Transmission electron microscopy on meteoritic troilite     

Jutta Töpel-Schadt  Wolfgang Friedrich Müller 《Physics and Chemistry of Minerals》1982,8(4):175-179

Phase transitions and associated domains of meteoritic troilite (FeS) have been studied by means of transmission electron microscopy (TEM). Three polymorphs have been found, two of which can be described by superstructures of the NiAs-type structure (A, C subcell). The P \(\overline 6\) 2c (√3A, 2C) polymorph, stable at room temperature, displays antiphase domains with the displacement vector 1/3< \(\overline {\text{1}}\) 10>. In situ heating experiments showed that the P \(\overline 6\) 2c polymorph changes at temperatures of 115°–150° C into an orthorhombic pseudohexagonal transitional phase with the probable space group Pmcn (A,√3A, C). It contains antiphase domains with the displacement vector 1/2 [110] and twins with a threefold twin-axis parallel c. When heated above 210° C the transitional phase transforms into the high-temperature modification with NiAs structure (P6 ₃/mmc). All observed phase transitions are reversible. The occurrence of antiphase and twin domains, respectively, agrees with the symmetry reductions involved in the subsolidus phase transitions. This is demonstrated by group-subgroup relationships among the space groups P6 ₃/mmc, Pmcn, and P \(\overline 6\) 2c.  相似文献   

Structural similarity of glasses and melts relevant to petrological processes     

Friedrich A. Seifert  Bjorn O. Mysen  David Virgo 《Geochimica et cosmochimica acta》1981,45(10):1879-1884

High-temperature Ramun spectroscopic investigations of melts in the systems GeO₂. Na₂O-SiO₂ and Na₂O-Al₂O₃-SiO₂ demonstrate the close structural relationships between melts and glasses. No coordination changes with temperature and no new anionic species have been observed in the melt relative to those in the glass. The relative concentration of different anionic units changes only slightly in the temperature range 100–1350 C. With increasing temperature, compositions with three-dimensionally connected structural units exhibit an intensity increase of a band characteristic of a defect in the network structure, but the network still remains essentially intact.  相似文献   

Comparison of Models for Delineating Wellhead Protection Areas in Confined to Semiconfined Aquifers in Alluvial Basins   总被引：2，自引：0，他引：2  

Craig B. Forster  Thomas E. Lachmar  Douglas S. Oliver 《Ground water》1997,35(4):689-697

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Sources and geochemical constraints of polycyclic aromatic hydrocarbons (PAHs) in sediments and mussels of two Northern Irish Sea-loughs.   总被引：4，自引：0，他引：4  

J Guinan  M Charlesworth  M Service  T Oliver 《Marine pollution bulletin》2001,42(11):1073-1081

Polycyclic aromatic hydrocarbons (PAHs) were sampled from 30 sediment and 8 mussel (Mytilus edulis) stations in two Northern Irish Sea-toughs. Analysis was performed by gas chromatography coupled to mass spectrometry (GC-MS). Sedimentary organic carbon, % silt/clay and mean grain size were analysed in order to assess the role of geochemistry on PAH distribution. With the exception of two sites in Larne Lough representing localized regions of high contamination, sum(PAHs) in sediments ranged between 83 and 2300 ng g(-1). Regression analysis indicated that particle size and organic C were dominant factors in controlling the distribution of PAHs throughout the sediments. Sources of PAHs in both sea-loughs were dominated by pyrogenic inputs suggesting that diffuse sources such as atmospheric deposition may be a major source to both water bodies. The sum of PAHs in mussels ranged between 95 and 184 ng g(-1). Variations in concentrations may be explained by differences in the condition between mussel populations. Mussels in both sea-loughs exhibit similar metabolic activities towards the PAH compounds which were predominantly bioaccumulated from sediments.  相似文献   

Gravimetric examination of Hagia Sophia's subsurface structure     

Jürgen Friedrich  Carl Gerstenecker  Onur Gürkan 《Journal of Geodesy》1996,70(10):645-651

The subsurface structure of Hagia Sophia, one of the oldest sacred monuments in the world built between 532–537 under the reign of Justinian in today's Istanbul, has been investigated by using two relative LaCoste-Romberg gravimeters in order to detect hidden cavities which have also served as earthquake dampers in similar constructions. On the building's ground floor a grid of 100 points with a grid size of about 4.m was measured. The mean gravimetric point error was ± 3.10^–8 ms^–2. The result of the examination is that cavities were not detected in the inner central part of Hagia Sophia with a larger diameter than 8.m down to a depth of about 20.m, and Hagia Sophia's foundation was found to be a slope of natural rock with a downward inclination to the East that has a small crest symmetrical to the building's East-West axis.  相似文献   

The windows of SETI--frequency and time in the search for extraterrestrial intelligence     

Oliver BM 《The Planetary report》1987,7(6):23-25

On Earth intelligent life evolved as a natural consequence of the events set in motion when the planet formed over 4 billion years ago. Since chemical evolution and solar-system formation appear to be occurring throughout the universe, we theorize that our universe may be rich with planets populated by intelligent beings who, like us, can search for evidence of other technological civilizations. Terrestrial civilization now has this capability. But if we do not begin the search soon, we'll lose the opportunity to do it from Earth as interfering signals of Earthly origin rapidly close the microwave window.  相似文献   

A two-dimensional model for a solar prominence: Effect of an external magnetic field     

R. Oliver  J. L. Ballester  E. R. Priest 《Solar physics》1991,134(1):123-144

Using analytical approximations we study the effects of different external magnetic configurations on the half-width, mass, and internal magnetic structure of a quiescent solar prominence, modelled as a thin vertical sheet of cool plasma. Firstly, we build up a zeroth-order model and analyse the effects produced by a potential coronal field or a constant- force-free field. This model allows us to obtain the half-width and mass of the prominence for different values of the external field, pressure and shear angle. Secondly, the effects of these external magnetic configurations on a two-dimensional model proposed by Ballester and Priest (1987) are studied. The main effects are a change of the half-width with height, an increase of the mass, a decrease of the magnetic field strength with height and a change in the shape of the magnetic field lines.  相似文献   

Monsoon response of the Somali Current and associated upwelling   总被引：1，自引：0，他引：1  

Friedrich Schott 《Progress in Oceanography》1983,12(3):357-381

The Somali Current typically develops in different phases in response to the onset of the summer monsoon. Each of these phases exists quasistationary for some time ranging from weeks to months. These periods of rather constant circulation patterns are separated by periods of rapid transition.In the early phase of the monsoon response, during May, with weak southerly winds off Somalia, a cross equatorial inertial current develops which turns offshore a few degrees north of the equator with a coastal upwelling wedge just north of the offshore flow. North of that region, an Ekman upwelling regime exists all the way up the coast. At the onset of strong winds in June, a northern anticyclonic gyre develops north of 5°N and a second cold wedge forms north of 8°–9°N, where that current turns offshore. The most drastic change of upwelling pattern occurs in the late phase of the summer monsoon, August/September, when the southern cold wedge propagates northward, indicating a break-down of the two-gyre pattern and development of a continuous boundary current from south of the equator to about 10°N. The wedge propagation during 1976–1978 is discussed, based on satellite observations (EVANS and BROWN, 1981), moored station data during 1978, 1979 and shipboard hydrographic data during 1979. A simple relation between the decrease of local monsoon winds offshore and wedge propagation cannot be determined.The southward coastal undercurrent, which is part of the Ekman upwelling regime north of 5° during the early summer monsoon, seems to turn offshore between 3° and 5°, probably due to a zonal excursion of depth contours in that area. With the spin-up of the deep-reaching northern gyre the undercurrent is extinguished during July to August but seems to get reestablished after the coalescence of the two gyres.  相似文献   

Seasonal and daily variation of mercury evasion at coastal and off shore sites from the Mediterranean Sea   总被引：3，自引：0，他引：3  

Maria E. Andersson  Katarina Grdfeldt  Ingvar Wngberg  Francesca Sprovieri  Nicola Pirrone  Oliver Lindqvist 《Marine Chemistry》2007,104(3-4):214-226

Dissolved gaseous mercury (DGM) was measured continuously using two newly developed techniques and a manual technique. The continuous techniques were based on the equilibrium between the aqueous and gaseous phase (DGM = Hg_extr / H', Hg_extr is the measured mercury concentration in the gas phase, H' is the Henry's Law coefficient at the desired temperature). In order to calculate the annual mercury evasion from the Mediterranean Sea, diurnal and seasonal measurements of DGM, total gaseous mercury in air (TGM), water temperature and wind speed were performed. During August 2003, March–April 2004 and October–November 2004 measurements of these parameters were conducted on board the RV Urania. The continuous measurements of DGM showed a diurnal variation in concentration, at both coastal and off shore sites, with higher concentrations during daytime than nighttime. The concentration difference could be as large as 130 fM between day and night. The degree of saturation was calculated directly from the measurements, S = Hg_extr / TGM and was found to vary between the different seasons. The highest average degree of saturation (850%) and the largest variation in saturation (600–1150%) was observed during the summer. The spring showed the lowest variation (260–360%) and the lowest average degree of saturation (320%). The autumn also showed a large variation in saturation (500–1070%) but a lower average (740%) compared to the summer cruise. This might be explained by the temperature difference between the different seasons, since that parameter varied the most. The flux from the sea surface was calculated using the gas exchange model developed by Nightingale et al. [Nightingale, P.D., Malin, G., Law, C.S., Watson, A.J., Liss, P.S., Liddicoat, M.I., Boutin, J., Upstill-Goddard, R. C., 2000. In situ evaluation of air–sea gas exchange parameterization using novel conservative and volatile tracers. Global Biogeochemical Cycles, 14(1):373–387]. The evasion varied between the different seasons with the highest evasion during the autumn, 24.6 pmol m^− 2 h^− 1. The summer value was estimated to 22.3 pmol m^− 2 h^− 1 and the spring to 7.6 pmol m^− 2 h^− 1. Using this data the yearly evasion from the Mediterranean Sea surface was estimated to 77 tons.  相似文献   

The pattern of large-scale bed forms in the Langeland Belt (Baltic Sea)     

Friedrich Werner  Robert S. Newton 《Marine Geology》1975,19(1):29-59

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