866 research outputs found
Oxygen isotope fractionation in Apollo 12 rocks and soils
Oxygen isotopic compositions and fractionations between coexisting minerals found in lunar samples from Apollo 11 and Apollo 12 flight
Heterogeneities in the solar nebula
Oxygen isotopic compositions of the high-temperatue phases in carbonaceous chondrites define a mixing line with an O-16 rich component and show little superimposed chemical isotope fractionation. Within a single inclusion in Allende, variations of delta O-18 and delta O-17 of 39% are found. The ordinary chondrites are slightly displaced from the terrestrial fractionation trend, implying that at least 0.2% of the oxygen in terrestrial rocks was derived from the O-16 rich component
Unusual superexchange pathways in a Ni triangular lattice of NiGaS with negative charge-transfer energy
We have studied the electronic structure of the Ni triangular lattice in
NiGaS using photoemission spectroscopy and subsequent model
calculations. The cluster-model analysis of the Ni 2 core-level spectrum
shows that the S 3 to Ni 3 charge-transfer energy is -1 eV and the
ground state is dominated by the configuration ( is a S 3 hole).
Cell perturbation analysis for the NiS triangular lattice indicates that
the strong S 3 hole character of the ground state provides the enhanced
superexchange interaction between the third nearest neighbor sites.Comment: 10 pages, 5 figures, accepted to PR
Extracellular calcification of Braarudosphaera bigelowii deduced from electron microscopic observations of cell surface structure and elemental composition of pentaliths
We have performed morphological and crystallographic studies of Braarudosphaera bigelowii using various light and electron microscopy techniques. A study by light microscopy revealed that B. bigelowii has a haptonema, and can use it for adhesion to external substrates. A study of the pentaliths by transmission electron microscopy indicates that the well-known trapezoidal lamina is formed with foliate crystals having perfectly identical crystallographic orientation. A cytological study shows that the pentaliths of B. bigelowii are surrounded by an organic structure consisting of a pentalith-substrate and thin organic layers. The pentalith-substrate underlies the proximal surface of the pentaliths and extends between the sides of the individual pentaliths, it also extends between the five trapezoidal segments forming a pentalith. Thin organic layers, which apparently originate from ridges of pentalith-substrate, cover the distal surface of the trapezoidal segments. The close association between the pentalith-substrate, organic layers, and pentaliths leads us to the hypothesis that calcification of the pentaliths occurs between the pentalith-substrate and organic layers, extracellularly. The relatively high Mg content observed in pentaliths supports our hypothesis of extracellular calcification
Random amplification of polymorphic DNA reveals clonal relationships among enteropathogenic Escherichia coli isolated from non-human primates and humans
Enteropathogenic Escherichia coli ( EPEC) strains are important agents of infantile diarrhea all over the world, gaining even greater importance in developing countries. EPEC have also been isolated from various animal species, but most isolates belong to serotypes that differ from those recovered from humans. However, it has been demonstrated that several isolates from non- human primates belong to the serogroups and/ or serotypes related to those implicated in human disease. The objective of this study was to evaluate the genetic differences between thirteen strains isolated from non- human primates and the same number of strains isolated from human infections. Human isolates belonged to the same serogroup/ serotype as the monkey strains and the evaluation was done by analysis of random amplified polymorphic DNA. Dendrogram analysis showed that there was no clustering between human and monkey strains. Human and non- human isolates of the EPEC serotypes O127:H40 and O128:H2 shared 90 and 87% of their bands, respectively, indicating strong genomic similarity between the strains, leading to the speculation that they may have arisen from the same pathogenic clone. To our knowledge, this study is the first one comparing genomic similarity between human and non- human primate strains and the results provide further evidence that monkey EPEC strains correlate with human EPEC, as suggested in a previous investigation
Crystallization behavior of iron- and boron-containing nepheline (Na2 O●Al2 O3 ●2SiO2 ) based model high-level nuclear waste glasses
This study focuses on understanding the relationship between iron redox, composition, and heat‐treatment atmosphere in nepheline‐based model high‐level nuclear waste glasses. Glasses in the Na2O–Al2O3–B2O3–Fe2O3–SiO2 system with varying Al2O3/Fe2O3 and Na2O/Fe2O3 ratios have been synthesized by melt‐quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N2), and reducing (96%N2–4%H2). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass‐ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass‐ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass‐ceramics produced as a result of heat treatment in air and N2 atmospheres developed a golden/brown colored iron‐rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron‐rich layer was observed in glass‐ceramics with varying Al2O3/Fe2O3 ratio, it was absent from glass‐ceramics with varying Na2O/Fe2O3 ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron‐rich layer on the surface of glass‐ceramics on the chemical durability of high‐level nuclear waste glasses have been discussed
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