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+/Fetotal, with the peraluminous compositions having a large Fe2+/Fetotal (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 [Be/Sconf(Tg)] 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 Be/Sconf(Tg) term.

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