Viscosity-temperature relations and structure in fully polymerized aluminosilicate melts from ion dynamics simulations

Results of a series of molecular dynamics simulations indicate that the Al:Si ratio (and to a lesser extent the type of charge-balancing countercation) present in aluminosilicate melts induces a "chemical pressure" that significantly influences the temperature dependence of the melt viscosity. The temperature dependence of the T-O bond length and T-O-T angle in the melt has been calculated for albite, anorthite, nepheline, and MgAl₂Si₂O₈ (a magnesium analog of anorthite) at their experimental melt densities at atmospheric pressure and for pure silica at two different densities corresponding to V V₀ = 1.0 ( 1 atm) and V V₀ = 0.8 ( ~4 GPa). These simulations show that melt fragility can be correlated to increasing T-O length and decreasing T-O-T angle, both of which are characteristics of silicate melts under increasing pressure. Framework cations with coordination numbers greater than four are observed in these aluminosilicate melts and are shown to be related to fragile behavior. Finally, for nepheline and anorthite we infer a large contribution to fragility from configurational disordering of Al-O-Si links which are known to be dominant in the glasses but are found to be absent from the simulated melts.