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AMORPHOUS MATERIALS: PROPERTIES, STRUCTURE, AND DURABILITY

The viscosity of hydrous NaAlSi₃O₈ and granitic melts: Configurational entropy models

¹ Department of Geological Sciences, 101 Geology Building, University of Missouri-Columbia, Columbia, Missouri 65211, U.S.A.
² Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, U.K.
³ Physique des Minéraux et des Magmas, IPGP, 4 place Jussieu, 75252 Paris Cedex 05, France

Correspondence: ^* E-mail: whittingtona{at}missouri.edu

We used configurational entropy theory to model the viscosity () of hydrous melts of NaAlSi₃O₈, haplogranite (SiO₂-KAlSi₃O₈-NaAlSi₃O₈), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log = A_e + B_e/TS^conf(T), where S^conf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables A_e, B_e, and S^conf(T_g), where T_g is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi₃O₈, haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined -T-X_H2O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi₃O₈ (n = 77), 0.16 log units for haplogranite (n = 55), and 0.28 log units for peraluminous granites (n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in A_e, B_e, and S^conf(T_g) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.

Key Words: Viscosity • configurational entropy • water • silicate melt • albite • granite