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Cation distribution and optical properties of MgAl2O4-CoAl2O4 flux-grown single crystals
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Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)(3)(OH)(3)O, a new mineral species of the tourmaline supergroup
No full textVanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)(3)(OH)(3)O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and muscovite-celadonite-chromphyllite-roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately TA, and a calculated density of 3.3 g/cm(3). In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: omega = 1.767(5), epsilon = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) angstrom, V=1661.11(4) angstrom(3), Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: (X)(Na0.89K0 06 square(0.05)) (Y)(V2.773+Mg0.17Fe0.063+) (Z)(Cr1.853+Al1.59V0.783+Mg1.78) (T)[(Si5.95Al0.05)O-18] B(BO3)(3) (V)(OH2.91O0.09) (W)(O0.86F0.14). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoK alpha X-radiation. Ideally, vanadio-oxy-chromium-dravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ Cr3+. Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr3+-dominant or V3+-dominant as a result of the compositional boundaries along the solid solution between Cr3+ and V3+ that are determined at (Y+Z)(V5Cr2), corresponding to Na-Y(V-3)(Z)(V2Cr2Mg2)Si6O18(BO3)(3)(OH)(3)O, and (Y+Z)(V1.5Cr5.5), corresponding to Na-Y(V1.5Cr1.5)(Z)(Cr4Mg2)Si6O18(BO3)(3)(OH)(3)O
Vanadio-oxy-dravite, NaV3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup
No full textVanadio-oxy-dravite, NaV3(Al4Mg2)(Si6O18)(BO3)(3)(OH)(3)O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and mica, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-dravite has a Mohs hardness of approximately 7 1/2, and a calculated density of 3.14 g/cm(3). In plane-polarized light, vanadio-oxy-dravite is pleochroic (O = yellow green and E = pale olive green) and uniaxial negative: omega = 1.693(5), epsilon = 1.673(5). Vanadio-oxy-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.0273(3), c = 7.2833(1) angstrom, V = 1620.24(5) angstrom(3), Z = 3. Crystal-chemical analysis resulted in the empirical structural formula: (X)(Na0.70Ca0.23 square K-0.05(0.02))(Sigma 1.00) (Y)(V1.393+Mg1.16Al0.35Fe0.043+Ti0.044+Fe0.022+)(Sigma 3.00) (Z)(Al3.74Mg1.28V0.783+Cr0.203+)(Sigma 6.00) (T)(Si6.00O18) (B)(BO3)(3) (v)(OH)(3) (W)[O-0.74(OH)(0.26)](Sigma 1.00). The crystal structure of vanadio-oxy-dravite was refined to an R1 index of 1.70% using 1800 unique reflections collected with MoK alpha X-radiation. Ideally, vanadio-oxy-dravite is related to oxy-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ Al3+. Tourmaline with chemical compositions classified as vanadio-oxy-dravite can be either Al dominant or V dominant as a result of the compositional boundaries along the solid solution between Al and V3+ that are determined at (Y+Z)(V1.5Al5.5), corresponding to Na-Y(V1.5Al1.5)(Z)(Al4Mg2)Si6O18(BO3)(3)(OH)(3)O, and (Y+Z)(V5Al2), corresponding to Na-Y(V-3)(Z)(V2Al2Mg2)Si6O18(BO3)(3)(OH)(3)O
Optical absorption spectroscopy study of the causes for color variations in natural Fe-bearing gahnite: Insights from iron valency and site distribution data
No full textFour gahnite single crystals with variable colors from pale blue to green have been studied by a multi-analytical approach with the aim to evaluate existing assignments of optical absorption bands. Combined information from electron microprobe analyses, Mössbauer spectroscopy, IR-spectroscopy, single-crystal X‐ray structure refinements, and optical absorption spectroscopy confirms the conclusions of earlier studies that the absorption bands recorded in the visible spectral region up to ~540 nm (above ~18 500 cm–1) are related to electronic d-d transitions in tetrahedrally coordinated Fe2+. It also demonstrates that a set of absorption bands between ~550–625 nm (~16 000–18 200 cm–1) are caused by spin-allowed and spin-forbidden d-d transitions in tetrahedrally coordinated Co2+. Two absorption bands at higher wavelengths (~680 and ~800 nm, i.e., ~14 700 and ~12 500 cm–1) are assigned to electronic transitions in exchange coupled VIFe3+-IVFe2+ pairs and a band at ~950 nm (~10 500 cm–1) is assigned to a spin-allowed electronic transition in VIFe2+. Low-Fe gahnite crystals owe their blue color to traces of cobalt at concentration levels in the order of 200 ppm and less, while the green color of gahnite crystals with higher Fe-contents is due to a combination of electronic ligand-metal transitions causing strong UV-absorption and electronic transitions in exchange coupled Fe2+-Fe3+ cation pairs that absorb in the red region of the visible spectrum. A detailed characterization of samples that includes cation site occupancy and iron valency data is demonstrated to be crucial for interpreting optical absorption spectra. Also electronic transitions in trace element chromophores below the detection limit of electron microprobe may participate to light absorption. All this information contribute to the comprehension of the causes of crystal color of minerals, gemstones, and ceramic pigments
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
The elasticity of MgAl2O4–MnAl2O4 spinels by Brillouin scattering and an empirical approach for bulk modulus prediction
No full textThe elastic constants Cij of a set of synthetic single crystals belonging to the join MgAl2O4–MnAl2O4 (spinel sensu stricto–galaxite) were determined by Brillouin spectroscopy at ambient conditions. The C11 component tends to remain constant for Mg-rich compositions (XMn < 0.5) and decreases in Mn-rich compositions, whereas C12 increases and C44 decreases almost linearly from MgAl2O4 to MnAl2O4. The bulk modulus KS is weakly dependent upon Mg-Mn substitution within experimental uncertainties, whereas the shear modulus G decreases with increasing Mn2+ content. For MnAl2O4, C11 = 271.3(1.3) GPa, C12 = 164.8(1.3) GPa, C44 = 124.9(5) GPa, KS = 200(1) GPa, and G = 88.7(5) GPa.
Based on the “polyhedral approach,” we developed a model that describes the crystal bulk moduli of the MgAl2O4–MnAl2O4 spinels in terms of their cation distribution and the polyhedral bulk moduli of the different cations. We refined a set of values for the effective polyhedral bulk modulus of Mg, Mn2+, and Al in tetrahedral (T) and octahedral (M) sites, which span from 153 to 270 GPa ranking as follows: KMMn < KMMg < KTMg ≈ KTMn < KMAl << KTAl.
Crystal bulk modulus was perfectly reproduced within 0.1% for all Mn2+-bearing samples. We also found a high linear correlation between the effective polyhedral bulk modulus and the ionic potential, IP, of the coordinating cations: Kij (GPa) = 20(2) IP + 108(10) (where i indicates the cation and j the polyhedral site). We tested this simple correlation by calculating the specific effective polyhedral bulk modulus of several cations in T and M coordination and then predicting the crystal bulk modulus for several spinel compositions. The success of our simple correlation in modeling the bulk modulus of spinels outside the MgAl2O4–MnAl2O4 system is encouraging, and suggests that the relationships between chemical composition, cation distribution and elastic properties in spinel-structured minerals and materials can indeed be expressed by relatively simple models
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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