1,721,176 research outputs found
Silicified cone-in-cone structures from Erfoud (Morocco): A comparison with impact-generated shatter cones
No full text40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Cònero Riviera, Ancona, Italy.
No full textIron oxidation state in impact glass from the K/T boundary at Beloc, Haiti, by high-resolution XANES spectroscopy
No full textWe examined the local iron environment in nine impact glasses from the Cretaceous-
Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during
the Chicxulub impact event. The samples have been analyzed by Fe K-edge high-resolution X-ray
absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state
and the coordination number. The pre-edge peak of our high-resolution XANES spectra display
noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range
from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing
lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four- and
five-coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra can be interpreted
as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. There is no evidence for six-fold coordinated Fe;
however, its presence in small amounts cannot be excluded from XANES data alone. Our
observations can be explained by two possible scenarios: either these impact glasses formed under
very reducing conditions and, because of their small size, were easily oxidized in air while still
molten, or they formed under a variety of different oxygen fugacities resulting in different Fe
oxidation states. In the first case, the oxidation state and coordination number would imply similar
formation conditions as splash-form tektites, followed by progressive oxidation
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Iron oxidation state in the Fe-rich layer and silica matrix of Libyan Desert Glass: a high-resolution XANES study
No full textLibyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert. Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown. Some rare samples present dark-colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component. We have studied the iron local environment in an LDG sample by means of Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe-poor matrix and Fe-rich layers. The pre-edge peak of the high-resolution XANES spectra of the sample studied displays small but reproducible variations between Fe-poor matrix and Fe-rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number. Comparison with previously obtained data for a very low-Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination ([4]Fe3+) in the low-Fe sample, the sample containing the Fe-rich layers display a mixture of tetra-coordinated trivalent iron ([4]Fe3+) and penta-coordinated divalent iron ([5]Fe2+), with the Fe in the Fe-rich layer being more reduced than the matrix. From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra-sample variations in the Fe-oxidation state, confirm that LDG is an impact glass and not a tektite-like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe-rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions
No full textWe determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe
K-edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si-K impact spherule from the same impact layer. The pre-edge peak of our high-resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe3+. Yellow K/T glasses have significantly higher Fe3+/(Fe2+ + Fe3+) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe3+) and high Si-K glass (20 mole% Fe3+). In particular, all the pre-edge peak data on
these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of [4]Fe2+ and [5]Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of
[4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+.
Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe3+/(Fe2+ + Fe3+) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca-sulfate- and Ca-carbonate-bearing sedimentary rocks at the impact site
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
The influence of mafic and felsic crust on the seawater chemistry ca. 3.0 billion years ago: Evidence from Nd isotopes in banded iron formations from the Murchison Greenstone Belt
No full texthttp://dx.doi.org/10.13039/501100002428 Austrian Science Fun
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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