Multi-spectroscopic study of Fe(II) in silicate glasses: Implications for the coordination environment of Fe(II) in silicate melts

The coordination environment of Fe(II) has been examined in seven anhydrous ferrosilicate glasses at 298 K and 1 bar using ⁵⁷Fe Mössbauer, Fe K-edge X-ray near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS), UV-Vis-NIR, and magnetic circular dichroism (MCD) spectroscopies. Glasses of the following compositions were synthesized from oxide melts (abbreviation and nonbridging oxygen:tetrahedral cation ratio (NBO/T) in parentheses): Li₂FeSi₃O₈ (LI2: 1.33), Rb₂FeSi₃O₈ (RB2: 1.33), Na_l.08Fe_l.l7Si_3.l3O₈ (NAl: 1.09), Na_l.46Ca_0.24Fe_l.08Si_2.97O₈ (NC6: 1.38), Na_l.09Ca_0.51Fe_0.72Si_3.10O₈ (NC2: 1.15), Na_0.99Ca_0.92Fe_0.24 Si_3.17O₈ (NCl: 1.04), and Na_0.29Mg_0.53Ca_0.52Fe_0.56Al_0.91Si_2.44O₈ (BAS: 1.05). Mössbauer, XANES, and EXAFS information suggests that iron is dominantly ferrous in all glasses (<10 atom% Fe(III)) with an average first-neighbor Fe(II) coordination varying from ∼ 4 to 5.2 (±0.2) oxygens. The UV-Vis-NIR spectrum of each sample exhibits intense absorption centered near 8100-9200 cm⁻¹ and weak absorption near 5000 cm^−l, which cannot be assigned unambiguously. The MCD spectrum of NC6 glass, which is the first such measurement on a silicate glass, shows three transitions at ∼8500 cm⁻¹, ∼6700 cm⁻¹, and ∼4500 cm⁻¹. The behavior of these MCD bands as a function of temperature (1.6 K to 300 K) and magnetic field strength (1 T to 7 T) indicates that they most likely arise from three distinct Fe(II) sites with different ground states, two of which are 5-coordinated and one of which is 4-coordinated by oxygens.The combined results suggest that Fe(II) predominantly occupies 5- and 4-coordinated sites in each glass, with the ratios differing for the different compositions. Small amounts of 6-coordinated Fe(II) are possible as well, but primarily in the more basic glass compositions such as BAS. The substitution of Li(I) for Rb(I) in the M₂FeSi₃O₈ base glass composition causes a weakening of the average Fe(II)-O bond, as indicated by the longer Fe(II)-O distance in the latter. The basalt composition glass was found to have the largest Fe(II) sites relative to those in the other glasses in this study. A bond valence model that helps predict the coordination number of Fe(II) in silicate glasses is proposed. The structural information extrapolated to Fe(II)-bearing melts is parameterized using bond valence theory, which helps to rationalize the melt-crystal partitioning behavior of ferrous iron in natural and synthetic melt-crystal systems.     

Development and the Household: Missing the point?

Over the past two decades development has been shifting its focus toward smaller scales and particular problems. As a result, the household has become an increasingly important institution for development, and has come under increased scrunity as development practitioners and scholars seek to better understand this institution’s functions and foundations, trying to ‘get the household right’. These efforts, rather than clarifying the character and the function of the household, have contributed to its indeterminacy by reifying the very institution they seek to analyse. Arguing that these efforts fundamentally miss the point of examining the household, this paper introduces a different framework of analysis that addresses the household not as a fixed object of research, but as a local construction that embodies flows of power and knowledge both within and transcending the local. Using the example of varying constructions of the household in two villages in Ghana’s Central Region, this article illustrates how such an approach allows us to address the various functions of the household as particular embodiments of these flows, an approach that better explains the endurance or ephemerality of these functions. Such an approach provides a stronger foundation for the consideration of how particular constructions of this institution may have troubling implications for issues like gender equity and sustainability.     

Immigrant Profiles of U.S. Urban Areas and Agents of Resettlement*

This article targets the role of intermediaries, such as refugee resettlement programs, in altering the geography of the foreign‐born. It argues that, under such intermediaries, destination choice within the United States is largely determined not by economic mechanisms but instead by information‐related factors such as friction of distance, migration chains, labor procurement, and resettlement intermediaries. Metropolitan statistical area (MSA) destinations are grouped into four profiles based on their mix of foreign‐born. The result is sets of MSAs differentiated by the era of immigration, immigrant origins, geographic pattern, and place characteristics that draw migrants. To evaluate intermediary impacts, monies allocated to states by the Office of Refugee Resettlement, refugee resettlement by state, and refugee movements to MSAs are expressed as a Refugee Resettlement Index and linked to MSA profiles. We conclude that although refugees constitute only a portion of total immigration, their effects are disproportionately large in terms of changing the foreign‐born profiles of MSAs and other communities, changing the fabric of society, and changing the geography of the foreign‐born in all its ramifications.     

Smithsonian Institution’s Global Volcanism Network

Information included in this summary is based on more detailed reports published in the Bulletin of the Global Volcanism Network, vol. 31, no. 5, May 2006 (on the Internet at ). Edited by scientists at the Smithsonian, this bulletin includes reports provided by a worldwide network of correspondents. The reports contain the names and contact information for all sources. Please note that these reports are preliminary and subject to change as events are studied in more detail. The Global Volcanism Program welcomes further reports of current volcanism, seismic unrest, monitoring data, and field observations.     

LandScan USA: a high-resolution geospatial and temporal modeling approach for population distribution and dynamics

High-resolution population distribution data are critical for successfully addressing important issues ranging from socio-environmental research to public health to homeland security, since scientific analyses, operational activities, and policy decisions are significantly influenced by the number of impacted people. Dasymetric modeling has been a well-recognized approach for spatial decomposition of census data to increase the spatial resolution of population distribution. However, enhancing the temporal resolution of population distribution poses a greater challenge. In this paper, we discuss the development of LandScan USA, a multi-dimensional dasymetric modeling approach, which has allowed the creation of a very high-resolution population distribution data both over space and time. At a spatial resolution of 3 arc seconds (～90 m), the initial LandScan USA database contains both a nighttime residential as well as a baseline daytime population distribution that incorporates movement of workers and students. Challenging research issues of disparate and misaligned spatial data and modeling to develop a database at a national scale, as well as model verification and validation approaches are illustrated and discussed. Initial analyses indicate a high degree of locational accuracy for LandScan USA distribution model and data. High-resolution population data such as LandScan USA, which describes both distribution and dynamics of human population, clearly has the potential to profoundly impact multiple domain applications of national and global priority.     

A note on the top-down and bottom-up gradient functions over a forested site

The dimensionless bottom-up and top-down gradient functions in the convective boundary layer (CBL) are evaluated utilizing long-term well-calibrated carbon dioxide mixing ratio and flux measurements from multiple levels of a 447-m tall tower over a forested area in northern Wisconsin, USA. The estimated bottom-up and top-down functions are qualitatively consistent with those from large-eddy simulation (LES) results and theoretical expectations. Newly fitted gradient functions are proposed based on observations for this forested site. The integrated bottom-up function over the lowest 4% of the CBL depth estimated from the tower data is about five times larger than that from LES results for a ‘with-canopy’ case, and is smaller than that from LES results for a ‘no-canopy’ case by a factor of 0.7. We discuss the uncertainty in the evaluated gradient functions due to stability, wind direction, and uncertainty in the entrainment flux and show that while all of these have a significant impact on the gradient functions, none can explain the differences between the modelled and observed functions. The effects of canopy features and atmospheric stability may need to be considered in the gradient function relations.     

Strontium-90 in surface and groundwater of the reclamation systems at the Chernobyl exclusion zone

The left bank of the Pripiat' river, opposite of the Chernobyl Nuclear Power Plant (ChNPP) is covered with a developed network of drainage canals, built for the reclamation of the swampy floodplain. This area is highly contaminated after the disaster in 1986. Concentrations of the most mobile radionuclide, ⁹⁰Sr, are comparable with ¹³⁷Cs, and reach many MBq m^-2. Their ratio in the upper 10 cm of soil is about 1. The high surface contamination as well as precipitation and flooding have been responsible for the water pollution in this area. Some specifics of ⁹⁰Sr behaviour in surface and groundwater, controlled by the hydrological regime, are described.     

NAZARETH METEORITES: CLASSIFICATION AND NOMENCLATURE

Six meteorites have been found in Castro County, Texas and named after the town of Nazareth, but each of the four classified specimens has been given two different full names. To clarify this, we have classified the other two specimens and helped to establish approved names for all six specimens. The classes and names of the six specimens, which probably come from six separate falls, are as follows: Nazareth (a), L6; Nazareth (b), L6; Nazareth (c), H5; Nazareth (d), H5; Nazareth (e), H6; Nazareth (iron), IIIA.