1,721,019 research outputs found
Al-Fe disorder in synthetic epidotes: a single-crystal X-ray diffraction study
No full textSynthetic epidotes were produced using a Tuttle type hydrothermal vessel in the temperature range from 500 to 700 degrees C and at 4.5 to 5.0 kbars pressure. Single-crystal X-ray diffraction structure refinements yielded intracrystalline cation distributions for two crystals grown at 600 degrees C (0.68 and 0.73 Fe apfu) and five crystals grown at 700 degrees C (0.88 to 1.08 Fe apfu). The resulting Fe occupancies were compared with those calculated according to a thermodynamic model: The samples formed at 700 degrees C display a Fe distribution between M3 and M1 sites that match the calculation; nevertheless, we found the presence of a small but appreciable amount of Fe in the M2 sites that increases with the total Fe content (up to 0.08 Fe apfu). The crystals formed at 600 degrees C show a much higher disorder than expected, bur have no Fe in the M2 site. Crystal-chemical features of the samples studied were compared with those of a previously published set of natural epidotes displaying a much lower Fe disorder: In particular, the unit-cell volume vs. Fe content relationship is not affected by the intracrystalline Fe distribution. This relation suggests that the Al-Fe intracrystalline distribution is independent of pressure
Rietveld refinement of okayamalite, CA2SiB2O7: structural evidence for the B/Si ordered distribution
No full textThe structure of okayamalite from Arendal, Norway, was refined using the Rietveld method, with CoKα powder X-ray diffraction data (R(F) = 3.69%). Okayamalite exhibits a melilite-type structure, space group P4̄21m, with cell edges a = 7.1248(2), c = 4.8177(2) Å. Si and B are ordered on the T1 and T2 sites respectively, in agreement with the refined tetrahedral distances ( = 1.657 Å and =1.498 Å). In comparison with the other melilite-type compounds, the cation population in okayamalite leads to the minimum structural misfit between tetrahedral and square-antiprism layers
High-pressure structural behavior of ingersonite, Ca 3 Mn 2+ Sb 5+ 4 O 14 : an in situ single-crystal X-ray study
No full textCiprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy)
No full textTwo new minerals of the hellandite group were found within alkali-syenitic ejecta enclosed in pyroclastic formations of the Roman Comagmatic Province (Latium, Italy). Mottanaite-(Ce) [ideally,Ca-X(4)Y(CeCu)Al-Z(T) Be-2(Si4B4O22)O-W(2)] and ciprianiite [ideally Ca-X(4)Y[(Th,U)(REE)](Z)Al(T)square(2)(Si4B4O22) (W)(OH, F)(2)] occur as transparent, brown-colored, tabular euhedral crystals in miarolitic cavities and voids of the ejecta, which consist mainly of sanidine and plagioclase (An ranging from 20 to 80%), with minor amounts of feldspathoid, clinopyroxene and/or clinoamphibole, magnetite, titanite, and zircon. Locally, accessory minerals include britholite-(Ce), baddeleyite, phosphate to silico-phosphate phases close in composition to the brabantite-cheralite series, thorite, fluorite, danburite, and vonsenite. The genesis of the new hellandite end-members can be related to late-stage post magmatic hydrothermal fluids enriched in Zr, Ti, REEs, and actinide elements.
Both mottanaite-(Ce) and ciprianiite have a vitreous luster and are non-fluorescent. Cleavage is absent in mottanaite-(Ce), fair to good in ciprianiite, {100}. Twinning is frequently observed in ciprianiite. Due to the strong intra-crystalline chemical zoning and twinning, physical properties could be measured only for mottanaite-(Ce). D-meats is 3.61(4) g/cm(3), D-calc is 3.88 g/cm(3). Mottanaite-(Ce) is biaxial negative, with alpha = 1.680(5), beta = 1.694(2), gamma = 1.708(5); 2V(meas) similar to90degrees.
Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals Studied are: a = 19.032(9) Angstrom, b = 4.746(3) Angstrom, c = 10.248(5) Angstrom, beta = 110.97(5)degrees, V = 864.3(8) Angstrom(3) for mottanaite-(Ce), and a = 19.059(5) Angstrom, b = 4.729(1) Angstrom, c = 10.291(4) Angstrom, beta = 111.33(2)degrees, V = 864.0(5) Angstrom(3) for ciprianiite. Single-crystal structure refinement confirmed the presence of a further distorted tetrahedral site which was first detected in a hellandite-(Ce) sample from Latium (Oberti et al. 1999). This site is occupied by Be (+/-Li) in stoichiometric mottanaite-(REE), whereas it mainly hosts hydrogen (bonded to the O5 oxygen atom) in ciprianiite and hellandite-(REE); solid solution between the end-members is possible, as shown by the studied samples. The chemical composition of the refined crystals was obtained by combining EMPA (for medium-Z elements) and SIMS analyses (for low- and high-Z elements); their results are in excellent agreement with the chemical information obtained from the structure refinements, The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are Ca-M3.4(4)M2[REE1.45Ca0.37(Th,U)(0.17)Y-4+(0.01)](Sigma=2)(M1)(Al-0.50 Fe0.383+Mg0.03Ti0.074+)(Sigma=0.99)(T)(Bc(1.18)Li(0.02)square(0.37))B3.99Si3.98O220.5(O1.042-F0.53OH0.43)(Sigma=2) for mottanaite-(Ce); and Ca-M3.4(4)M2[REE0.723+(Th,U)(0.66)4+Ca0.60Y0.02](Sigma=2)(M1)(Al0.48Fe0.383+Ti0.104+Mg0.05Mn0.023+)(Sigma=1.03)(T)(Be(0.82)square(0.60)Li(0.04)) B4.00Si4.00O22O5((O0.97OH0.54F0.49)-O-2)(Sigma=2) for ciprianiite
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
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