1,721,073 research outputs found
The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling
No full textThe magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg2 + undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg2 + undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO3+ and MgCO3° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg2 +(aq) and Mg2 + bound to a number of inorganic and organic ligands show a strong enrichment of 26Mg in MgCO3°, MgHCO3+ and MgSO4° compared to Mg2 + (i.e. 1000ln?26/24MgCO3°-1000ln?26/24Mg2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 ?, Mg(oxalate)22 ?, Mg(citrate)?) or depletion (Mg(EDTA)2 ?, Mg(citrate)24 ?) of 26Mg in Mg-carboxylate complexes compared to Mg2 +. Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg2+, Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg2+ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments
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
Stabilité de la magnésite et de la dolomite ; interprétation des résultats de mise en solution et de synthèse par thermodiffusion
No full textAttempts to put protractedly ground calcite, magnesite and dolomite into solution show the impossibility of determining the solubility of a mineral under conditions in which it cannot be synthetized.
On the other hand, synthesis experiments by means of thermal diffusion have enabled crystallization of minerals which should be metastable if Robie's free-energy data of carbonates are accepted.
As a result, free-energy data of carbonates in the systems MgO-CO₂-H₂O and CaO-MgO-CO₂-H₂O have been reevaluated from solubility measurements, except those obtained outside the stability fields of minerals. Stability diagrams based on new free-energy values appear to be in good agreement with our experimental results and with the geological behavior of these minerals.Des essais de mise en solution aqueuse, réalisés dans les conditions normales (T = 22° C ; P[CO₂] = 10⁻³ֹ‧⁵ atm.), à partir de calcite, magnésite et dolomite longuement broyées, font apparaître l'impossibilité de déterminer la solubilité d'un minéral dans des conditions où il n'est pas synthétisable.
D'autre part des expériences de synthèse, effectuées par thermodiffusion, à partir de carbonates magnésiens, à différentes températures et pressions de CO₂, aboutissent à la cristallisation de minéraux dont l'apparition est en désaccord avec les diagrammes de stabilité couramment admis.
L'ensemble de ces observations nous a conduit à recalculer les valeurs d'enthalpies libres de formation des carbonates dans les systèmes MgO-CO₂-H₂O et CaO-MgO-CO₂-H₂O, en excluant les données obtenues à partir de mises en solutions effectuées hors des domaines de stabilité. Les valeurs ainsi calculées permettent de tracer des diagrammes de phases qui rendent mieux compte de nos résultats expérimentaux ainsi que des conditions naturelles de formation de ces minéraux.Dandurand Jean-Louis, Schott Jacques. Stabilité de la magnésite et de la dolomite ; interprétation des résultats de mise en solution et de synthèse par thermodiffusion. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 100, 2, 1977. pp. 94-99
Solubilité de la silice dans des solutions aqueuses très concentrées de formamide et de chlorure de lithium. Détermination du coefficient d'activité de la silice en solution
No full textAmorphous silica solubility measurements were performed in formamide and lithium chloride solutions of various concentrations. Each of the two kinds of experiments shows a different behavior. In lithium chloride solutions, the silica molality decreases when the salt concentration increases. In formamide solutions an opposite behavior is observed and the silica molality increases when the activity of water decreases until zero. The question of how to calculate the activity coefficient of silica as a function of water activity and the possible role of various parameters influencing the silica solubility are discussed.La mesure de la solubilité de la silice amorphe dans des solutions de formamide et de chlorure de lithium pour différentes concentrations montre des comportements différents. Dans les solutions de chlorure de lithium, la molalité de la silice diminue lorsque la concentration augmente. Dans les solutions de formamide, l'inverse se produit et la molalité de la silice augmente quand l'activité de l'eau décroît. Le problème de la détermination expérimentale du coefficient d'activité de la silice en fonction de l'activité de l'eau des solutions est posé ; l'influence des différents paramètres intervenant pour régler la solubilité de la silice est discutée.Dandurand Jean-Louis, Schott Jacques, Tardy Yves. Solubilité de la silice dans des solutions aqueuses très concentrées de formamide et de chlorure de lithium. Détermination du coefficient d'activité de la silice en solution. In: Bulletin de Minéralogie, volume 105, 4, 1982. pp. 357-363
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
Etude des mécanismes de transport sélectif de matière par effet thermogravitationnel dans les roches considérées comme milieux poreux
No full textCosteseque Pierre, Dandurand Jean-Louis, Fortuné Jean-Pol, Gout Robert, Grezes-Besset Raoul, Mouysset-Espagne Marguerite, Pérami René, Schott Jacques, Tollon Francis. Etude des mécanismes de transport sélectif de matière par effet thermogravitationnel dans les roches considérées comme milieux poreux. In: Phénomènes de transport de matière dans l'écorce terrestre. ATP - CNRS. Action thématique Programmée du Centre National de la Recherche Scientifique. Résultats scientifiques. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1979. pp. 87-94. (Sciences Géologiques. Mémoire, 53
Etude des mécanismes de transport sélectif de matière par effet thermogravitationnel dans les roches considérées comme milieux poreux
No full textCosteseque Pierre, Dandurand Jean-Louis, Fortuné Jean-Pol, Gout Robert, Grezes-Besset Raoul, Mouysset-Espagne Marguerite, Pérami René, Schott Jacques, Tollon Francis. Etude des mécanismes de transport sélectif de matière par effet thermogravitationnel dans les roches considérées comme milieux poreux. In: Phénomènes de transport de matière dans l'écorce terrestre. ATP - CNRS. Action thématique Programmée du Centre National de la Recherche Scientifique. Résultats scientifiques. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1979. pp. 87-94. (Sciences Géologiques. Mémoire, 53
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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