HR: 0830h
AN: V41C-0303    [PDF]
TI: Enthalpies of Mixing in Sodium Silicate Glasses and Relevance to Adam-Gibbs Theory
AU: * Hovis, G
EM: hovisguy@lafayette.edu
AF: Lafayette College, Department of Geology and Environmental Geosciences, Easton, PA 18042 United States
AU: Jarry, P
EM: p.jarry@euroglas.com
AF: Laboratoire de Physique des G‚omat‚riaux, Institut de Physique du Globe, 4, place Jussieu, Paris, 75252 France
AU: Toplis, M
EM: mtoplis@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-les-Nancy, F-54501 France
AU: Richet, P
EM: richet@ipgp.jussieu.fr
AF: Laboratoire de Physique des G‚omat‚riaux, Institut de Physique du Globe, 4, place Jussieu, Paris, 75252 France
AB: Solution calorimetric measurements have been made in 20.1 wt % hydrofluoric acid at 50$\deg$C on binary Na$_{2}$O-SiO$_{2}$ glasses ranging in composition from 0 to 50 mol % Na$_{2}$O. The initial calorimetric data for compositions between 20 and 50 mol % Na$_{2}$O were highly variable, determined later to be due to the development of Na carbonate on the glass surfaces. However, additional measurements using minimally-ground specimens that had been remelted shortly before the calorimetric dissolutions produced highly reproducible results. After adjustment of the data for glass transition temperature (Richet et al., 1984, J. Amer. Ceram. Soc.), the results show nearly linear ("ideal") behavior of the heats of solution between 50 % Na$_{2}$O and 30-35 % Na$_{2}$O. However, positive enthalpies of mixing (H$_{ex}$) are evident in the compositional region between 0 and 30-35 mol % Na$_{2}$O, with maximum magnitudes of H$_{ex}$ on the order of 5 kJ/mol relative to 0 and 35 % Na$_{2}$O end members. Within the framework of the Adam-Gibbs theory of structural relaxation, viscosity and heat capacity data may be combined to determine configurational entropies of silicate glasses. When applied to sodium silicate glasses (Toplis, 2001, Chemical Geology), positive entropies of mixing (S$_{ex}$) are calculated for compositions between 0 and 30 mole % Na$_{2}$O, a compositional region where liquid immiscibility also is known to occur (Haller et al., 1974, J. Amer. Ceram. Soc.) and where Gibbs free energies of mixing (G$_{ex}$) therefore must be positive. Because positive entropies of mixing contribute negatively (-TS$_{ex}$) to G$_{ex}$, the observed immiscibility requires positive enthalpies of mixing (H$_{ex}$) in the silicic part of the compositional range. The present study confirms such positive enthalpies of mixing and supports the usefulness of Adam-Gibbs theory for the prediction of viscosity/entropy behavior of silicate glasses and liquids.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3699 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting