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A Crystal-chemical Model For Pbca Ortho-pyroxene
No full textThe study of a crystal-chemical model for Pbca orthopyroxene was undertaken using the results of X-ray single-crystal structure refinements and chemical analyses of about 200 samples with different compositions and degrees of order. Multiple linear correlations between compositional and structural variables were searched for using the statistic package SPSS. The coefficients and constant terms of linear equations that allow the prediction of cell parameters and interatomic distances for any orthopyroxene, starting from its crystal-chemical formula, were calculated. From the predicted distances, the geometric refinement program DLS-76 yields the relevant atomic positions, which agree satisfactorily with those measured experimentally. The same results were obtained more quickly using coefficients that directly express the correlation between the atomic fractions at the structural sites and the atomic positions. The calculation of atomic positions makes it possible to study the effects induced on the structure by any variation of chemical composition and cation distribution; in particular it has allowed the prediction of structural properties for fictive end-members. The coefficients for the calculation of cation-O mean bond distances of the regular polyhedra M1 and SiB appear to be nearly correlated to the ionic radii of the cations at these sites, thus justifying the introduction of (M1-O) and (SiB-O) bond distances in the linear equations used for determining or checking M1 and SiB site populations. The prediction of atomic positions can be used for lattice-energy calculations, in order to develop a structure-energy model for orthopyroxene. The coefficients and constant terms of linear equations for the atomic fractions as a function of cell parameters, atomic positions, and mean atomic numbers at M1 and M2 sites were also calculated in order to test the possibility of predicting, for any orthopyroxene, its cation distribution in the absence of chemical analysis
Crystal-chemical Implications of the Mg-2+-Fe-2+ Distribution In Orthopyroxenes
No full textStructure reflnement data of natural and heated orthopyroxenes with different Fe/
(Fe + Mg) ratios were used for a systematic analysis of structural variations as a function of
chemical composition and ordering degree. The mean T-O bond distances do not vary
significantlyw ith increasingir on content and thereforea ny changeo f (TB-O) distancesm ay
provide a measure of Si/Alrv substitution, which occurs only at the TB site. The mean Ml-O
distancei ncreasesli nearly with iron content as a consequencoef a homogeneouse xpansiono f
Ml site.T he mean M2-O distancea lso increasesw ith iron content,b ut the linear plot is the
sum of very different trends of the individual bond lengths; among these, the M2-O3 distancese
xhibit a peculiarb ehaviorc onsistentw ith a 4 + 2 coordination of the M2 polyhedron.
The R3+ cations, which enter the structure with the Si/Altr substitution, appear to order
totally in the Ml site
High-pressure behavior of space group P2/n omphacite
No full textA single-crystal X-ray diffraction (XRD) study, using a diamond-anvil cell at high pressure and room temperature, was performed on a crystal from a natural space group P2/n omphacite sample with composition very close to Jd 55Di 45 and with a high degree of order in cation distribution. Unit-cell pa-rameters were determined at 13 different pressures up to about 7.5 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) fitted to the P-V data yielded V 0 = 421.43(4) Å 3, K T0 = 122(1) GPa, and K' = 5.1(3). The K T0 value for this sample lies between the data obtained for the two end-members jadeite and diopside, and describes a slight positive curvature trend. During the same experiment, intensity data were collected and crystal structures were refined at 5 pressures up to 7.3 GPa. Both M1 and M2 polyhedra volumes showed a slight but significant change in slope at about 4 GPa. This behavior can likely be explained in terms of tilt angle variation of TA and TB tetrahedral, which also showed a change in slope with pressure, rather than in terms of bond length compression anomaly
New thermoelastic parameters of natural C2/c omphacite
No full textThe compressibility at room temperature and the thermal expansion at room pressure of two disordered crystals (space group C2/c) obtained by annealing a natural omphacite sample (space group P2/n) of composition close to Jd 56Di 44 and Jd 55Di 45, respectively, have been studied by single-crystal X-ray diffraction. Using a Birch-Murnaghan equation of state truncated at the third order [BM3-EoS], we have obtained the following coefficients: V 0 = 421.04(7) Å 3, K T0 = 119(2) GPa, K′ = 5.7(6). A parameterized form of the BM3 EoS was used to determine the axial moduli of a, b and c. The anisotropy scheme is β c ≤ β a ≤ β b, with an anisotropy ratio 1.05:1.00:1.07. A fitting of the lattice variation as a function of temperature, allowing for linear dependency of the thermal expansion coefficient on the temperature, yielded α V(1bar,303K) = 2.64(2) × 10 -5 K -1 and an axial thermal expansion anisotropy of α b ≫ α a > α c. Comparison of our results with available data on compressibility and thermal expansion shows that while a reasonable ideal behaviour can be proposed for the compressibility of clinopyroxenes in the jadeite-diopside binary join [K T0 as a function of Jd molar %: K T0 = 106(1) GPa + 0.28(2) × Jd (mol%)], the available data have not sufficient quality to extract the behaviour of thermal expansion for the same binary join in terms of composition
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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