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Stability and dynamics of crystals and glasses of motorized particles

admin — Mon, 08 Mar 2010 07:02:45 +0000

Many of the large structures of the cell, such as the cytoskeleton, are assembled and maintained far from equilibrium. We study the stabilities of various structures for a simple model of such a far-from-equilibrium organized assembly in which spherical particles move under the influence of attached motors. From the variational solutions of the many-body master equation for Brownian motion with motorized kicking we obtain a closed equation for the order parameter of localization. Thus, we obtain the transition criterion for localization and stability limits for the crystalline phase and frozen amorphous structures of motorized particles. The theory also allows an estimate of nonequilibrium effective temperatures characterizing the response and fluctuations of motorized asemblies.

Stability and dynamics of crystals and glasses of motorized particles
Tongye Shen and Peter G. Wolynes*
+ Author Affiliations

Departments of Chemistry and Biochemistry and of Physics, University of California at San Diego and Center for Theoretical Biological Physics, La Jolla, CA 92093-0371
Contributed by Peter G. Wolynes, April 12, 2004

Bulk-solvent and hydration-shell fluctuations, similar to α- and β-fluctuations in glasses, control protein motions and functions

admin — Mon, 08 Mar 2010 07:01:40 +0000

The concept that proteins exist in numerous different conformations or conformational substates, described by an energy landscape, is now accepted, but the dynamics is incompletely explored. We have previously shown that large-scale protein motions, such as the exit of a ligand from the protein interior, follow the dielectric fluctuations in the bulk solvent. Here, we demonstrate, by using mean-square displacements (msd) from Mössbauer and neutron-scattering experiments, that fluctuations in the hydration shell control fast fluctuations in the protein. We call the first type solvent-slaved or α-fluctuations and the second type hydration-shell-coupled or β-fluctuations. Solvent-slaved motions are similar to the α-fluctuations in glasses. Their temperature dependence can be approximated by a Vogel-Tammann-Fulcher relation and they are absent in a solid environment. Hydration-shell-coupled fluctuations are similar to the β-relaxation in glasses. They can be approximated by a Ferry or an Arrhenius relation, are much reduced or absent in dehydrated proteins, and occur in hydrated proteins even if embedded in a solid. They can be responsible for internal processes such as the migration of ligands within myoglobin. The existence of two functionally important fluctuations in proteins, one slaved to bulk motions and the other coupled to hydration-shell fluctuations, implies that the environment can control protein functions through different avenues and that no real protein transition occurs at ≈200 K. The large number of conformational substates is essential; proteins cannot function without this reservoir of entropy, which resides mainly in the hydration shell.

P. W. Fenimore*, Hans Frauenfelder*,†, B. H. McMahon*, and R. D. Young‡
+ Author Affiliations

*Theory Division, Los Alamos National Laboratory, Los Alamos, NM 87545; and ‡Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011
Contributed by Hans Frauenfelder, August 19, 2004

A simple empirical method for high-quality electron microprobe analysis of fluorine at trace levels in Fe-bearing minerals and glasses

admin — Mon, 08 Mar 2010 07:00:09 +0000

We present a new, high-quality electron microprobe method for analyzing F in Fe-bearing minerals and glasses down to trace levels (<1000 ppm F). This method is empirically based and corrects for the problem of overlap of the FK peak onto the "shoulder" of the FeL 1 peak that arises when using synthetic multi-layered diffraction crystals. It also achieves high precision and accuracy while maintaining low detection limits and a small beam diameter. Analytical conditions for F using the new method and a synthetic W/Si diffraction crystal are: 10 kV accelerating voltage, 180 nA beam current, 8 µ m beam diameter, and 400 second total peak count time. An iterative beam exposure method is employed to minimize potential damage to the sample due to beam heating. The method presented here can be used to address problems related to degassing of F at active volcanoes, the reservoirs of F in the mantle, and the partitioning of F between minerals and silicate liquid.

Jeffrey B. Witter* and Scott M. Kuehner
Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, U.S.A.

Correspondence: * E-mail: witt_98103@yahoo.com

Coloured glasses for high dose dosimetry using the thermoluminescent technique

admin — Mon, 08 Mar 2010 06:58:49 +0000

Coloured glasses produced commercially by Cebracê, São Paulo, were analysed by the thermoluminescent (TL) method, to verify the possibility of their use as high-dose dosemeters or irradiation indicators in industrial areas, due to their easy handling and their low cost. The samples were exposed to different radiation doses, using the Gamma-Cell 220 system (60Co) of IPEN. The TL emission curves presented main peaks at 135, 150 and 145°C in the bronze, brown and green glass samples, respectively. Calibration curves were obtained for the glasses between 50 Gy and 360 kGy. Reproducibility of TL response and the lower detection doses were determined for each kind of glass. All tested glasses showed their usefulness as irradiation indicators and as high-dose dosemeters.

Linda V.E. Caldas* and Maria Inês Teixeira
+ Author Affiliations
Instituto de Pesquisas Energéticas e Nucleares, Comissão Nacional de Energia Nuclear, C.P. 11049, 05422-970, São Paulo, SP, Brazil
*Corresponding author: lcaldas@ipen.br

A XANES determination of the oxidation state of chromium in silicate glasses

admin — Mon, 08 Mar 2010 06:55:14 +0000

Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al2O3 + SiO2 ± TiO2, quenched from melts equilibrated as a function of oxygen fugacity (fO2) at 1400°C. The spectra vary systematically with fO2 (log fO2 ~0 to –16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1s 4s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr2+/Cr. The resulting Cr2+/Cr values are in agreement with the theoretical dependence on log fO2 suggesting that the 1s 4s feature is diagnostic of Cr2+ in a silicate glass and (Cr2+/Cr) = ± 0.015. The Cr2+/Cr ratio for a given log fO2 changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr2+. Cr2+/Cr varies between ~0.3 and 0.8 over the log fO2 range bounded by the Ni/NiO and Fe/FeO fO2 buffers, suggesting that Cr2+ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr2+/Cr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.
Andrew J. Berry* and Hugh St.C. O’Neill
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Correspondence: * E-mail: andrew.berry@anu.edu.au

Ca-Mg and K-Mg mixing around non-bridging O atoms in silicate glasses: An investigation using 17O MAS and 3QMAS NMR

admin — Mon, 08 Mar 2010 06:50:09 +0000

In an effort to improve the physical accuracy of models of the thermodynamics of silicate melts, we describe a systematic study of the extent of modifying cation mixing, using 17O 3QMAS NMR, in a series of Ca-Mg and K-Mg silicate glasses. The spectra for the mixed cation Ca2–2xMg2xSi2O6 glass show that only one large non-bridging O atom (NBO) peak occurs that encompasses the entire range of chemical shifts ranging from Ca-NBO to Mg-NBO. Comparison of the isotropic projections from 3QMAS NMR to spectra predicted by a random model show that mixing in these glasses is highly disordered, but may contain a small amount of ordering at the glass transition temperature. In contrast, cation mixing in K-Mg silicate glasses is very ordered, confirming previous results; however, the results of this study disagree with the interpretation of the previous study and show that the NBO in K-Mg silicate glasses contain mostly Mg-NBO, not a highly ordered K-Mg-NBO species. These order-disorder results have direct implications in constraining entropy models and therefore allowing better predictions of mineral-melt equilibria in silicate melts.

Jeffrey R. Allwardt* and Jonathan F. Stebbins
Department of Geological and Environmental Sciences, Stanford University Stanford, California 94305-2115, U.S.A.

Correspondence: * E-mail: allwardt@pangea.stanford.edu

Determination of the molar absorption coefficient for the infrared absorption band of CO2 in rhyolitic glasses

admin — Mon, 08 Mar 2010 06:43:50 +0000

A new calibration was performed for the molecular CO2 band at 2349 cm –1 in the infrared absorption spectra of rhyolitic glasses. Glasses with varying amounts of CO2 (730–3900 ppm by weight) and H2O (0.5–7.0 wt%) were synthesized in an internally heated pressure vessel at 200–800 MPa and 1100 and 1200 °C. The CO2 content of the glasses was measured by coulometric CO2-titration after thermal extraction at 1200 °C. It is shown that the entire CO2 content cannot be extracted from a rhyolitic glass at this temperature. Using the Lambert-Beer law, and taking into account the residual CO2 still present in the glasses after extraction, we have calculated a linear molar absorption coefficient of 1214 ± 78 L · cm –1/mol for the band at 2349 cm –1 . Strictly, this value is a practical absorption coefficient because the IR band intensity is related to the total CO2 content and not to the molecular CO2 content. However, no direct evidence for other carbon species such as carbonate groups is found in the IR spectra. Hence, we suggest that the concentration of carbonate in the studied metaluminous rhyolite is negligible and the derived value is close to the real absorption coefficient for molecular CO2. The absorption coefficient does not vary noticeably with changing water content of the glasses. With the new calibration, CO2 concentrations in rhyolitic glasses are 13% higher than data based on the previous calibration of Blank (1993) for water-poor glasses.

Harald Behrens*, Nathalie Tamic and Francois Holtz
Institut für Mineralogie, UNI Hannover, Welfengarten 1, D-30167 Hannover, Germany

Correspondence: * E-mai;: H.Behrens@mineralogie.uni-hannover.de

V oxidation state and coordination number in silicate glasses by XAS

admin — Mon, 08 Mar 2010 06:40:33 +0000

The local structure of vanadium (V) in synthetic glasses of basaltic composition has been studied by means of high-resolution V K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to obtain quantitative data on distances, V coordination number (CN), and oxidation state. The compositions and experimental conditions were chosen so as to verify the effect of bulk-glass composition (using a diopside-anorthite composition, a sodium disilicate glass, and an iron-titanium-bearing basaltic glass) and V content (from 0.1 to 5 V2O5 wt%) on the structural role of V in these glasses.

The combined analysis of high-resolution XANES spectra and EXAFS data indicate that on average, the Fe-free glasses synthesized in air show vanadium in the V5+ state, mainly in tetrahedral coordination (less than 20% [5]V5+) and with distances of 1.697 (± 0.020) Å, in agreement with the values found for tetrahedral V in minerals. In contrast, the Fe-bearing basaltic glasses display a mixture of V5+ in fourfold and fivefold coordination, 40% [4]V5+–60% [5]V5+ in proportion, and the EXAFS-derived distances and coordination numbers are in agreement with this interpretation. No significant changes in the V local structure were found in the glasses analyzed as a function of V-contents in the 0.1 to 5 V2O5 wt% range. The data obtained suggest that the structural role of vanadium in these melts is rather insensitive to bulk composition, in terms of V and alkali content, but can be strongly affected by the presence of other transition elements, e.g., Fe3+ competing with V to enter the tetrahedral framework.

Gabriele Giuli1,*, Eleonora Paris1, Jim Mungall2, Claudia Romano3 and Donald Dingwell4
1 Dipartimento di Scienze della Terra, and Unità Istituto Nazionale Fisica della Materia (INFM), Università di Camerino, Italy
2 Department of Geology, University of Toronto, Toronto, Canada
3 Dipartimento di Scienze Geologiche, Università Roma III, Italy
4 Department of Earth Sciences, University of Munich, Germany

Correspondence: * E-mail: gabriele.giuli@unicam.it

Incinerator waste as secondary raw material: examples of applications in glasses, glass-ceramics and ceramics

admin — Mon, 08 Mar 2010 06:39:50 +0000

The recovery and beneficiation of solid waste residues (bottom and fly ash) produced during incineration of municipal wastes are very important goals from the environmental and economic point of view. Some technologies allow not only to reduce the volume of incinerator waste but also to generate products of economic value by vitrification, devitrification, and ceramic processes. Bottom ash is preferable compared to fly ash as a secondary raw material. Mixed with glass cullet, bottom ash can be easily transformed into homogeneous and inert glasses, which can be transformed into glass fibres or sintered glass-ceramics after controlled thermal treatments. Alternatively, bottom ash can be used to produce tiles, in particular glazed tiles containing vitrified bottom ash in the ceramic body, and possibly bricks. The results presented in this review are promising and in accordance with the waste minimization policy as well as resources conservation.

Luisa Barbieri & Isabella Lancellotti
Università di Modena e Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Modena, Italy barbieri.luisa@unimore.it

Volatiles in submarine basaltic glasses from the Ontong Java Plateau (ODP Leg 192): implications for magmatic processes and source region compositions

admin — Mon, 08 Mar 2010 06:38:49 +0000

Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H2O, CO2, S, Cl). At four of the sites (1183, 1185, 1186, 1187) the glass is from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. Glassy pillow rims from Site 1187 and the upper group of flows at Site 1185 have 8.3–9.3 wt% MgO compared with values of 7.2–8.0 wt% MgO for glasses from Sites 1183, 1184 1186, and the lower group of flows at Site 1185. Low-MgO glasses have slightly higher H2O contents (average 0.22 wt% H2O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H2O (average 0.16 wt%). Average S concentrations are 910 ± 60 ppm for the high-MgO glasses v. 1030 ± 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeOT. This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K2O and S/TiO2 with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S K peak in the glasses indicate low oxygen fugacities comparable to MORB values. Chlorine contents of the glasses are very high compared with MORB, and Cl/K ratios for all glasses are relatively high (>0.7). This ratio is sensitive to assimilation of hydrothermally altered material, so the high values indicate assimilation during shallow-level crystallization of OJP magmas. Ratios of H2O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H2O/Ce values are probably also caused by the same assimilation process that results in high Cl. The water content of the high MgO-magmas before contamination is estimated to be approximately 0.07 wt% H2O, corresponding to H2O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H2O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H2O content of the OJP mantle source region (170 ± 30 ppm H2O) is similar to that of the depleted MORB source (140 ± 40 ppm H2O). Instead, large extents of melting beneath the OJP must have been caused by a relatively high mantle potential temperature, consistent with upwelling of a hot mantle plume.

Julie Roberge1, Rosalind V. White2 & Paul J. Wallace1
1 Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272, USA pwallace@darkwing.uoregon.edu
2 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK