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Fragilities of liquids predicted from the random first order transition theory of glasses

admin — Mon, 08 Mar 2010 02:53:41 +0000

A microscopically motivated theory of glassy dynamics based on an underlying random first order transition is developed to explain the magnitude of free energy barriers for glassy relaxation. A variety of empirical correlations embodied in the concept of liquid “fragility” are shown to be quantitatively explained by such a model. The near universality of a Lindemann ratio characterizing the maximal amplitude of thermal vibrations within an amorphous minimum explains the variation of fragility with a liquid’s configurational heat capacity density. Furthermore, the numerical prefactor of this correlation is well approximated by the microscopic calculation. The size of heterogeneous reconfiguring regions in a viscous liquid is inferred and the correlation of nonexponentiality of relaxation with fragility is qualitatively explained. Thus the wide variety of kinetic behavior in liquids of quite disparate chemical nature reflects quantitative rather than qualitative differences in their energy landscapes.

Xiaoyu Xia and Peter G. Wolynes
+ Author Affiliations

Departments of Physics and Chemistry, University of Illinois, Urbana, IL 61801
Contributed by Peter G. Wolynes

Fluoride sites in aluminosilicate glasses: High-resolution 19F NMR results

admin — Mon, 08 Mar 2010 02:53:08 +0000

We present here high-resolution, 19F NMR spectra, collected with sample spinning rates to 25 kHz, for sodium and calcium silicate and aluminosilicate glasses. Several distinct fluoride ion sites are well resolved and can be assigned to various coordination environments based on clear similarities to crystalline model compounds. In aluminosilicates, fluoride with one Al and several Na or Ca neighbors predominate, but silicon-fluoride groups may also be significant. Small concentrations of the latter can also be detected in sodium silicate glasses, suggesting a possible role in reduction of viscosity. Fluoride sites with no Si neighbors are, however, predominant in Al-free sodium silicates and probably in Ca silicates as well.

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

Correspondence: E-mail: stebbins@pangea.stanford.edu

Magnesium coordination environments in glasses and minerals: New insight from high-field magnesium-25 MAS NMR

admin — Mon, 08 Mar 2010 02:49:16 +0000

A comparison of 25Mg magic angle spinning (MAS) NMR spectra of crystalline and glassy diopside (CaMgSi2O6) reveals that the chemical shift in the disordered phase corresponds to that of the mineral, suggesting that sixfold coordination is essentially retained upon vitrification. Likewise, a crystalline leucite analogue (K2MgSi5O12) known to possess fourfold-coordinated Mg has the same peak position as the corresponding glass. In addition to being a sensitive probe of local structure by analogy with crystalline phases, these data may be understood in terms of the effect of competing cation field strengths: competing network modifier cations with higher field strength induce higher magnesium coordination numbers, with a consequent increase in bond length and decrease in chemical shift. This work demonstrates the utility of NMR at 14.1 Tesla for probing the coordination environment of Mg in glasses and minerals, and suggests great potential for high-field NMR investigations of quadrupolar nuclides with low resonance frequencies in amorphous systems.

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

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

Water speciation in silicate glasses and melts: Langmuir limited site model

admin — Mon, 08 Mar 2010 02:48:42 +0000

The distribution of dissolved water in a haplogranitic glass between molecular water and reacted OH groups, as measured by Sowerby and Keppler (1999) at temperature and pressure, is compared to a Langmuir model. In this model the number of reactive sites in the glass is limited, so as the total water content increases, the concentration of reacted OH groups saturates. The comparison between the model and experimental measurements is good with two fitting parameters. The total water solubility in NaAlSi3O8 melts as measured by Holtz et al. (1995) is the sum of molecular water dissolved from the gas phase and reacted OH groups; the concentration of OH groups calculated from these solubility measurements also fits the Langmuir model. The partial molar volumes for water in NaAlSi3O8 melts are constant above about 4 wt% total water, but decrease at lower water contents; this dependence can be understood from the speciation measurements.

Robert H. Doremus*
Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, U.S.A.

Correspondence: * E-mail: doremr@rpi.edu

Si CPMAS NMR investigations of silanol-group minerals and hydrous aluminosilicate glasses

admin — Mon, 08 Mar 2010 02:48:03 +0000

One-pulse magic angle spinning (MAS) 29Si and 1H-29Si cross-polarization (CP) MAS nuclear magnetic resonance (NMR) spectroscopy was performed on minerals with OH groups and on hydrous aluminosilicate glass samples. The silanol-group samples used were krauskopfite, rosenhahnite, thaumasite, ussingite, and KHSi2O5 with known Si to H distances and ellenbergerite, the proton positions of which are not as well defined. The Si-H distances from the minerals can be compared with the cross-polarization time constants (SiH) and the proton spin-relaxation times in the rotating frame [1(H)], and a rough correspondence exists between a mineral’s shortest Si-H distance and its SiH value. Also, fast [1(H)] values correspond to large bulk H densities. The CP spectra of the hydrous aluminosilicate glass samples were fitted with two peaks, representing two different Si environments within the glass structure. The contact-time curves of the higher frequency peak imply SiH similar to the mineral samples with short Si-H distances, and this suggests that the glasses could contain a large fraction of either Si-OH groups or protonated bridging O atoms.

Jane V. Oglesby* and Jonathan F. Stebbins
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.

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

Fivefold-coordinated aluminum in tectosilicate glasses observed by triple quantum MAS NMR

admin — Mon, 08 Mar 2010 02:47:24 +0000

Eight glasses with molar Mg/2Al 1 in the system MgO-Al2O3-SiO2 have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. Calculations of the polymerization state of these liquids made assuming that all aluminum is in tetrahedral coordination charge balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.

Michael J. Toplis1,*, Simon C. Kohn2, Mark E. Smith3 and Iain J.F. Poplett3
1 CRPG-CNRS, BP20, F-54501, Vandoeuvre-lès-Nancy, France
2 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ U.K.
3 Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K.

Correspondence: * E-mail: mtoplis@crpg.cnrs-nancy.fr

Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

admin — Mon, 08 Mar 2010 02:38:01 +0000

The 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.

Claudia Romano1,*,, Eleonora Paris2, Brent T. Poe3, Gabriele Giuli2, Donald B. Dingwell3 and Annibale Mottana1,
1 Dipartimento di Scienze Geologiche, Universitá degli Studi Roma Tre, Largo S. Leonardo Murialdo, I-00146 Rome, Italy
2 Dipartimento di Scienze della Terra, and INFM, Universitá di Camerino, I-62032, Camerino, Italy
3 Bayerisches Geoinstitut Universität Bayreuth, D-95440 Bayreuth, Germany

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

admin — Mon, 08 Mar 2010 02:37:21 +0000

Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia–Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation–inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (47·4–51·1 wt % SiO2, 5·65–8·75 wt % MgO) is the broad range of K2O (0·1–1·8 wt %) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g. TiO2 0·97–2·1%; Na2O 2·4–4·3%; P2O5 0·08–0·7%; H2O 0·25–1·5%; La 4·3–46·6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (87Sr/86Sr = 0·70255–0·70275) and Pb (206Pb/204Pb = 18·951–19·493; 207Pb/204Pb = 15·528–15·589; 208Pb/204Pb = 38·523–38·979), and negatively with Nd (143Nd/144Nd = 0·51310–0·51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number–K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63–69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo86–89·5). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo90·5). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase ± clinopyroxene, and have lower mg-number (57–67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale. We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.

KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry

VADIM S. KAMENETSKY1,*, JOHN L. EVERARD2, ANTHONY J. CRAWFORD1, RICK VARNE1, STEPHEN M. EGGINS3,4 and RUTH LANYON1,4
1SCHOOL OF EARTH SCIENCES AND CENTRE FOR ORE DEPOSIT RESEARCH, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TAS. 7001, AUSTRALIA
2TASMANIAN GEOLOGICAL SURVEY, PO BOX 56, ROSNY PARK, TAS. 7018, AUSTRALIA
3DEPARTMENT OF GEOLOGY, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
4RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA

Received October 1, 1998; Revised typescript accepted September 2, 1999

Relationships between the medium-range structure of glasses and crystals

admin — Mon, 08 Mar 2010 02:36:45 +0000

P. H. Gaskell*
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK

* E-mail: phg1@cam.ac.uk

The known structure of a crystalline phase is almost always useful in investigating the unknown structure of the compositionally equivalent glass. For the local environment around elements like Si, B and P, the correspondence between site geometry and symmetry can be impressively close. Beyond near neighbours, any relationship becomes less obvious – at least in real-space data. Progress in understanding the medium-range structures of glasses has been painfully slow as a result. One essential clue is given by reciprocal-space features at low Q (scattering vector) in X-ray or neutron scattering data, which are clearly related to the medium-range structure. Interpretation of these features as ‘quasi-Bragg’ scattering allows direct comparison between the structures of the glass and equivalent crystalline phases. Applications of this method will be illustrated in borates and silicates, together with some chalcogenide glasses. Correspondence between low-Q features for these glasses and compositionally-equivalent crystals is qualitatively good. In some cases there is semi-quantitative agreement too. Thus the essential flavour of the medium-range structure of several typical glasses appears to be interpretable, rather easily.

KEYWORDS: crystalline phase, glass, X-ray scattering data

Water in Zr-bearing synthetic and natural glasses

admin — Mon, 08 Mar 2010 02:36:13 +0000

François FARGES1,* and Stéphanie ROSSANO2

1 Laboratoire des Géomatériaux, Université de Marne-la-Vallée, (and Stanford University and Institut Universitaire de France) 5 Boulevard Descartes – Champs S/Marne 77454 – Marne la Vallée Cedex, France
2 Laboratoire de Minéralogie-Cristallographie, Universités de Paris 6, 7 et IPGP, UMR CNRS 7590 – 2 place Jussieu, 75252 – Paris cedex 05, France (now at the Laboratoire des Géomatériaux, Université de Marne-la-Vallée)

* email:farges@univ-mlv.fr

This paper was presented at «Fluid/minerals Interactions in the Crust», a symposium in honour of Martine Lagache, Paris, October 1999

The local environment of 1400 ppm Zr in hydrous synthetic glasses (sodium trisilicate and albitic) has been probed at the Zr K-edge by X-ray Absorption Fine Structure (XAFS) spectroscopy and compared to that for natural glasses (volcanic and tektites). In most glasses (anhydrous and hydrous), Zr is found to be 6-coordinated by oxygen, with Zr-O distances of 2.08 Å. However, in anhydrous albite glass, the Zr-O distance increases to 2.12 Å, related to the presence of highly-coordinated Zr-environments (7,8) in the glass. Network-forming, next-nearest neighbour T (T = Si/A1) contributions are measured near 3.6 Å, suggesting corner-sharing ZrO6-TO4 polyhedra. We observed no large effect due to the presence of water in these glasses, suggesting that molecular water and hydroxyls are not likely to complex Zr in these compositions. However, a slight structural relaxation of the tetrahedral framework (increased network fragility with a small production of NBO’s) is observed around Zr in albitic compositions with more than 1 wt.% water. Then, no strong evidence for depolymerization induced by water is observed around Zr (Zr being used in that study as a probe for trace amounts of NBO’s). By comparison with transition elements (Fe and Ni), Zr appears to be much less affected by the introduction of water. The conclusions for Zr are likely to be extrapolated to natural magmas, as the natural glasses probed show similar environments around Zr. Then, oxygen appears the most efficient anion to «complex» Zr, as compared to water, hydroxyls, halogens, CO2 or S.

Key-words: Zr, silicate glasses, natural glasses, XANES, EXAFS..