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On the complex H-bonding network in paravauxite, Fe2+Al2(PO4)2(OH)2 · 8 H2O
Full text linkPhosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, playing an essential
geochemical role in the evolution processes of pegmatites. A good knowledge of their crystal chemistry is therefore necessary to better understand the genesis of pegmatites. Paravauxite is a mineral found in hydrothermal tin veins and granite pegmatites. Its ideal chemical formula is Fe2+Al2(PO4)2(OH)2·8H2O. Its crystal structure
was solved and refined by Baur in 1969 on the basis of single-crystal X-ray diffraction data. This
structure model appears to be consistent. However, due to the technical limitations of X-ray diffraction, the refinement only provided the isotropic displacement parameters, and the positions of nine independent proton sites were assigned but
not refined. This led to a poor description of (the expected) complex H-bonding scheme in the paravauxite structure. In light of this, the crystal structure of a natural paravauxite was reinvestigated using electron microprobe analysis in wavelength dispersive mode (EPMA-WDS) and single-crystal neutron diffraction in an attempt to resolve these open questions
On the complex H-bonding network in paravauxite, Fe+Al(PO)(OH)·8HO: A single-crystal neutron diffraction study
No full textThe crystal structure and the chemical composition of a paravauxite from the Siglo Veinte Mine, Llallagua, Bustillo Province, Potosi Department, Bolivia [Fe(Formula Formula Mg0.064Ca0.002)Σ0.998Al(1)Al(2)Al2.005P(P1.998Si0.002)Σ2O8(OH)2·8H2O, a = 5.242(1) Å, b = 10.569(2) Å, c = 6.970(2) Å, α = 106.78(3)°, β = 110.81(2)° and γ = 72.29(2)°, space group P1İ], was investigated by single-crystal neutron diffraction and electron microprobe analysis in wavelength-dispersive mode. Neutron-intensity data were collected at 293 K and anisotropic structure refinement was performed. At the end of the refinement no peak larger than 1.3 fm/ Å3 was present in the final difference-Fourier map of the nuclear density. The final statistical index was R1 = 0.0495 for 194 refined parameters and 1678 unique reflections with Fo > 4σ(Fo). Eleven independent H sites (i.e. H(1), H(2), H(3), H(4A), H(4B), H(5), H(6), H(7), H(8), H(9A) and H(9B)), all at ~1 Å from the respective O sites, were located successfully. H(4A) and H(4B) and H(9A) and H(9B) are two mutually exclusive subsite couples only 0.4–0.6 Å apart. The complex H-bonding scheme in the paravauxite structure is now well defined and 12 independent H bonds, with an energetically favourable bonding configuration, are described. A comparison between the previous experimental findings based on Raman and infrared spectroscopy and those obtained in this present study is carried out. Paravauxite provides the rare opportunity to investigate the H-bond configuration of coexisting hydroxyl groups and H2O molecules in minerals by single-crystal neutron diffraction. H2O is present as zeolitic (i.e. lying in the cavities) and non-zeolitic H2O (i.e. bonded to Al or Fe to form Al or Fe octahedra)
Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1070K
Full text linkThe effects of temperature on the crystal structure of a natural epidote [Ca-1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) and beta = 115.36(5)A degrees, Sp. Gr. P2(1) /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M-3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)center dot center dot center dot O(2) and O(10)-H(1)center dot center dot center dot O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)center dot center dot center dot O(4) is energetically more favourable, as H(1)center dot center dot center dot O(4) = 1.9731(28) , O(10)center dot center dot center dot O(4) = 2.9318(22) and O(10)-H(1)center dot center dot center dot O4 = 166.7(2)A degrees, whereas H(1)center dot center dot center dot O(2) = 2.5921(23) , O(10)center dot center dot center dot O(2) = 2.8221(17) and O(10)-H(1)center dot center dot center dot O2 = 93.3(1)A degrees. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 . The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)(1,070K) = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)(1,070K) = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)center dot center dot center dot O(4) and H(1)center dot center dot center dot O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed.</p
A single-crystal neutron and X-ray diffraction study of pezzottaite, Cs(BeLi)AlSiO
No full textThe chemical composition and the crystal structure of pezzottaite [ideal composition Cs(Be2Li) Al2Si6O18; space group: R3c, a = 15.9615(6) A ° , c = 27.8568(9) A ° ] from the type locality in Ambatovita (central Madagascar) were investigated by electron microprobe analysis in wavelength dispersive mode, thermo-gravimetric analysis, Fourier-transform infrared spectroscopy, single-crystal X-ray (at 298 K) and neutron (at 2.3 K) diffraction. The average chemical formula of the sample of pezzottaite resulted Cs1,Cs2(Cs0.565Rb0.027K0.017)R0.600 Na1,Na2(Na0.101Ca0.024)R0.125Be2.078Li0.922 Al1,Al2(Mg0.002 Mn0.002Fe0.003Al1.978)R1.985 Si1,Si2,Si3(Al0.056Si5.944)R6O18 0.27H2O. The (unpolarized) IR spectrum over the region 3,800–600 cm-1 was collected and a comparison with the absorption bands found in beryl carried out. In particular, two-weak absorption bands ascribable to the fundamental H2O stretching vibrations (i.e. 3,591 and 3,545 cm-1) were observed, despite the mineral being nominally anhydrous. The X-ray and neutron structure refinements showed: (a) a non-significant presence of aluminium, beryllium or lithium at the Si1, Si2 and Si3 sites, (b) the absence (at a significant level) of lithium at the octahedral Al1, Al2 and Al3 sites and (c) a partial lithium/beryllium disordering between tetrahedral Be and Li sites
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
A single-crystal neutron and X-ray diffraction study of elbaite
No full textThe crystal chemistry and crystal structure of gem-quality elbaite from the Pederneira mine, São José da Safira, Minas Gerais, Brazil, were investigated by electron microprobe analysis in wavelength dispersive mode, un-polarized Fourier transform infrared spectroscopy, single-crystal neutron and X-ray diffraction. The average chemical formula of the sample resulted X (Na0.73Ca0.06□0.21) Y (Al1.24Li1.02Fe0.41Mn0.18Mg0.07) Z Al6T Si6O18B (BO3)3V (OH)3W (OH0.46F0.54) and the chemical analysis showed an almost homogeneous distribution of the principal elements over the crystal, with significant amounts of Fe (wt% FeO ~ 3.1) and Mn (wt% MnO ~ 1.3) which substitute YAl. The infrared spectrum over the diagnostic range of fundamental hydroxyl stretching vibrations is characterized by three strong absorption bands at 3,591 (ν1), 3,562 (ν2), and 3,491 (ν3) cm−1, respectively. The neutron structure refinement showed: (a) the presence of boron at the B site only and (b) the absence (at a significant level) of aluminum at the T site, which appears to be fully occupied by silicon. Only one proton site was successfully located. The H bond configuration is described with O3 as “donor” and O5 as “acceptor.” The O3–H bond distance corrected for “riding motion” is 0.9834 Å, and H···O5 = 2.238(7) Å, O3···O5 = 3.152(4) Å and O3–H···O5 = 157.7(5)°. The reasons of the positional disorder at the O1 and O2 sites, observed in both neutron and X-ray refinements, are discussed
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