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The real structures of clinotobermorite and tobermorite 9Å: OD character, polytypes, and structural relationships
No full textClinotobermorite, Ca 5Si 6O 17·5H 2O, is a rare mineral, structurally related to tobermorite 11 Å. It is characterized by the presence of structural disorder (evidenced by the diffuseness of the reflections with k odd) which, until now, prevented the determination of its real structure. In this paper, a model for the real structure of clinotobermorite is proposed on the basis of OD theory, through examination of the X-ray diffraction pattern of a sample coming from the Wessels mine (South Africa). The proposed model, which assumes the presence of silicate double chains of wollastonite-type, is confirmed by structural refinements carried out for the two polytypes with maximum degree of order (MDO). The MDO 1 polytype of clinotobermorite (monoclinic, space group Cc, a = 11.276(2), b = 7.3427(8), c = 22.642(4) Å, β = 97.28(1)°) was refined up to R = 0.15, whereas the two refinements performed on the MDO 2 polytype (triclinic, space group C1, a = 11.274(2), b = 7.3439(7), c = 11.468(2) Å, α = 99.18(1), β = 97.19(1), γ = 90.09(1)°) converged to R = 0.12 and R = 0.10, respectively. In clinotobermorite infinite calcium polyhedral layers parallel to (001) are connected through double silicate chains [Si 6O 17] 10- running along b; additional calcium cations and H 2O molecules are placed in the channels of the resulting framework. By dehydration at 225°C, clinotobermorite transforms topotactically into a new phase, which also displays an OD character. The results of the structural refinement carried out for its triclinic MDO 2 polytype (space group C1̄, a = 11.156(5), b = 7.303(3), c = 9.566(5) Å, α = 101.08(4), β = 92.83(5), γ = 89.98(4)°) indicate that this phase, with crystal chemical formula Ca 5Si 6O 16(OH) 2, exhibits single chains of wollastonite-type, resulting from decondensation of the double chains. On the basis of the new detailed structural information a possible explanation for the enigmatic thermal behaviour to tobermorite 11 Å has been proposed
The real structure of tobermorite 11A: normal and anomalous forms, OD character and polytypic modifications
No full textSpecimens of tobermorite 11Å from Wessels mine, South Africa, an 'anomalous' tobermorite, and from Bašcenov, Urals, Russia, a 'normal' tobermorite, have been studied through X-ray diffraction. Their structural disorder, evidenced by the diffuseness of the reflections with k odd, has been dealt with through an OD approach, which allowed us to derive a reliable model of the structure, to define the two main polytypes (MDO structures), which were subsequently refined, revealing the distinctive structural details of the two specimens. The MDO1 polytype of tobermorite 11Å from Wessels mine [orthorhombic, space group F2dd, a = 11.265(2), b = 7.386(1), c = 44.970(9) Å] was refined to R = 0.128, whereas the refinement carried on the MDO2 polytype [monoclinic, space group B11m, a = 6.735(2), b = 7.385(1), c = 22.487(4) Å, Υ = 123.25(1)°] converged to R = 0.051. Two refinements were carried out for the MDO2 polytype of the specimen from the Urals [space group B11m, a = 6.732(2), b = 7.368(1), c = 22.680(4) Å, Υ = 123. 18(1)°] with data collected with conventional source (R = 0.089) and with synchrotron radiation (R = 0.110). Common structural features are the infinite layers of calcium polyhedra, parallel to (001), with tetrahedral chains of wollastonite-type running along b and attached on both sides of these layers. The composite layers are stacked along c and connected through formation of double tetrahedral chains, with 'ideal' composition [Si6O15(OH)2] and [Si6O16(OH)] in the specimens from Wessels mine and Urals respectively. The most distinctive feature of the two specimens is the content of the structural cavities: whereas only water molecules were found in the specimen from Wessels mine, 'zeolitic' calcium cations and water molecules were found in that from Urals. These structural aspects are compared and their relevance in explaining the different thermal behaviour of the two types of tobermorite 11Å is indicated
Crystal structure of clinotobermorite and modelling of tobermorite 11Å by means of the OD theory
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Mobility of acidic protons in zeolites: A neutron diffraction study of d-heulandite
No full textNeutron Rietveld refinement of a natural heulandite (Si/Al = 3.1) in its deuterium form was performed in space group C2/m. Two Brønsted acid sites were identified. One was on framework oxygen O1, occupied to 20% and headed toward the center of the 8-membered ring channel running parallel to [1 0 2]; the other was on O6, occupied to 30% and headed toward the 10-membered ring channel running parallel to the c-axis. Three other extraframework sites, located around a distance of 3 Å from the framework oxygens, were attributed to reabsorbed H2O molecules. On the whole, about 3.2 Brønsted acid sites were located representing about 37% of the value expected on the basis of the aluminium content. This discrepancy is attributed to proton transfer from the Brønsted site to reabsorbed H2O molecules, forming either a hydroxonium ion (H3O+) or charged clusters such as (H5O2)+ and (H7O3)+
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
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