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  • 1
    Electronic Resource
    Electronic Resource
    Crystallography, chemistry and structural disorder in the new high- T c BiCaSrCuO superconductor (1988)
    [s.l.] : Nature Publishing Group
    Nature 332 (1988), S. 334-337 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The present observations are from samples BCSCO-a and BCSCO-b of Hazen et al.2. All selected-area electron diffraction microscopy (SAED), X-ray analytical electron microscopy (AEM), and high-resolution transmission electron microscopy (HRTEM) experiments were performed with a Philips 420 ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/332334a0
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Structural modulations and phase transitions in β-eucryptite: an in-situ TEM study (1999)
    Springer
    Physics and chemistry of minerals 26 (1999), S. 633-643 
    Details
    ISSN: 1432-2021
    Keywords: Key words Modulated structure ; Order-disorder ; β-eucryptite ; Transmission electron microscopy ; electron diffraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Beta-eucryptite as grown by the flux method has been investigated by in-situ cold- and hot-stage transmission electron microscopy (TEM). Using electron diffraction and dark-field TEM imaging, we found no evidence for the merohedral twinning that accompanies structural collapse at the β-to-α-quartz transition, suggesting a true hexagonal symmetry for the aluminosilicate framework of β-eucryptite. Selected-area electron diffraction (SAED) patterns exhibited a variety of incommensurate structures along the three a axes with an average modulation period of about 6.5 a. These modulated structures arise from superperiodic stacking parallel to (100) of two structural units with different Li configurations. In-situ heating experiments show that the modulated structures undergo a reversible transition to the disordered modification at elevated temperatures. This transition arises from positional order-disorder of Li ions along the main structural channels. In addition, it appears that at constant low temperature, electron beam irradiation alone can induce a sequence of transitions similar to those that are thermally induced from the ordered modification to modulated states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050228
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  • 3
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    100-K superconducting phases in the Tl-Ca-Ba-Cu-O system (1988)
    American Physical Society
    Details
    Publication Date: 1988-04-18
    Print ISSN: 0031-9007
    Electronic ISSN: 1079-7114
    Topics: Physics
    Published by American Physical Society
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  • 4
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    Superconductivity in the high-TcBi-Ca-Sr-Cu-O system: Phase identification (1988)
    American Physical Society
    Details
    Publication Date: 1988-03-21
    Print ISSN: 0031-9007
    Electronic ISSN: 1079-7114
    Topics: Physics
    Published by American Physical Society
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  • 5
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    Presentation of the Distinguished Public Service Award of the Mineralogical Society of America for 2015 to J. Alexander Speer (2016)
    Mineralogical Society of America
    Details
    Publication Date: 2016-04-02
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 6
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    A refined monoclinic structure for a variety of "hydrohematite" (2015)
    Mineralogical Society of America
    Details
    Publication Date: 2015-02-13
    Description: In ferruginous soils, nano- to microscale hematite (α-Fe 2 O 3 ) plays a central role in redox processes and contaminant cycling. Hematite is known to incorporate structural OH – and water, and the requisite charge balance is achieved by iron vacancies. Prior researchers have suggested that the defective hematite structures form unique phases called "protohematite" and "hydrohematite." Infrared and Raman spectroscopic studies have assigned a lower-symmetry space group to "hydrohematite" ( R 3 c ) relative to that of stoichiometric hematite ( R c ). However, the existence and structure of these phases have been contentious, largely due to the lack of in situ X-ray diffraction data Here we present a new structure refinement for "hydrohematite" precipitated hydrothermally at 200 °C in a monoclinic space group ( I 2/ a ) using time-resolved synchrotron X-ray diffraction (TR-XRD) data collected during the in situ hydrothermal precipitation of akaganeite and its transformation to stoichiometric hematite. Distinct peak splitting was observed in the "hydrohematite" diffraction patterns, indicating a violation of the threefold rotational symmetry. A monoclinic unit cell with parameters of a = 7.3951(10), b = 5.0117(5), c = 5.4417(7) Å, β = 95.666(5)° provided a good fit and significant reduction in 2 and R wp relative to space group R c . Rietveld analyses revealed that water concentrations in the first-formed crystals of "hydrohematite" were comparable to water contents of akaganeite and goethite. Thus, the hydrothermal transformation of akaganeite to "hydrohematite" is promoted not by dehydration but by reconstruction of the oxygen framework.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 7
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    Synthesis and structure of a stuffed derivative of {alpha}-quartz, Mg0.5AlSiO4 (2015)
    Mineralogical Society of America
    Details
    Publication Date: 2015-10-02
    Description: A structural derivative of quartz with the composition Mg 0.5 AlSiO 4 has been grown from glass and characterized using synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM), and 29 Si nuclear magnetic resonance (NMR) spectroscopy. Rietveld analysis of the XRD data indicates that the framework of Mg 0.5 AlSiO 4 is isostructural with α-quartz, rather than β-quartz, as is consistent with previous theoretical modeling ( Sternitzke and Müller 1991 ). Al and Si exhibit long-range disorder over the framework tetrahedral sites, indicated by the absence of the superlattice reflections corresponding to the doubling of c relative to that of quartz. Nevertheless, 29 Si NMR measurements show that Al and Si exhibit partial short-range order with an ordering degree of 56%. Electron diffraction reveals superlattice reflections indicative of doubled periodicities along the a -axes. Fourier electron density maps show that Mg occupies channel sites that each are bonded to six O atoms, in contrast to the tetrahedral coordination of Li in the β-quartz-type framework for β-eucryptite, LiAlSiO 4 . Furthermore, the concentrations of Mg in adjacent channels are different, resulting in framework distortions that generate the superstructures along a .
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    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 8
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    Jianshuiite in oceanic manganese nodules at the Paleocene-Eocene boundary (2016)
    Mineralogical Society of America
    Details
    Publication Date: 2016-02-03
    Description: Synchrotron powder X-ray diffraction and scanning electron microscopy examinations of manganese oxide concretions/nodules (~0.3–1.0 mm diameter) from ODP Site 1262 on Walvis Ridge in the Southeastern Atlantic Ocean revealed that they consist primarily of the layered Mn oxide phase jianshuiite [(Mg,Mn,Ca)Mn 3 4+ O 7 ·3H 2 O]. The nodules are from an interval with severe carbonate dissolution that represents the Paleocene/Eocene (P/E) thermal maximum (~55.8 Ma). Most nodules from the middle of the carbonate dissolution interval contain internal open space, and consist almost entirely of euhedral plate-like jianshuiite crystals, 2–4 μm in diameter and ~0.1–0.5 μm thick. Backscattered electron images and energy-dispersive X-ray analyses revealed stacks of interleaved Al-rich and Al-poor jianshuiite crystals in some nodules. The crystals in other nodules contain predominantly Mg (with trace K and Al) in addition to Mn and O, making them near "end-member" jianshuiite. Rietveld refinements in space group R confirmed the isostructural relationship between jianshuiite and chalcophanite, with Mg occupying the interlayer position above and below the vacant sites in the Mn/O octahedral sheet, and coordinated to 3 octahedral layer O atoms (1.94 Å) and 3 interlayer water O atoms (2.13 Å). Final refined occupancy factors suggest that small quantities of Ni and possibly Mn 2+ are located on the Mg site. The transient appearance of the Mg-rich birnessite-like phase jianshuiite, probably abiotically produced, must indicate an exceptional transient change in the chemistry of the pore fluids within deep ocean sediments directly following the P/E boundary, possibly as a result of decreasing oxygen levels and pH, followed by a return to pre-event conditions.
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    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 9
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    Time-resolved synchrotron X-ray diffraction study of the dehydration behavior of chalcophanite (2014)
    Mineralogical Society of America
    Details
    Publication Date: 2014-10-02
    Description: Time-resolved synchrotron X-ray powder diffraction data were used to investigate the dehydration behavior of the chalcophanite (ZnMn 3 O 7 ·3H 2 O) structure from 300 to 1060 K. Rietveld refinements revealed two obvious phase changes, at ~450 and ~950 K, corresponding to the dehydration of chalcophanite followed by transformation to a spinel structure (Mn-hetaerolite). Only small changes were observed in the chalcophanite unit cell from RT to ~438 K; the volume increased by ~0.8%, mostly caused by thermal expansion of ~0.5% along c . Above ~427 K, the interlayer water molecules were lost, resulting in a collapse of the interlayer spacing from ~7 to ~4.8 Å. The newly formed anhydrous phase (ZnMn 3 O 7 ) retained chalcophanite’s R space group and a dimension, but the c dimension decreased from ~21 to 14.3 Å, and the Zn coordination changed from octahedral to tetrahedral. Above ~775 K the anhydrous chalcophanite began to transform to a spinel structure, corresponding to a Mn-rich hetaerolite [(Zn 0.75 Mn 2+ 0.25 )Mn 2 3+ O 4 ]. By ~973 K the transformation was complete. The diffraction patterns did not show a significant increase in background during the transformation, indicating that the reaction did not involve transient amorphization. The phase change was likely triggered by loss of 1.25 of seven O atoms in the original anhydrous chalcophanite structure with a corresponding reduction of Mn 4+ to Mn 3+ and Mn 2+ .
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    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 10
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    Times arrow in the trees of life and minerals (2016)
    Mineralogical Society of America
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    Publication Date: 2016-05-03
    Description: Charles Darwin analogized the diversification of species to a Tree of Life. This metaphor aligns precisely with the taxonomic system that Linnaeus developed a century earlier to classify living species, because an underlying mechanism—natural selection—has driven the evolution of new organisms over vast timescales. On the other hand, the efforts of Linnaeus to extend his "universal" organizing system to minerals has been regarded as an epistemological misfire that was properly abandoned by the late nineteenth century. The mineral taxonomies proposed in the wake of Linnaeus can be distinguished by their focus on external character (Werner), crystallography (Haüy), or chemistry (Berzelius). This article appraises the competition among these systems and posits that the chemistry-based Berzelian taxonomy, as embedded within the widely adopted system of James Dwight Dana, ultimately triumphed because it reflects Earth’s episodic but persistent progression with respect to chemical differentiation. In this context, the pioneering work of Hazen et al. (2008) in mineral evolution reveals that even the temporal character of the phylogenetic Tree of Life is rooted within a Danan framework for ordering minerals.
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