Viscosities of melts in the Na 2OFeOFe 2O 3SiO 2 system and factors controlling relative viscosities of fully polymerized silicate melts

Abstract

The viscosity-temperature relationships of nine melts in the Na 2OFeOFe 2O 3SiO 2 system (in equilibrium with air) have been measured in the temperature range of 1450–800°C, using the concentric cylinder method. 57Fe Mössbauer spectra were obtained on quenched samples and show that all melts with ≥ 20 mole% ferrite component contain ≥ 95% Fe as tetrahedrally-coordinated ferric ions. The compositions investigated lie along the SiO 2NaFeO 2 and Na 2Si 4O 9Na 6Fe 4O 9 joins. The viscosities of m decrease strongly with decreasing silicate content along these joins. In contrast, the temperature dependence of viscosity does not vary significantly along these joins. Comparison with equivalent melt compositions (related by the trivalent cation exchange operators AlFe −1, BFe −1 and GaFe −1) in the Na 2OAl 2O 3SiO 2, Na 2OB 2O 3SiO 2 and Na 2O-Ga 2O 3 viscosities decrease in the order aluminosilicate > ferrosilicate, (galliosilicate ?) > borosilicate. Thp electronegativities of the trivalent cations are inversely correlated with the relative viscosities of melts in these systems. Similarly, the electronegativities of network-stabilizing cations are inversely correlated with melt viscosity for alkali and alkaline-earth aluminosilicate melt systems. The variation in the viscosity of tectosilicate melts is correlated with estimated average T-O-T bond angles, and exothermic heats of solution of quench glasses. Structural controls of viscosity discussed are tetrahedral ordering and relative bond strengths. The acmite component in natural, peralkaline, silicic volcanics will not contribute directly to high melt viscosities for these lavas.