAMGBR produces very good results in the difficult challenge of modelling both non-cyclonic and tropical cyclone waves in the geographically complex environment of the Great Barrier Reef. 相似文献
7.
Area usage estimation and spatiotemporal variability in distribution patterns of southern right whales,Eubalaena australis,of southern Brazil
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Eduardo Pires Renault‐Braga Karina Rejane Groch Paulo A. de Carvalho Flores Eduardo R. Secchi Luciano Dalla‐Rosa 《Marine Ecology》2018,39(3)
Southern right whales—Eubalaena australis (Desmoulins, 1822)—migrate seasonally from high‐latitude feeding grounds to coastal breeding and calving grounds at lower latitudes such as the southern coast of Brazil. Understanding how these whales are distributed along the coast is important for monitoring their postwhaling recovery and defining management strategies. In this study, we applied Kernel density estimators to aerial survey data to determine main occurrence and concentration areas of right whales in southern Brazil and investigate inter‐ and intra‐annual distribution patterns between 2003 and 2012. Our results show considerable variation in area usage within and among years, and changes in the general distribution pattern of right whales in the last years of the study. Intra‐annually, higher concentration area tended to expand from July to September and decrease in November. Some areas stood out as high‐density areas for right whales: Ribanceira/Ibiraquera, Itapirubá Sul/Sol, and from Arroio to Gaivota. Some evidences also suggest preferential areas for mother–calf pairs. The higher concentration area of right whales in southern Brazil was estimated at 52,541 km2 and the occurrence area was 682.69 km2, which is the whole study area. As right whale distribution in the region is likely expanding due to this population's current recovery, our study provides essential information for management plan of the Right Whale Environmental Protection Area. 相似文献
8.
Manuela Temmer Julia K. Thalmann Karin Dissauer Astrid M. Veronig Johannes Tschernitz Jürgen Hinterreiter Luciano Rodriguez 《Solar physics》2017,292(7):93
We analyze the well-observed flare and coronal mass ejection (CME) from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of effects – from Sun to Earth – to better understand the dynamic evolution of the CME and its embedded magnetic field. We study in detail the solar surface and atmosphere associated with the flare and CME using the Solar Dynamics Observatory (SDO) and ground-based instruments. We also track the CME signature off-limb with combined extreme ultraviolet (EUV) and white-light data from the Solar Terrestrial Relations Observatory (STEREO). By applying the graduated cylindrical shell (GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO (Solar and Heliospheric Observatory) coronagraph data, we track the temporal and spatial evolution of the CME in the interplanetary space and derive its geometry and 3D mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the magnetic cloud (MC) from in situ measurements from Wind. This is compared to nonlinear force-free (NLFF) model results, as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are lower in the interplanetary space by ~?50% and 75%, respectively, possibly indicating an erosion process. A CME mass increase of 10% is observed over a range of \({\sim}\,4\,\mbox{--}\,20~\mathrm{R}_{\odot }\). The temporal evolution of the CME-associated core-dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME. 相似文献
9.
L. Secco 《Mineralogy and Petrology》1988,39(3-4):175-185
Summary The clinopyroxene suites from lherzolite nodules from Mts. Leura and Noorat (Victoria, Australia) have been investigated by X-ray diffraction and electron probe microanalyses (Dal Negro et al., 1984; Cundari et al., 1986).The evolution of the host nodule is shown by general depletion of AlIv, AVI, Ti4+, Fe2+ and enrichment of Si, Mgm,, Fe3+, Cr3+, Ca, while Na depletion occurs only in the clinopyroxenes from Mt. Noorat. Different mechanisms of cation substitution are thus involved in the two clinopyroxene suites, suggesting different total pressures of equilibration in the stability field of spinel.Modifications involving the M1 octahedron are mainly ascribed to variable amounts of trivalent cations, the volume of the M 1 site increasing with decreasing content of trivalent cations in each suite. The differences in M 1 site configuration between the Mt. Leura and Mt. Noorat clinopyroxene suites are ascribed to the different amounts of A1v1 and FeM,.The volume of the T tetrahedral site is generally related to A1Iv in each suite. An increase in T site volume from Mt. Leura to Mt. Noorat clinopyroxenes was found however, due to lengthening of the T-Obrg bond lengths, for similar AIIv contents. The volume of the M2 site, generally related to Ca content, was generally higher in the Noorat clinopyroxenes for similar Ca (and Ca + Na) contents, due to the longer M2-O3C1 bond length, strongly related to Na content. Cell volume is linearly correlated to M 1 volume in each suite, but is definitely higher in the Noorat clinopyroxenes for similar M 1 volume. All the structural data show that the total pressure of equilibration in the Noorat clinopyroxenes was lower than that in the Leura clinopyroxenes, as suggested by chemical data.
With 2 Figures 相似文献
Die Kristallchemie von hochdruck-klinopyroxenen aus spinell-lherzolithknollen von Mt. Leura und Mt. Noorat, Victoria, Australien
Zusammenfassung Die Klinopyroxenparagenesen aus Lherzolitheinschlüssen von Mt. Leura und Mt. Noorat (Victoria, Australien) wurden mittels Röntgendiffraktometrie und Elektronenmikrosonde untersucht (Dal Negro et al., 1984; Cundari et al., 1986). Die Lherzolitheinschlüsse zeigen generell eine Abnahme von AlIV, AlVI, Ti4+, Fe MI 2+ und eine Zunahme von Si, Mgm,, Fe3+, Cr3+ und Ca. Eine Abnahme von Na tritt nur in Klinopyroxenen von Mt. Noorat auf. Verschiedene Substitutions-Mechanismen der Kationen weisen auf verschiedene Equilibrationsdrucke im Stabilitätsbereich der Spinelle hin und sind daher in den beiden Klinopyroxenserien zu berücksichtigen. Modifikationen in der oktaedrischen M1-Position wurden vor allem dem wechselnden Anteil an dreiwertigen Kationen zugeschrieben, wobei in jeder der beiden Serien das Volumen der Ml-Position mit abnehmenden Gehalten dreiwertiger Kationen zunimmt. Die unterschiedliche Konfiguration der M1-Position der Klinopyroxene von Mt. Leura und Mt. Noorat wird mit unterschiedlichen Gehalten an AlVI und Fe MI 2+ , in Zusammenhang gebracht.Das Volumen der tetraedrischen T-Position ist normalerweise mit den Gehalten an AllIV in jeder Serie verknüpft. Ein Vergleich der Klinopyroxene von Mt. Leura und Mt. Noorat zeigte jedoch, daß bei gleichen Gehalten an AlVI das Volumen der tetraedrischen Position infolge einer Aufwertung der T-Obrg.-Bindungen zunimmt. Das Volumen der M2-Position, üblicherweise mit dem Ca-Gehalt korreliert, ist in den Klinopyroxenen von Mt. Noorat bei ähnlichen Ca (und Ca f Na) Gehalten infolge größerer M2--O3C1-Abstände größer. Sie stehen also mit den Na-Gehalten in Verbindung.Das Volumen der Elementarzelle korreliert mit dem der M1-Position in jeder Serie; es ist aber in den Klinopyroxenen von Mt. Noorat deutlich höher. Die Ergebnisse der Strukturuntersuchungen zeigen-wie auch die chemischen Daten-, daß die genannten Equilibrationsdrucke für die Klinopyroxene von Mt. Noorat niedriger waren als für die von Mt. Leura.
With 2 Figures 相似文献
10.
Time-clustering of natural hazards 总被引:1,自引:0,他引:1
Luciano Telesca 《Natural Hazards》2007,40(3):593-601
Natural hazards can be represented as point processes that are characterized by the occurrence times of the events and their
intensities. It is crucial to investigate the correlation properties of these processes in order to gain a deep knowledge
of the dynamical mechanisms which underlie hazardous phenomena. To this end, suitable methodologies must be developed to perform
these correlation analyses on processes, which are described as point-like processes. The concept of time-clustering implies
a time-structured organization of these processes, and is in direct opposition to the pure randomness typical of Poissonian
processes in which the events are uncorrelated. This article reports several examples of natural hazards within the framework
of time-clustering. 相似文献