Structural relaxation of vitreous albite near Tg and implications for transport properties of the supercooled liquid at high pressure

We estimate the glass transition temperature, T_g, for vitreous/amorphous albite between 0 and 7.7 GPa by tracking the progress of densification following high-temperature annealing experiments with run durations equal to 5τ (when τ=100 s). T_g decreases by 54 K/GPa up to 2.6 GPa, and thereafter shows a weak negative pressure dependence. This behavior mimics the negative pressure dependence of viscosity of albite liquid shown by [1]; however, we do not find a change in the sign of ∂T_g/∂P at least up to 7.7 GPa as reported in some isothermal ∂η/∂P, and ∂D_O/∂P data sets. Our high field (21.8 T) ²⁷Al MAS NMR measurements of recovered glasses rapidly quenched from super-T_g conditions possess trace amounts of high coordinated Al at 2.6 GPa and only ∼17% by 5.5 GPa. This suggests that the decrease in T_g (and viscosity at low temperature) results dominantly from topological rearrangement of the supercooled melt structure and not changes to Al or Si coordination number and connectivity of the network. In fact, at T_g from 0 to 8 GPa, the X_NBO, or network connectivity, is unchanged [2] and at 7.7 GPa, we find the proportion of high coordinated Al is still ∼35%. Convergence in the timescales of relaxation at T_g(P) and the onset of Na mobility to 6 GPa documented by high-pressure electrical conductivity measurements [3] implies that the fragility of albite melt increases with pressure up to ∼4-5 GPa, without changing the effective polymerization of the melt. In contrast, fragility appears to decrease with pressure in partially depolymerized silicate melts. Such differences in fragility can be used for extrapolation of activation energy based models for viscous flow to high pressure. [1] Kushiro, 1978, EPSL, 41; Brearley et al., 1986, GCA, 50; Brearley and Montana, 1989, GCA, 53; Poe et al., 1997, Science, 276; Suzuki et al., 2002, Phys. Chem. Miner., 29; Funakoshi et al., 2002, J. Phys.: Condens. Matter., 14; Behrens and Schulze, 2003, Am. Min., 88. [2] Lee et al. 2004, GCA, 68; [3] Bagdassarov et al., 2004, Phys. Chem. Glasses, 45.