A study of the bonded interactions in nitride molecules in terms of bond critical point properties and relative electronegativities

Bond critical point properties calculated for the MN bonds in a number of geometry optimized nitride molecules containing first- and second-row M cations are compared with those calculated for a number of oxide molecules. As reported for the oxides, the value of the electron density, ρ(r _c ), at the bond critical points, r _c , increases with decreasing bond length while for the more electronegative cations, the local energy density, H(r _c ) decreases nonlinearly in value as the relative electronegativities of the M-cations, χ _M , tend to increase. In the majority of cases, χ_M, |λ₁|/λ₃ and ∇²ρ(r _c ) increase with decreasing minimum energy bond lengths. The bond lengths adopted by the molecules are indicated to be an important determinant of the critical point properties of the electron density distributions. The relative electronegativities derived from the electron density distributions of the nitrides agree with those derived for the oxides and Pauling’s electronegativities to within ∼5%, on average. Received: 3 February 1997 / Revised, accepted: 11 July 1997     

The eclipses of EX Hydrae

Two short runs of spectral data on EX Hya have been obtained. One of these provides a high time and wavelength resolution data set covering a cycle in the 67 min modulation and containing a deep eclipse. The other was acquired shortly preceding one of the relatively rare outbursts of this system. Examination of these data sets confirms features noted in other studies. In particular, a broadV/R eclipse effect' is seen in the Balmer lines, and a sharp peak in Balmer line equivalent width is detected at mid-eclipse. Also, the correlation with 67 min phase of the (O-C) residuals for optical eclipse time is examined in relation to the location of the eclipsed structure. The accretion curtain model of Rosenet al. (1988) is discussed and shown to be qualitatively consistent with the observational data in both the optical and X-ray bands.     

Tasmante cruise: Swath-mapping and underway geophysics south and west of Tasmania

The 1994 Tasmante swath-mapping and reflection seismic cruise covered 200 000 km² of sea floor south and west of Tasmania. The survey provided a wealth of morphological, structural and sedimentological information, in an area of critical importance in reconstructing the break-up of East Gondwana.The west Tasmanian margin consists of a non-depositional continental shelf less than 50 km wide and a sedimented continental slope about 100 km wide. The adjacent 20 km of abyssal plain to the west is heavily sedimented, and beyond that is lightly sedimented Eocene oceanic crust formed as Australia and Antarctica separated. The swath data revealed systems of 100 m-deep downslope canyons and large lower-slope fault-blocks, striking 320° and dipping landward. These continental blocks lie adjacent to the continent ocean boundary (COB) and are up to 2500 m high and have 15°–20° scarps.The South Tasman Rise (STR) is bounded to the west by the Tasman Fracture Zone extending south to Antarctica. Adjacent to the STR, the fracture zone is represented by a scarp up to 2000 m high with slopes of 15–20°. The scarp consists of continental faultblocks dipping landward. Beyond the scarp to the west is a string of sheared parallel highs, and beyond that is lightly sedimented Oligocene oceanic crust 4200–4600 m deep with distinct E-W spreading fabric. The eastern margin of the bathymetric STR trends about 320° and is structurally controlled. The depression between it and the continental East Tasman Plateau (ETP) is heavily sedimented; its western part is underlain by thinned continental crust and its central part by oceanic crust of Late Cretaceous to Early Tertiary age. The southern margin of the STR is formed by N-S transform faults and south-dipping normal faults.The STR is cut into two major terrains by a N-S fracture zone at 146°15E. The western terrain is characterised by rotated basement blocks and intervening basins mostly trending 270°–290°. The eastern terrain is characterised by basement blocks and intervening strike-slip basins trending 300°–340°. Recent dredging of basement rocks suggests that the western terrain has Antarctic affinities, whereas the eastern terrain has Tasmanian affinities.Stretching and slow spreading between Australia and Antarctica was in a NW direction from 130–45 Ma, and fast spreading was in a N-S direction thereafter. The western STR terrain was attached to Antarctica during the early movement, and moved down the west coast of Tasmania along a 320° shear zone, forming the landward-dipping continental blocks along the present COB. The eastern terrain either moved with the western terrain, or was welded to it along the 146°15 E fracture zone in the Early Tertiary. At 45 Ma, fast spreading started in a N-S direction, and after some probable movement along the 146°15E fracture zone, the west and east STR terrains were welded together and became part of Australia.     

Overview of calcite/opal deposits at or near the proposed high-level nuclear waste site,Yucca Mountain,Nevada, USA: Pedogenic,hypogene, or both?

Calcite/opal deposits (COD) at Yucca Mountain were studied with respect to their regional and field geology, petrology and petrography, chemistry and isotopic geochemistry, and fluid inclusions. They were also compared with true pedogenic deposits (TPD), groundwater spring deposits (GSD), and calcite vein deposits (CVD) in the subsurface. Some of the data are equivocal and can support either a hypogene or pedogenic origin for these deposits. However, Sr-, C-, and O-isotope, fluid inclusion, and other data favor a hypogene interpretation. A hypothesis that may account for all currently available data is that the COD precipitated from warm, CO₂-rich water that episodically upwelled along faults during the Pleistocene, and which, upon reaching the surface, flowed downslope within existing alluvial, colluvial, eluvial, or soil deposits. Being formed near, or on, the topographic surface, the COD acquired characteristics of pedogenic deposits. This subject relates to the suitability of Yucca Mountain as a high-level nuclear waste site.     

Petrogenesis of complex veins in the Chantonnay (L6f) chondrite

Two cross-cutting veins in the Chantonnay (L6f) chondrite illustrate different patterns of fractionation of total chondritic shock melts. The earlier vein, which is dark-colored and bears abundant host rock xenoliths, is strongly reduced and sodium-poor relative to the bulk meteorite. It resembles and may be cogenetic with melt pockets in Chantonnay. The later vein, which is lighter-colored and somewhat vesicular, lacks evidence of either Na loss or reduction but shows modest internal differentiation. Its metal and total iron contents (26.5 wt.%) are higher than normal for L-group chondrites.The trend of chemical fractionation recorded in the earlier Chantonnay vein resembles that reported for chondrules in ordinary chondrites, suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts.     

Detrended correspondence analysis of heathland vegetation: The identification of runoff contributing areas

Detrended correspondence analysis is applied to information on vegetation composition for the heathland zone of a New Forest catchment. A map of runoff contributing areas is constructed from a soil moisture related gradient in the vegetation data and the resulting map compares favourably with other information on runoff contributing areas in a subcatchment.     

Humic substances,a possible source of environmental goitrogens

The by-products of humic substances (HS), resorcinol, orcinol, phloroglucinol, pyrogallol, 3,4- and 3,5-dihydroxybenzoic acids, and ortho- and meta-phthalic acids, have been tested for antithyroid and goitrogenic activities. By two in vitro assays all compounds, except the phthalic acids, were potent inhibitors of the thyroid peroxidase enzyme, and of thyroidal ¹²⁵I uptake and/or its incorporation into thyroid hormones using thyroid slices. Resorcinol in drinking water was goitrogenic as indicated by in vivo assay in rats. These findings, the identification of resorcinol in the water supply of an endemic goiter district in western Colombia, where increased goiter prevalence is associated with watersheds rich in organic matter, and results of elemental analysis of goitrogenic water extracts from the same district showing a composition similar to that of aquatic HS, suggest that HS degradation by-products may play an important role in the etiology of endemic goiter.     

Land use change

Despite decades of interest by geographers in patterns and processes of land use change, the data base for such studies is at best fragmentary. In the absence of an adequate land inventory, it is difficult to see how an effective land planning and allocation system can be devised to meet future social, economic and ecological needs. It is argued that a more comprehensive system for monitoring land use change is required, and the role and accuracy of remote sensing in providing a land use survey base is assessed.     

Crystal structure refinement and electron density distribution in diaspore

Diaspore from Dilln, Hungary, AlOOH, is orthorhombic with space group Pbnm, a=4.4007(6), b=9.4253(13), c=2.8452(3) Å, and Z=4. The crystal structure and electron distribution have been refined from 791 graphite-monochromatized MoKα data (maximum 2θ=130°) to R=0.035 (R _w =0.029). Difference maps show substantial electron density ascribed to covalent bonding in the hydroxyl group, O(2)-H, but no residual density is observed along the Al-O(1,2) bonds. An analysis of the charge distribution implies net charges of +1.47(26), ?1.08(16), ?0.59(13) and +0.20(5) for Al, O(1), O(2) and H respectively. Semi-empirical molecular orbital calculations of the Hückel type agree with the experimentally determined atomic charge distribution and also allow a rationalization of the observed bond length variations.