1,721,006 research outputs found
Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
No full textThe thermoelastic behavior of a crystal of Fe-rich holmquistite with crystal-chemical formula A(K0.01Na0.01)B(Li1.88Mg0.10Na0.02)C(Mg1.68Fe1.422+ Mn0.022+ Al1.88)TSi8.00O22W[(OH)1.97F0.03] was studied by single-crystal X‐ray diffraction at temperatures up to 1023 K, where isothermal annealing in air for 160 h yielded the loss of 0.85 H apfu coupled with oxidation of M1Fe. A complex pattern of cation exchanges was deciphered by comparing structure refinements done before and after annealing. Li migration from the M4 to M3 site is responsible for nonlinearity of the c parameter around 600 K during the first annealing. Cooling of the partially deprotonated crystal to room temperature (RT) showed discontinuities in trends of the b and c parameters around 820-800 K, which cannot be ascribed to a phase transition and can be explained by a rearrangement of the structural units affecting the geometry of the M4 polyhedron. Such discontinuities have never been observed in amphiboles before and must be related to dimensional constraints deriving from the peculiar composition of this amphibole, which contains the smallest possible homovalent constituents, i.e., BLi, CAl, and TSi. The calculated thermoelastic parameters are: Fe-rich holmquistite: αa = 1.36(2)×10-5; αb = 0.55(1)×10-5; αc = 1.5(1)×10-5 - 6.7(9)×10-9; αV = 3.5(3)×10-5 - 0.8(3)×10-8 (polynomial); 2.58(6)×10-5 (linear); partially deprotonated Fe-rich holmquistite: αa = 1.324(9)×10-5 (RT-1023 K); αb = 0.60(1)×10-5 (RT-773 K); αc = 0.68(2)×10-5 (RT-773 K); αV = 2.59(2)×10-5 (RT-773 K). Fe-rich holmquistite is much stiffer than all the previously studied orthorhombic Pnma and Pnmn amphiboles. The results of this work allow progress toward a general model that may explain how the amphibole structure responds to non-ambient conditions, and allows the release of water in diverse geological environments
Structure refinement and new crystal-chemical data for tiragalloite (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH) from the Scerscen glacier, Val Malenco, Italy
No full textA new sample of tiragalloite from the Scerscen Glacier of the Malenco valley (Lombardy, Italy) has been studied by single-crystal X-ray diffraction and microchemical analyses. Structure refinement in space group P21/n converge to R1=0.0373 for 6149 reflections with I ≥ 2 Σ(I) and 0.0583 for all 8242 data. The refined unit-cell parameters are: A=6.6702(2) Å, b=19.9336(7) Å, c=7.5759(2) Å, β=95.518(1)°, V=1002.63(5) Å; and the crystal-chemical formula is (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH), with Z=4. The new data of tiragalloite from Scerscen confirm the general organization of the crystal structure, as previously reported in literature for two samples from Graveglia valley, Liguria, Italy, which contained a different amount of V5+ or As5+ cations. However, refinement of the site populations at the Mn(3) and Mn(4) sites suggests that the distribution of Ca among these atom sites might be different than that reported in literature. In particular, in tiragalloite from Scerscen, Ca seems to be preferentially located at the [6]Mn(4)(O5OH) octahedron rather than within the large [7]Mn(3)O7 polyhedron. The topology of the [AsSi3O12(OH)]8- unit of tiragalloite is compared with natural and synthetic phases containing similar groups of four tetrahedra
The crystal structure of beryllonite from the type locality and comparison with isopointal structures
No full textThe crystal structure of beryllonite, NaBePO4, has been re-evaluated based on high-resolution X-ray diffraction data using a single crystal from the type locality, Stoneham (Maine, USA). Beryllonite is monoclinic, with a = 8.1309(2) Å, b = 7.7974(2) Å, c = 14.1918(4) Å, ß = 90.039(1)°, V = 899.76(4) Å3, space group P21/n (no. 14), Z = 12. Full anisotropic crystal-structure refinement converged to an R1 value of 0.0504 based on all 9435 unique reflections in the θ range 2–50°. The crystal structure consists of a framework of PO4 and BeO4 tetrahedra sharing all vertexes with adjacent tetrahedra. On the (010) plane, each tetrahedron shares three vertexes with three different tetrahedra forming a quasi-ditrigonal 63 net with alternating PO4 and BeO4 tetrahedra. The remaining vertex of each PO4 and BeO4 tetrahedra points outside the (010) plane and allows corner-linkage among the tetrahedral sheets. Na-centred polyhedra with six-and nine-fold coordinations are located in channels defined by the framework of tetrahedral rings. Beryllonite is isostructural with several natural and synthetic compounds and for these, a quantitative comparison of the crystal-chemical features is reported
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
Thermal behaviour of a Ti-rich phlogopite from Mt. Vulture (Potenza, Italy) : An in situ X-ray single crystal diffraction study
No full textThe structural evolution of a trioctahedral mica from Cava St. Antonio, Mt. Vulture, Potenza, Italy, has been studied in the temperature range 100–1023 K using in situ single-crystal X-ray diffraction techniques. The sample used shows a Ti-rich composition close to the phlogopite-annite join with the following crystal-chemical formula: (K0.85Na0.11Ba0.03)(Al0.17Mg1.98Mn0.01Fe2+0.29Fe3+0.37Ti0.18)(Si2.75Al1.25) O10.66(F0.13OH1.20Cl0.01). In the present study, the chemical and structural changes and the deprotonation process involved during heating have been characterized. Analysis of the data showed that a, b, and c cell parameters expand almost linearly up to 823 K, while the β angle remains constant. A sharp decrease in the unit-cell dimensions was observed at 873 and 1023 K. Linear trends obtained during reversal experiments from 1023 K to room temperature demonstrated the irreversibility of these events.
Structure refinements of single-crystal XRD data collected at 100, 200, 298, 473, 673, 873, and 1023 K converged to 2.14 ≤ R (%) ≤ 8.47, 2.47 ≤ Rw (%) ≤ 10.83. In the temperature range 100–673 K, the thermal expansion along the c direction is mainly due to interlayer thickness dilation. The tetrahedral ring approaches the ideal hexagonal shape with increasing temperature to match the expanding octahedral sheet. In the range 873–1023 K, a strong shrinking of the interlayer is associated with the shortening of the M1-O4 and M2-O4 distances and to the consequent reduction of octahedral thickness. Such structural features indicate the occurrence of Fe oxidation process, involving loss of structural H, which is responsible for a phase transition. Mössbauer spectroscopy supported this hypothesis
Kinetics of Fe-oxidation/deprotonation process in Fe-rich phlogopite under isothermal conditions
No full textOrthopyroxene as a Geospeedometer: Thermal History of Kapoeta, Old Homestead 001 and Hughes 002 Howardites
No full textSingle crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and
Hughes 002 howardites were studied by x-ray diffraction and microprobe analyses. The Fe-Mg equilibrium
distribution coefficients k~ of the crystals were used to calculate the closure temperatures (T,) using the
calibration by Stimpfl et al. (1 999). The compositions, the presence of exsolved augite lamellae, and the T,
values (from 365 to 385 "C) obtained for Kapoeta orthopyroxenes suggest that our fragment comes from a
diogenitic cumulate clast. The more Fe-rich composition, the absence of exsolved lamellae, and the higher
T, values (from 583 to 605 "C) measured in the Old Homestead 001 orthopyroxenes suggest that this
fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002
orthopyroxenes, close in composition to Old Homestead 001, the different T, values (339, 358, and 607 "C)
recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest T, support
the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic
orthopyroxenes with different thermal histories
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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