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Understanding the changes in viscosity due to the different oxidation states of iron ions is important for simulating phenomena related to molten slags and also for understanding the structure of iron-oxidecontaining silicate melts. However, these viscosity changes are not well understood. Here, we show the viscosity changes of R2O-SiO2-FexO (R = Li, Na, or K) melts due to changes in the oxidation states of the iron ions by systematically varying the oxygen partial pressure using several Ar-based gases at 1 773 K. The initial compositions of the samples were 30R2O-60SiO2-10Fe2O3 (mol%), and the ratio of Fe 3+ to Fe 2+ in the R2O-SiO2-FexO melts increased with increasing oxygen partial pressure in all samples. Meanwhile, the viscosity of all the R2O-SiO2-FexO melts decreased with increasing Fe 2+ to total-Fe ratio. The data indicate that the increase in the amount of Fe 2+ ions, which behave as network modifiers, would result in depolymerization of the silicate melts. It should also be noted that the viscosity of the melts increased in the order of alkali cationic radius (K > Na > Li) when the ratios of Fe 2+ to Fe 3+ in the melts were comparable. This was due to the change in the coordination structure of Fe 3+ in the melts. Not only the oxidation state of the iron ions but also the coordination structure of Fe 3+ may be important for understanding the viscosity.KEY WORDS: viscosity; alkali ferrosilicate melts; redox equilibria of Fe; oxygen partial pressure.

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