1,720,975 research outputs found
The Crystal-chemistry of Pumpellyite - An X-ray Rietveld Refinement and Fe-57 Mossbauer Study
No full textPumpellyite of the general formula W8X4Y8Z12O56-n(OH)n contains Fe, Al and Mg in two crystallographically different octahedral sites. Three different pumpellyite samples covering the known compositional field from Al- to Fe-rich have been studied to determine the valence state and intracrystalline partitioning of the Fe cations between the two independent octahedral sites. Fe+2 and Fe+3 cation partitioning is interpreted on the basis of results obtained by Fe-57 Mossbauer spectroscopy at 293 and 77 K and from Rietveld structure analysis performed on powder X-ray diffraction data. Pumpellyite from low-grade metamorphic rocks typically contains a majority of iron in the Fe+3 oxidation state, which is found in the smaller and less symmetrical octahedral Y-site. Fe+2 was also present in all pumpellyite samples studied and it is located in the larger and more symmetrical octahedral X-site
The crystal chemistry of julgoldite-Fe3+ from Bombay, India, studied using synchrotron X-ray powder diffraction and Fe-57 Mossbauer
No full textThe crystal structure of julgoldite-Fe3+ from Bombay, India, was investigated by Fe-57 Mossbauer spectroscopy and synchrotron X-ray powder diffraction. Only ferric iron was detected in the Mossbauer measurements and it occurs at two different octahedral sites in the atomic ratio 20:80. Based on Rietveld refinements, the Fe3+ cations are located at the X- and Y-octahedral sites with atomic percentages of about 25% and 75%, respectively. The resulting chemical formula of the Bombay julgoldite sample is Ca-8(Fe2.73+Al1.1Mg0.2)(Fe-8.0(3+))Si12O42(OH)(14). The oxidation state of Fe is not the same as that arrived at through simple crystal-chemical considerations. Such analysis cannot give quantitative results for the valence state of mixed-valence cations in pumpellyite-type minerals and their intracrystalline partitioning behavior. Assignments of the Mossbauer absorption doublets and an analysis of Fe-intracrystalline partitioning behavior are discussed with reference to previous works on different pumpellyite-type minerals
Microscopic strain in synthetic pyrope-grossular solid solutions determined by synchrotron X-ray powder diffraction at 5 K: The relationship to enthalpy of mixing behavior
No full textA series of synthetic pyrope-grossular garnets (Mg3Al2Si3O12-Ca3Al2Si3O12) were investigated by powder X-ray synchrotron radiation at 5 K to determine their microscopic structural strain, which may be responsible for the observed excess enthalpy of mixing for this binary. This substitutional solid solution provides an excellent system for investigating microscopic-macroscopic relationships and the physical nature behind non-ideal thermodynamic mixing behavior in silicates, because of
the measurable nonidealities shown by its enthalpy and volume of mixing. An analysis of the X-ray refection profiles, based on theoretical considerations of X-ray line broadening, permits for the first time a direct experimental determination of crystallite size and the root-mean-square structural strain for a mineral solid solution. The measured microscopic strain shows positive and asymmetric deviations from linearity across the join with the largest excess in pyrope-rich compositions. There is a
good correlation between the structural strain and the macroscopic enthalpy of mixing behavior for pyrope-grossular garnets as measured by calorimetry
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
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Temperature dependence of disorder and correlation effects in almandine X-site: XAFS study and comparison with XRD data
No full textXAES measurements in -t.ile fem~r4tlitè range :20473 K ai Ihe F~,K,edge of synt.!ìeti<:: aìmandine (pe3:-\i2Sig01 V '
allow us to deterrùine thJ~T-pependen~ of the XAFSDebye-Waller (DW) Iactors fòr eachof the-twocrystallographically
independent Fe'-Qbond dìstanees (Fe( 1)"0 and Fe{:ì).-Q). The XAFS DW factor of the longer Fe(2)-{)bondis 'Iarger and. has a
grearer T-dependence ihan the shorter F~(l)-O bond, whieh reraains constant over the measured T ranae, The XAFS DW
faetors are compared with the uncorrelared part of the DW factors derivcd from a single-crystal X-ray-diffraetipQ: .(XPJ)
studiy on almandine and are interpreted.on the basis of theEinsìeìn .model. A cemparison with dìffractiorr data shows that
the shoner Fe(l)-O bond is significantly affected by correlation in the atomic displacements
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