Vacancy and defect structures in metal oxides

The self-consistent local density (LD) theory is used to model vacancy and defect structures in metal oxides by use of the embedded cluster scheme. LCAO-MO expansions of cluster orbitals are obtained by the DV-Xα discrete variational method, and used to characterize energy levels, charge distributions, X-ray absorption and cohesive energies. Examples are drawn from recent work on transition metal monoxides such as Fe_1?x O, ceramic materials such as yttrium-stabilized zirconia, and the alumina/ruby system. It is shown that in addition to predicting spectroscopic properties, LD theory has now reached a stage of implementation where lattice stability and local site geometry can be usefully explored.     

Intermittent Aeolian Saltation: A Protocol For Quantification

The quantification of saltation intermittency—the proportion of time when sand transport occurs—provides valuable information about the nature of aeolian systems, including insights concerning threshold conditions for sand transport. Intermittency has been measured in numerous studies, but a lack of measurement standardization often makes the comparison of results difficult. Four methodological factors influence estimates of intermittency: sample frequency, sample duration, sample area, and sensor elevation. For a given transport rate, the value of intermittency may decrease with sensor elevation, increased sample frequency, or decreased sensor area; it may increase or decrease as a consequence of sample duration. We therefore simulate the influences of these factors and suggest a common protocol for measuring and reporting intermittency based on measurements at 1 Hz with 100 mm² sensor area at 50 mm elevation above the surface, or equivalent values.     

CHANGING SPATIAL CHARACTERISTICS OF WILLAMETTE VALLEY FARMS

This article reflects on the racial configuration of urban space. Previous research tends to posit racial segregation and diversity as either endpoints on a continuum of racial dominance or mirror images of one another. We argue that segregation and diversity must be jointly understood; they are necessarily related, although not inevitably as binary opposites. Our view is that the neighborhood geographies of U.S. metropolitan areas are simultaneously and increasingly marked by both racial segregation and racial diversity. We offer an approach that classifies neighborhoods based jointly on their compositional diversity and their racial dominance, illustrated by an examination of the neighborhood racial structure of several large metropolitan areas for 1990 and 2000. Compositional diversity increased in all metropolitan areas in ways rendered visible by our approach, including a sharp reduction in the number of highly segregated white neighborhoods, transitioning mostly into moderately diverse yet still white-dominated neighborhoods, and a fourfold increase in the number of highly diverse neighborhoods. Even so, many highly segregated spaces remain, especially for whites and blacks. Latino-dominated spaces show a mix of persistence and emergence. Although compositional diversity is increasing, highly diverse neighborhoods are still rare and are the least persistent of all racial configurations. Our approach clearly demonstrates the “both/and”-ness of segregation and diversity.     

Livestock responses to droughts and severe winter weather in the Gobi Three Beauty National Park, Mongolia

We investigated how both droughts and dzuds (severe winter weather) control livestock mortality in a non-equilibrium steppe ecosystem of Mongolia, Gobi Three Beauty National Park. These steppe ecosystems have developed under high interannual variability of rainfall and nomadic grazing systems. Interannual precipitation variation was 39%, with 128 mm mean annual precipitation. The effect of climate variability and extreme events on livestock mortality is a critical aspect for the Mongolian economy. Analysis of drought and precipitation variability on livestock mortality rate was not significantly influenced by the index of mean annual precipitation and annual winter temperature. Overall, unlike hot dry regions, pastoral livestock mortality in the cold dry regions was affected more by dzuds and annual growing seasonal rain than by droughts. Dzuds can be frequent events, occurring as often as once every 2 and 3 years within a decade. The average annual livestock mortality for the combined drought and dzuds years (18%) was 4.8% greater than the years with dzuds alone, and 7% greater than in years with only drought. Thus livestock mortality appears to be more sensitive to dzuds than to droughts, and that dzuds contributes more to livestock mortality even years where combined drought and winter storms occur.     

Preservation of oxygen isotope compositions in granulites from Northwestern Canada and Enderby Land,Antarctica: implications for high-temperature isotopic thermometry

 Preservation of high-temperature mineral isotopic compositions is necessary for successful high-temperature isotopic thermometry. Other requirements include large fractionations between constituent minerals, well-calibrated equilibria, carefully designed sampling strategies and data handling techniques that quantitatively account for retrograde exchange. Here, we apply isotopic thermometry and data handling techniques to calculate and contrast mineral-pair apparent temperature data and observed closure temperature data (T _c-observed) (cf. Farquhar et al. 1993) for the very high temperature (>900°C), dry granulites of the Taltson Magmatic Zone of Northwestern Canada and the Napier Complex of Enderby Land, Antarctica. The isotopic compositions of garnet grains from both terrains reflect high temperature conditions (>950°C) and point to this mineral as an excellent candidate for isotopic thermometry. The isotopic compositions of quartz, pyroxene, ilmenite and magnetite indicate that they equilibrated to lower temperature conditions (<900°C) due to faster rates of oxygen diffusion in these minerals, possibly enhanced by exsolution and ductile deformation, compared with garnet. Our temperature data for garnet and pyroxene are ≈200°C higher than is possible to explain by existing “wet” diffusion data, but is consistent with “dry” diffusion data, suggesting that the extremely dry nature of these rocks may have played a significant role in the preservation of high-temperature isotopic compositions. Both quartz and magnetite exhibit subgrain features, indicative of ductile deformation. Quartz-magnetite temperatures from the Napier complex are similar to those inferred for a late (D3) deformation and are lower than those predicted by “dry” diffusion data. We infer that the quartz-magnetite isotopic fractionations reflect deformation-enhanced exchange that accompanied D3. Garnet in these same samples did not undergo ductile deformation and did not exchange oxygen with coexisiting phases during cooling. This may reflect strain partitioning between less easily deformed, low abundance garnet and more easily deformed matrix quartz and magnetite. The resistance of garnet to ductile deformation in these rocks is a second reason why garnet is suitable for isotopic thermometry. Received: 6 February 1996 / Accepted: 25 April 1996     

Stability and phase equilibria of chloride and carbonate bearing scapolites at 750°C and 4000 bar

At 750°C and 4000 bar scapolite is stable relative to plagioclase + calcite over the range of plagioclase compositions An₅₃–An₈₃. The assemblage plagioclase + scapolite + calcite is stable relative to plagioclase + calcite over the ranges of plagioclase composition An₄₈-An₅₃ and An₈₃–An_91.5. When NaCl is present in the coexisting fluid the range of scapolite compositions stable relative to plagioclase increases. High mole fractions of NaCl in the fluid stabilize scapolite relative to plagioclases from An₂₅ to An₈₇ in the presence of excess calcite. Determination of the $\text{Cl}\text{(Cl + CO}{}_3\text{)}$ ratios of the synthetic scapolites shows that the range of stable scapolite compositions is significantly larger than heretofore proposed, and that even the chloride and carbonate bearing scapolites must be considered a four component solid solution. The K_D for the exchange of NaCl and CaCo₃ between coexisting scapolite, fluid and carbonate is given by the equation In $\text{K}{}_{\mathrm{D}}\text{= (?0.0028) [}\text{Al}\text{(Al + Si)}\text{]}{}^{\mathrm{?5.5580}}$. This equation implies that Cl-poor natural scapolites coexisted with fluids low in NaCl, and that regional occurrences of Cl-rich scapolites are likely to represent metamorphosed evaporite sequences.     

Low frequency electromagnetic radiation associated with magnetic disturbances

Continuous observations of the amplitude and spectrum of naturally occurring radiation in the band 2–40 kc/s have been made during the period June to December 1958 near Sydney, Australia. A large number of isolated noise bursts lasting for some hours were detected. The intensity ranged from 6 × l0⁻¹⁹ to 6×10⁻-¹⁷W m⁻² (c/s)⁻¹ at 4·6 kc/s. Three main types of bursts were identified and classified on a basis of their spectra which usually extended from 3 to 5 kc/s, 4 to 8 kc/s and 2 to 30 kc/s, respectively. Major bursts, which were always of the latter two types, were clearly associated with strong auroral and magnetic activity and some showed a reproducible sequence of amplitude variation lasting about 36 hours. On three occasions, a detailed correspondence between the intensity of the noise and of simultaneously occurring red oxygen airglow was observed. Theories of the origin of the noise are discussed.