Structure and rheology of iron-bearing Na2O-Al2O3-SiO2 melts

The viscosity of a series of Na₂O-FeO-Fe₂O₃-Al₂O₃-SiO₂ melts has been measured by micropenetration methods. The viscosity of the Fe-bearing melts follows the same compositional trend as the Fe-free Na₂O-Al₂O₃-SiO₂ melts as a function of Na/Al at constant SiO₂ content: viscosity increases with decreasing peralkalinity and remains almost constant for all investigated compositions in the peraluminous field. The addition of 1 mol% Fe₂O₃ to the Fe-free peraluminous compositions causes ~1 log₁₀ Pa s decrease in viscosity, while ~3 mol% Fe₂O₃ needs to be added to the peralkaline compositions to produce a similar decrease in viscosity. The need for more charge-balancing cations in the peraluminous melt structure appears to control the Fe²⁺/Fe^total, with the peraluminous compositions having a large Fe²⁺/Fe^total (0.6) compared with that for the peralkaline compositions (0.3). Although triclusters probably exist in peraluminous composition melts, the configurational entropy term at a viscosity of 10¹² Pa s [B_e/S_conf(T_g)] calculated for these melts suggests that the flow process is not initiated at a tri-cluster site. Thus the peralkaline and peraluminous melts have not only different structures but also slightly different flow processes. These differences are reflected in the viscosity, activation energy for viscous flow and calculated B_e/S_conf(T_g) term.