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Bonn, Germany

Kroll H.,University of Munster | Kirfel A.,Steinmann Institute | Heinemann R.,University of Munster
European Journal of Mineralogy | Year: 2014

The successful parameterization of the volume thermal expansion in the Mg,Fe olivine solid-solution series stimulated us to carry the analysis further in view of the orthorhombic symmetry of the olivines and thus augment the available thermophysical data. To this purpose we studied the three axial expansivities, αa,b,c, on the basis of the same body of data and the same model as used for our parameterization of the volume expansion. The results are applied to various thermophysical parameters that possess a significant dependence on αa,b,c. In particular, the temperature variations of the axial adiabatic and isothermal moduli (a), the axial thermodynamic Grüneisen parameters (b), and the axial isothermal Anderson-Grüneisen parameters (c) are obtained for both forsterite (Fo) and fayalite (Fa). This allowed us to also study the response of αa,b,c axial expansivities to pressure. Since Fo and Fa are isomorphous, it is striking that the axial thermophysical properties of Fo are distinctly less anisotropic than those of Fa. Whereas in Fo, both the axial compressibilities and the axial expansivities follow the same sequence, i.e. βT; b . βT, b βT, b and αb <αc <αa, respectively, this expectation is violated in Fa where αc <αa <αb above 400 K. It is shown that the different anisotropies of the expansivities and hence of the α-dependent thermophysical properties can be related to differing structural responses to heating. © 2014 E. Schweizerbart'sche Verlagsbuchhandlung. Source


Pigozzi G.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Mukherji D.,TU Braunschweig | Elerman Y.,Ankara University | Strunz P.,Nuclear Physics Institute of Czech Republic | And 4 more authors.
Journal of Alloys and Compounds | Year: 2014

Nanostructured nickel silicides find application in electronics, high-temperature alloys, electrode materials and catalysis. In this work, the effect of size reduction on the structure and magnetic properties of β1-Ni3Si intermetallic phase nanoparticles is studied. Electrochemical selective phase dissolution (ESPD) was used to produce the β1-Ni3Si nanoparticles of different sizes (from 20 to 215 nm) by extracting β1 nano-size precipitates from two-phase Ni-Si and Ni-Si-Al precursor alloys. The extracted nanoparticles have a core-shell structure with β1-Ni3Si core and an amorphous silica shell. Particles size and shape are controlled by the composition and thermal treatment of the precursor alloys. Precipitates size is scaled without modifying the ordered L12 lattice structure. The bulk β1-Ni3Si is ferromagnetic below 260 K with low saturation magnetization (2 emu/g), while the core-shell Ni3Si/silica nanoparticles are superparamagnetic at low temperatures (<9-11 K) with low coercivity (<90 Oe) and magnetization >20 emu/g at 5 T. It is suggested that weak particle magnetic moments and low magnetic anisotropy of the L1 2 structure are responsible for these properties. The shell on one hand protects the core from degradation; however the oxidation of the core/shell interface region can influence the magnetic behavior of the nano-powders. © 2013 Elsevier B.V. All rights reserved. Source


Vrabec M.,University of Ljubljana | Janak M.,Slovak Academy of Sciences | Froitzheim N.,Steinmann Institute | De Hoog J.C.M.,University of Edinburgh
Lithos | Year: 2012

Phase relations among the mineral assemblages of UHP kyanite eclogite were investigated in the Pohorje Mountains of the Eastern Alps. Ultrahigh-pressure metamorphism resulted from intracontinental subduction during the Cretaceous (ca. 92Ma). Kyanite-bearing eclogites are associated with meta-ultramafic rocks including UHP garnet peridotites and are embedded in metapelitic gneisses and micaschists. The kyanite eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacite contains up to 5mol% of Ca-Eskola molecule. Breakdown of omphacite during decompression resulted in exsolution of oriented rods of silica. Phengite contains up to 3.5 Si a.p.f.u. Polycrystalline quartz inclusions in peak-pressure minerals - garnet, omphacite and kyanite - are surrounded by radial fractures diagnostic of the former presence of coesite. Peak-pressure minerals are replaced by symplectites of diopside+plagioclase+amphibole after omphacite, plagioclase+biotite after phengite and sapphirine+corundum+spinel+anorthite after kyanite. Sapphirine has composition close to (Mg, Fe) 12.4 Al 38.9 Si 4.5 O 80 in average, which is amongst the most aluminous yet reported. Peak metamorphic conditions were constrained from calculated phase equilibria in the NKCFMASH system with the fixed bulk-rock composition, and conventional geothermobarometry. This approach led to consistent results, the calculated peak P-T conditions of 3.0-3.7GPa and 710-940°C, in the stability field of coesite and the same range as metamorphic conditions recorded by the associated garnet peridotites. This implies that eclogites and their host rocks were subducted to depths of about 100km. The relatively high temperature at peak pressure, compared to UHP rocks of Tertiary age in the Western Alps where mostly oceanic crust was subducted, probably resulted from radiogenic heat production by subducting continental crust, in the intra-continental setting of the Cretaceous subduction zone in the Eastern Alps. © 2012 Elsevier B.V. Source


Hubner N.,RWE AG | Korner F.,Steinmann Institute | Schneider J.,Geologisches Institute
Zeitschrift der Deutschen Gesellschaft fur Geowissenschaften | Year: 2011

The intr amontane basins of Saint Affrique and Lodève contain each about 2000 m of Permian sediments. These continental deposits, formed within alluvial, fl uvial, lacustrine and playa systems, comprise mudstones, sandstones and fanglomerates. Facies development during the Permian is similar in both basins, but the sediments of the Saint Affrique Basin are coarser than those of the Lodève Basin, which contains mainly clay- and siltstones. The tectonic development of the Saint Affrique and the Lodève basins is closely coupled with the tectonic evolution of the Variscan Massif Central, in particular, the Montagne Noire. Two major tectonic events have been recognised, which commences with the development of a macro scale sedimentary cycle. By using climate relevant indicators from geochemical, petrological and sedimentological analyses, six wet and dry climate cycles could be identifi ed. These have been used for the fi rst time to correlate the two basins resulting in a detailed correlation chart, especially in the upper part of the Permian where lithostratigraphic marker beds and biostratigraphic indicators are almost absent. The results also show that the assumption of a direct connection is not necessary for explaining the similarities in facies development, if both basins were controlled by the same climatic and tectonic infl uences. © 2011 E. Schweizerbart'sche Verlagsbuchhandlung. Source


Kuhl G.,Steinmann Institute | Rust J.,Steinmann Institute
Palaontologische Zeitschrift | Year: 2010

The arthropod Mimetaster hexagonalis Gürich, 1931 from the Lower Devonian Hunsrück Slate (Germany) is re-described. With 123 available individuals, M. hexagonalis is the most abundant non-trilobite arthropod from this Lagerstätte. New (as well as old) specimen material shows new morphological features of M. hexagonalis. The ontogeny is described based on three juvenile specimens. The growth mode is similar to that in the marrellomorph Vachonisia rogeri Lehmann, 1955. The number of trunk segments increases from juvenile to adult. The mode of life of the arthropod is re-considered. The arthropod is considered to live in groups of several individuals. A mutualistic relationship with sponges of unknown systematic affinities is most likely. An ecological interaction with tentaculitoids is recognized but needs further investigation. The phylogenetic position of M. hexagonalis as a member of the Marrellomorpha is confirmed, but remains to be explored within the broader frame of euarthropod phylogeny. © 2010 Springer-Verlag. Source

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