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La collezione Gasser nel Museo di Mineralogia dell'Università di Padova
No full textNel capitolo viene descritta la collezione mineralogica Gasser presente nel Museo di Mineralogia dell'Università di Padova, e comprende:
- le vicende storiche legate all'acquisto della collezione;
- il lavoro di riordino e di inventariazione della collezione;
- i campioni rappresentativi della collezione;
- i falsi mineralogici
La collezione Gasser nel Museo di Mineralogia dell'Università di Padova
No full textNel capitolo viene descritta la collezione mineralogica Gasser presente nel Museo di Mineralogia dell'Università di Padova, e comprende:
- le vicende storiche legate all'acquisto della collezione;
- il lavoro di riordino e di inventariazione della collezione;
- i campioni rappresentativi della collezione;
- i falsi mineralogici
Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy) : crystal structure and genetic implications
No full textAn As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium- fluorine) geochemical type and served as the principal source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3, space group I2/a] and has the empirical formula (Y 0.70Dy0.07Er0.05Ca0.05Gd 0.02U0.02Yb0.01 Tb0.01Th 0.01Nd0.01)Σ0.95 (Nb 0.68As0.275+W0.06Ta 0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections with Fo/σFo > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs
The behavior at non-ambient conditions of colemanite: a hydrous Ca-borate
No full textColemanite, CaB3O4(OH)3·H2O, is a common hydrous borate of large economic relevance, as it is one of the major commodities of boron with applications in the fields of glass and ceramic industries. Colemanite-rich layers are usually found in stratigraphic successions related to lacustrine basins in semi-arid to arid environments, associated to a local volcanic activity, which provides the source for boron. Despite the large economic relevance, the behavior of this mineral at non-ambient conditions of temperature and pressure was almost unexplored, which can provide a basis for understanding its stability during diagenetic and metamorphic processes. In this contribution, we report the highpressure behavior of colemanite (Lotti et al., 2017), based on in situ single-crystal synchrotron X-ray diffraction data up to 24 GPa, and its low-T behavior by in situ X-ray and neutron single-crystal diffraction. Colemanite was found to be stable up ~ 14.5 GPa, where a reconstructive phase transition towards a high-pressure polymorph (colemanite-II) with same symmetry (space group P21/a), but a six times larger unit cell volume, occurs. The elastic behavior of colemanite was described by fitting the experimental data with a III-order Birchurnaghan equation of state, yielding the following refined elastic parameters: KV0 = 64(4) GPa and KV' 5.5(7). The colemanite-tocolemanite-II phase transition induces an increase in the average coordination number of both the Ca and B cations. In particular, a fraction of the boron sites increases its coordination from triangular to tetrahedral by making a further bond with a H2O-oxygen atom. Although the phase transition occurs (at ambient temperature) at pressures far from those associated with the usual geologic environments of colemanite, the reported results disclosed flexible deformation mechanisms that borate compounds may adopt to accommodate pressure, thus providing new insights on the behavior of borate minerals at non-ambient conditions. The complex hydrogen-bonding network of olemanite has also been characterized, at ambient and low temperature conditions, by means of in situ single-crystal synchrotron X-ray and neutron diffraction experiments. A positional disorder, related to the presence of two mutually exclusive configurations of the H2O-molecule hydrogen atoms, was found both above and below ~ 0°C, where a displacive phase transition from the P21/a to the P21 space group occurs.
Lotti, P., Gatta, G.D., Comboni, D., Guastella, G, Merlini, M., Guastoni, A., Liermann, H.-P. (2017): High-pressure behavior and Pinduced phase transition of CaB3O4(OH)3*H2O (colemanite), J. Am. Ceram. Soc., in press, DOI: 10.1111/jace.14730
High-temperature behavior and dehydration of the natural borate colemanite
No full textColemanite, CaB3O4(OH)3*H2O, is the most common natural hydrous calcium borate and one of the most important mineral commodities for boron. It is mainly extracted from Turkish deposits, which form for the chemical precipitation of colemanite from relatively diluted waters of lacustrine basins, hosted in continental semi-arid to arid environments and fed by B-rich hydrothermal springs related to a local volcanic activity (e.g. Helvaci & Alonso, 2000). Due to its common occurrence in waste rocks at extraction sites, several studies have been focused in the recent past on the exploitation of colemanite as raw material, for example as an additive in the production of lightweight cements (Targan et al., 2003) or radiation shielding materials (Glinicki et al., 2018). However, despite the relevance beared in the applications of colemanite, we still have only a partial knowledge of the high-temperature behavior of this mineral compound.
In this study we investigated the high-temperature behavior of a natural sample of colemanite from the Bigadiç deposit (Turkey), by means of in situ powder synchrotron X-ray diffraction, performed at the MCX beamline of Elettra (Trieste, Italy), using a hot blower device.
The refined unit-cell parameters and structures show a significantly anisotropic thermal expansion, which is only accommodated along the b and c crystallographic axes, whereas the a axis, corresponding to the direction of the chains of B-coordination polyhedra, is almost unaffected. Between 275 and 325 °C, a reduction in the unit-cell volume and a decrease in the refined occupancy of the H2O-oxygen site, reveal the occurrence of a dehydration mechanism, which is followed by a complete and irreversible amorphization between 325 and 375 °C.
Merging the V-T data of this experiment with those obtained in a previous low-T experiment on the same natural sample (Lotti et al., 2018), a thermal Berman equation of state was refined in the range from -171 to 250 °C (104-525 K), yielding a thermal expansion coefficient at ambient-T (αV298K) of 4.5(1)*10-5 K-1 and α1 = 5.7(8)*10-8 K-2.
Glinicki M.A., Antolik A. & Gawlicki M. 2018. Evaluation of compatibility of neutron-shielding boron aggregates with Portland cement in mortar. Constr. Build. Mater., 164, 731-738.
Helvaci C. & Alonso R.N. 2000. Borates Deposits of Turkey and Argentina: A Summary and Geological Comparison. Turkish J. Earth Sci., 9, 1-27.
Lotti P., Gatta G.D., Demitri N., Guastella G., Rizzato S., Ortenzi M.A., Magrini F., Comboni D., Guastoni A. & Fernandez-Diaz M.T. 2018. Crystal chemistry and temperature behavior of the natural hydrous borate colemanite, a mineral commodity of boron. Phys. Chem. Minerals, in press DOI: https://doi.org/10.1007/s00269-017-0929-7.
Targan S., Olgun A., Erdogan Y., & Sevinc V. 2003. Influence of natural pozzolan, colemanite ore waste, bottom ash, and fly ash on the properties of Portland cement. Cement Concrete Res., 33, 1175-1182
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
On the crystal structure and crystal chemistry of pollucite, (Cs,Na)16Al16Si32O96·nH2O: a natural microporous material of interest in nuclear technology
No full textThe crystal structure and crystal chemistry of two natural, pollucite samples, from Buckfield, Oxford County, Maine (sample M3 ), and from Kanzit Mawaie, Laghman, Nooristan, Afghanistan, (sample N5 ), have been investigated by means of wavelength-dispersive X-ray microanalysis, thermogravimetric analysis, single-crystal X-ray and neutron diffraction, and single-crystal Fourier-transform infrared spectroscopy. The X-ray and neutron diffraction patterns of the two pollucite crystals show a metrically cubic unit cell [with am3 = 13.6914(6) Å and aN5 = 13.6808(6) Å by neutron diffraction data; the deviation from isometry is 4σ(F0) for M3 and R1 = 0.0693 for 31 refined parameters and 331 unique reflections with, F0 > 4σ(F 0) for N5. The structure refinements show a disordered Si/Al-distribution in the tetrahedral framework. The analysis of the difference-Fourier maps of the nuclear density confirms the presence of extraframework water molecules with oxygen sharing the Cs site (at 1/8,1/8,1/8, Wyckoff-16ft position). However, the minima, ascribable to the proton sites, are very weak in density. Two possible proton, positions, leading to a reasonable H2O configuration, are given, and the possible hydrogen bonding is described. Sodium is located at 1/4,1/8,0 (Wyckoff-24c position). The main IR absorption bands in the regions typical of H2O are assigned, and the presence of hydroxyls in the studied samples is ruled out. Neutron diffraction and FTIR data agree with the presence of very weak hydrogen bonds in the structure. The detailed description of the crystal structure and crystal chemistry of pollucite (e.g., Si/Al-distribution, configuration of the extra-framework content, possible hydrogen bonding scheme) reported in this study is the key to understand the high thermo-elastic stability of pollucite, the immobility of Cs at non-ambient conditions, and the extremely low leaching rate of Cs, which, make this open-framework silicate a promising material with potential use for fixation and deposition of Cs radioisotopes
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