1,721,018 research outputs found
Guarinite, a new finding in the sanidinite ejecta of the IV Hydromagmatic Unit from Albano Lake crater, Latium, Italy.
No full textCrystal chemical investigations on rare zirconium-niobium silicates of the Alfred Lacroix collection of the Muséum National d’Histoire Naturelle (Paris)
No full textCrystal chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides structural group
Full text linkEcdemite from Harstigen, Värmland, Central Sweden was studied through Raman and
FTIR spectroscopy, electron microprobe techniques, synchrotron powder and single
crystal diffraction. The ideal mineral formula proposed by Palache et al. (1951),
Pb6Cl4As2O7, is confirmed. In contrast to previous suggestions, the present study
demonstrates that the mineral is monoclinic, with space group P21 and unit-cell
parameters a = 10.8276 (38) Å, b = 10.8265 (21) Å, c = 6.9705 (14) Å, Β = 113.261°
(24). The crystal structural study of ecdemite shows that the mineral can be considered
as made up by regularly alternating, with 1:1 ratio, Cl layers and litharge-like defective
layers, also hosting As cations. These features indicate that ecdemite can be
considered as a new member of the of layered lead oxyhalides group
Heliophyllite: A discredited mineral species identical to ecdemite
Full text linkThe type material for heliophyllite, preserved in the Swedish Museum of Natural History in Stockholm, was re-investigated through a combined EPMA (electron probe X-ray microanalysis), Raman, and X-ray powder diffraction (XRPD) and single-crystal study. EPMA chemical data, together with Raman and single-crystal structural studies, point to heliophyllite being identical to ecdemite. XRPD synchrotron data highlight the presence of a minor quantity of finely admixed finnemanite in the analyzed material, explaining the presence of some additional diffraction peaks, not indexable with the ecdemite unit cell, reported in the literature. The discreditation of heliophyllite has been approved by the IMA Commission on New Minerals and Mineral Names (proposal 19-H, 2019)
INCORPORATION OF Co IN THE DOLOMITE STRUCTURE: COUPLED EPMA AND SINGLE CRYSTAL STRUCTURAL STUDIES OF Co-RICH DOLOMITE FROM THE TENKE-FUNGURUME DISTRICT, DEMOCRATIC REPUBLIC OF CONGO
No full textStructural variations in Co-rich dolomite from the Tenke-Fungurume district, Democratic Republic of Congo, have been
examined with coupled EPMA and single-crystal structure refinements for Co contents up to 0.30 atoms per formula unit. Full
occupation of Ca at the A site is constantly found, with progressive Co incorporation at the B structural site leading to isotropic
expansion of the B coordination octahedra and a corresponding anisotropic contraction of the A polyhedra, mainly achieved
through the shortening of ‘‘basal’’ polyhedral edges. The simultaneous opposing variation of AO6 and BO6 polyhedra explains
the strongly anisotropic expansion of the unit-cell parameter
Rietveld refinement of chapmanite SbFe2Si2O8OH, a TO dioctahedral kaolinite-like mineral
No full textChapmanite, Sb(OH)Fe2(SiO4)2 is a rare mineral that generally occurs as yellow coatings on rocks from antimony mines. The very small dimensions of the crystals prevented X-ray single-crystal study of the mineral whose structure was derived through high voltage electron diffraction (Zhukhlistov & Zvyagin, 1977). The present study confirm the correctness of the structure determined by those authors and shows the effectiveness of the Rietveld method also in the case of highly diluted phases. The structure of chapmanite is similar to that of kaolinite with two major differences: a) the dioctahedral sheet of chapmanite contains Fe3+ instead of Al3+; b) chapmanite has Sb3+ cations grasped to the octahedral sheet. on the opposite side with respect to the tetrahedral sheet
Zoisite-(Pb), a new orthorhombic epidote-related mineral from the jakobsberg mine, värmland, sweden, and its relationships with hancockite
No full textThe new mineral zoisite-(Pb), ideally CaPbAl3 (SiO4)(Si2 O7)O(OH), was discovered in a sample from the Jakobsberg manganese-iron oxide deposit, Värmland, Sweden. Zoisite-(Pb) is found as pale pink subhedral prisms elongated on [010], up to 0.3 mm in size, associated with calcite, celsian, diopside, grossular, hancockite, hyalophane, native lead, phlogopite, and vesuvianite. Associated feldspars show one of the highest PbO contents (~7–8 wt%) found in nature. Electron-microprobe analysis of zoisite-(Pb) point to the empirical formula (Ca1.09 Pb0.86 Mn2+ 0.01Na0.01)∑1.97 (Al2.88 Fe3+ 0.10 Mn3+ 0.04)∑3.02 Si3.00 O12 (OH)1.00 . The eight strongest diffraction lines [dobs, Iobs, (hkl)] are 8.63 s (101), 8.11 mw (200), 4.895 m (011), 4.210 m (211), 3.660 s (112, 311), 3.097 mw (312), 2.900 s (013), and 2.725 m (511). Zoisite-(Pb) is isostructural with zoisite and its crystal structure was refined up to R1 = 0.0213 for 2013 reflections with Fo > 4σ(Fo). Pb shows a stereochemically active lone pair leading to a lopsided distribution of its coordinating oxygens. A full chemical and Raman characterization of zoisite-(Pb) and of the Pb-bearing epidote hancockite is reported, together with an improved crystal structural model of hancockite, refined up to R1 = 0.0254 for 2041 reflections with Fo > 4σ(Fo). The effects of the incorporation of Pb in the crystal structure of zoisite-(Pb), hancockite, and related synthetic and natural phases are described and discussed
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
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