MODELLING CONFIGURATIONAL ENTROPY OF SILICATE MELTS

The Adam-Gibbs theory provides a robust connection between the transport or relaxation properties of melts (i.e. viscosity) and their thermochemical properties (Adam and Gibbs, 1965):

log η = A + B/[T*S_^c(T)] (1)

The expanded form of (1):

log η = A + B/[ T*(S_^c(Tg) + Cp_^c ln(T/Tg) ) ] (2)

has adjustable unknown parameters A, B and S_c(Tg) which are routinely estimated by fitting (2) to experimental measurements of Cp_c, Tg and . This approach has been adopted and modified by a number of workers (e.g., Richet & Neuville, 1992; Richet & Bottinga, 1995; Toplis et al., 1997; Webb, 2008; Avramov, 2013). Here we follow this same approach and apply the method to hydrous melts and glasses. Our goal is explore the potential connections between the transport properties of silicate melts and their corresponding thermochemical properties.

We use recently published datasets for silicate melts (N~20) that comprise both measurements of melt viscosity, Cp_c and Tg. However, we assume that all silicate melts converge to a single, common, but unknown, value at high temperature. This implies a single unknown value of A for all melts. This strategy has a sound theoretical basis, strong empirical support, and creates substantially more reliable estimates of the other adjustable parameters (cf. Russell et al. 2002; 2003). In the case of derivative properties, including Tg and fragility (m), our model for the configurational entropy of melts returns similar values to those predicted by the viscosity model for multicomponent silicate melts of Giordano et al. (2008). This preliminary analysis provides a firm initial connection between the thermochemical properties of melts and their transport properties.

References: Adam G, Gibbs JH (1965) J Chem Phys 43:139–146. Avramov I (2013) J Non-Crystalline Solids 362:120-123. Giordano, D Russell, JK & Dingwell, D (2008) Earth & Planet Sci Letts 271, 123-134. Richet P, Bottinga Y (1995) Rev Mineral 32:67-94. Richet P, Neuville DR (1992) Adv Phys Geochem 10:132-161. Russell JK, Giordano D, Dingwell DB, Hess K-U (2002) Eur J Min 14:417–427. Russell JK, Giordano D, Dingwell D (2003) Am Min 88:1390-1394. Toplis MJ, Dingwell DB, Hess K.-U, Lenci T (1997) Am Min 82:979–990. Webb SL (2008) Chem Geol 256:92–101.