34,318 research outputs found
Nudged elastic band calculations of the (4H)XSi hydrogarnet type defect in Mg2SiO4forsterite
No full textFirst-principles calculations based on density functional theory (DFT) using the generalized gradient approximation (GGA) were performed to assess the energetic barriers separating different topological configurations of the (4H)XSi hydrogarnet type defect in Mg2SiO4 forsterite with the climbing image nudged elastic band (CI-NEB) method. Barrier heights are low (<0.6 eV) with respect to typical ac- tivation energies observed for H-diffusion but more comparable to those for electrical conductivity of H2O-bearing nominally anhydrous minerals. As can be expected, hydrogen bonding to O atoms both within the defect and belonging to adjacent tetrahedra plays a fundamental role in the stability of each configuration. Saddle points along the minimum energy path (MEP) typically correspond to the transition of one hydrogen bond breaking to form a new hydrogen bond such that one or more OH bonds have shifted in direction without themselves breaking. MEPs show that slightly out-of-plane torsional hopping from one configuration to another can reduce the height of the barrier. We illus- trate several different reaction coordinates between symmetry equivalent pairs of configurations and non-symmetry related pairs that can result in an effective means of local charge transport by shifting the center of mass of the (4H)4+ cluster within the defect site without proton transfer to an interstitial site. Especially at low temperatures in the absence of thermally activated processes that result in the breaking of stronger chemical bonds, these types of configurational transformation mechanisms are likely to be important contributors to the dielectric behavior of nominally anhydrous silicate minerals and also affect both electrical conductivity and electrical conductivity anisotropy when investigated by AC methods such as impedance spectroscopy. The NEB method can also be used to examine more effective charge and mass transport processes that involve the dissociation of the hydrogarnet defect into more complex chemical species, which might involve similar hydrogen bond breaking and forming processes observed in this study along with more significant atomic displacements
Electrical conductivities of pyrope-almandine garnets up to 19 Gpa and 1700°C
No full textElectrical conductivities of polycrystalline garnets ranging in chemical composition from almandine
(Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer
spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent
with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing
garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function
of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant
T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profi le of the mantle, using a recently developed laboratory-
derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only
a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle
Effect of Aluminum on Ti-coordination in Silicate Glasses: a XANES Study
No full textThe structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent
the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25,
0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios
crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption
features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali
titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated
Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation
The effect of Fe on the viscosity of silicate melts by PD-titration and XANES spectroscopy
No full textThe oxidation state of iron can exert considerable influence on magma phase equilibria and rheology. It is typically recognized that ferrous iron behaves as a network modifier in most silicate melts. Ferric iron, on the other hand, occurs both as a network former (coordination IV) and as a modifier. Despite of a wide range of spectroscopic investigations, the efficiency of iron and its oxidation state on modifying the viscosity of silicate melts is still unclear and under debate, mainly due to a lack of data on the melt structure. Here we present new viscosity data for a wide range of natural rhyolitic, trachytic, moldavitic, andesitic, latitic, pantelleritic, basaltic and basanitic compositions as well as wet chemistry and synchrotron analysis. Dry Newtonian shear viscosities were investigated at high temperature (1050-1600°C) and low temperature (616-860°C) using the concentric cylinder apparatus and the micropenetration technique, respectively. The glasses obtained by fast quenching (from different temperature) the high-T melts were measured for determining, via potassium dichromate (PD) titration, the Fe2+/Fe3+ ratio. The structural role of iron species was also investigated by Fe K-edge XANES spectroscopy performed at ESRF (Grenoble). Wet chemistry analyses were found consistent with high-T prediction of [1] and [2] empirical models. Fe K-edge XANES spectra allowed to interpret the rheological measurements in terms of structural played by iron species.[1] Kress and Carmichael 1991, Contrib. Min. Petrol. 108, 82; [2] Ottonello et al. 2001, Chem. Geol. 174, 157 Geophysical Research Abstracts, Vol. 7, 07275, 2005 SRef-ID: 1607-7962/gra/EGU05-A-07275 © European Geosciences Union 2005
High-pressure and high-temperature measurements of electrical conductivity in basaltic rocks from Mount Etna, Sicily, Italy
No full text[1] We have investigated the electrical properties of Etnean rocks by in situ complex impedance spectroscopy using a multianvil apparatus. From these measurements we determined the electrical conductivity of a lava flow sample, whose basaltic composition can be considered close to that of the parent magma and also that of a mafic nodule representative of the high-density cumulates interpreted as responsible for the main high-velocity anomaly observed beneath the volcano. The electrical conductivities of the two samples were measured at pressures of 0.9 and 1.5 GPa and temperatures from 400 to 1100degreesC at frequencies from 0.1 to 10(5) Hz. To investigate the electrical properties of the Etnean products as a function of partial melting, a few experiments were performed in a piston-cylinder apparatus prior to the electrical measurements. The obtained data were approximated using an equivalent circuit fitting technique. For the lava flow sample, electrical conductivity displays Arrhenian behavior over the entire investigated temperature range, with an activation energy of similar to0.8 eV Within the uncertainties of our measurements, we do not observe any effect of pressure on conductivity between 0.9 and 1.5 GPa. On the contrary, experiments performed on the series of partially molten samples indicate that conductivity increases with increasing quantity of glass. While conductivities of samples with minor amounts of glass are comparable to that of the lava flow starting material, with increased melting, conductivity can increase by as much as a factor of 3. The mafic nodule was observed to have a conductivity higher than the lava flow sample (e.g., at 800degreesC and 0.9 GPa, a factor of 4 higher). However, scatter in the data is somewhat higher in comparison to the lava flow sample, most likely because of the coarse texture of the samples and a consequence to their chemical and structural heterogeneity. Using our results, we illustrate the effects that a layer of hot magma surrounded by a cooler wall rock has on apparent resistivities determined by one-dimensional forward calculations. Our modeling demonstrates that both size and depth of magmatic intrusions strongly influence apparent resistivity and that these parameters can be extracted from field data if the electrical properties of the rocks below the surface are well understood
Electrical conductivity of hydrous wadsleite
No full textIn situ complex impedance spectroscopy of H2O-bearing wadsleyite was performed in a multianvil apparatus at 14 GPa at temperatures up to 950 °C in order to determine electrical conductivity. With increasing H2O content in wadsleyite the electrical
conductivity increases at a rate higher than observed in previous studies. The activation enthalpy in the temperature range studied
where proton conduction dominates is low (0.66 eV) suggesting an inevitable crossover to small polaron conduction at moderately
higher temperatures, depending on H2O concentration. Although the solubility of H2O in wadsleyite is significant for a silicate mineral (> 3 wt%), the presence of more than trace dissolved H2O in wadsleyite is likely to result in a conductivity too high compared to recent estimates of transition-zone conductivity. The use of complex impedance spectroscopy shows that the frequency dependence
of electrical properties is very different in the case of H2O-bearing silicate phases. At frequencies below 1000 Hz complex impedance spectra contain strong features which likely result from the sample–electrode interface such that including the low-frequency data would lead to artificially low conductivities
The Construction of the Subject in the Short Fiction of Edgar Allan Poe
No full textThis study is primarily concerned with the diverse processes of constitution
and deconstitution of subjectivity at work in the writing
of Edgar Allan Poe. The analysis is largely confined to the short
fiction, although some reference is made to Poe's other work; twentyone
tales are examined, in greater or lesser detail, with the aid of
various theoretical perspectives - sociological, structuralist and,
above all, psychoanalytic.
The aim is to present a new reading of Poe's texts which rejects
traditional "unity"-based interpretations. The thesis privileges the
psychological dimension, but in textual, not biographical terms; it
stresses the tales' often undervalued element of modernity as well as
their receptiveness to emergent processes and discourses.
The psychological dimensions analysed include: the explicit presentation
of mental splitting ('William Wilson') and institutionalised
madness ('The System of Doctor Tarr and Professor Fether'); the signification
of alienation ('The Man of the Crowd') and self-destruction
('The Imp of the Perverse', 'The Black Cat', 'The Tell-Tale Heart')
as constitutive of the subject at a determinate historical moment;
the simultaneous construction and subversion of mythical signifiers
of an illusory "full" subject, both metonyms (the detective, the
mesmerist) and metaphors (the artwork, the interior); the symbolic
emergence from repression of active female desire, perceived as
threatening in the male unconscious ('The Oval Portrait', 'Ligeia');
and the disintegration of the subject under the pressure of its own
repressions ('The Fall of the House of Usher').
Particular stress is laid throughout on the textual undermining of
the dividing-lines between "normal" and "abnormal", "sane" and "insane",
"respectable" and "criminal". It is concluded that Poe's work constitutes
a map of the vicissitudes and contradictions of subjectivity
in patriarchal culture; from the study of these texts, the "I" emerges
as formed out of a massive repression, and as therefore constantly
liable to fragmentation and rupture
Early growth response gene-2 (Egr-2) regulates the development of B and T cells
No full textThe study was supported by Arthritis Research UK.
Copyright @ 2011 Li et al.BACKGROUND: Understanding of how transcription factors are involved in lymphocyte development still remains a challenge. It has been shown that Egr-2 deficiency results in impaired NKT cell development and defective positive selection of T cells. Here we investigated the development of T, B and NKT cells in Egr-2 transgenic mice and the roles in the regulation of distinct stages of B and T cell development. METHODS AND FINDINGS: The expression of Egr1, 2 and 3 were analysed at different stages of T and B cell development by RT-PCT and results showed that the expression was strictly regulated at different stages. Forced expression of Egr-2 in CD2+ lymphocytes resulted in a severe reduction of CD4+CD8+ (DP) cells in thymus and pro-B cells in bone marrow, which was associated with reduced expression of Notch1 in ISP thymocytes and Pax5 in pro-B cells, suggesting that retraction of Egr-2 at the ISP and pro-B cell stages is important for the activation of lineage differentiation programs. In contrast to reduction of DP and pro-B cells, Egr-2 enhanced the maturation of DP cells into single positive (SP) T and NKT cells in thymus, and immature B cells into mature B cells in bone marrow. CONCLUSIONS: Our results demonstrate that Egr-2 expressed in restricted stages of lymphocyte development plays a dynamic, but similar role for the development of T, NKT and B cells.This article is provided by the Brunel Open Access publishing fund
- …
