Melt densities in the Na 2O-FeO-Fe 2O 3-SiO 2 system and the partial molar volume of tetrahedrally-coordinated ferric iron in silicate melts

Abstract

The densities of 12 melts in the Na 2O-FeO-Fe 2O 3-SiO 2 system have been determined in equilibrium with air, in the temperature range of 1000–1500°C, using the double bob, Archimedean technique. Ferrous iron determinations of 100–200 mg samples, “dip” quenched from high temperature, indicate that all the melts investigated were highly oxidized under these experimental conditions. 57Fe Mössbauer spectra of glasses obtained by drop quenching 80 mg melt samples from loop equilibration runs yield Fe 3+/Fe 2+ data equivalent to that for the densitometry (dip) samples for all but the most viscous melt, and confirm that all but one melt equilibrated with air during the densitometry measurements. Melt densities range from 2.17 to 2.88 g/cm 3 with a mean standard deviation (from replicate experiments) of 0.36%. Least squares regression of the density data at 1300, 1400 and 1500°C, was calculated, both excluding and including excess volume terms (herein named linear and nonlinear fits, respectively) and the root mean squared deviation (RMSD) of each regression was compared with the total experimental error. The partial molar volumes computed for linear fits for Na 2O and SiO 2 are similar to those previously reported for melts in the Na 2O-Al 2O 3-SiO 2 system ( Stein et al., 1986). The partial molar volumes of Fe 2O 3 obtained in these linear fits are equal to those obtained by Shiraishi et al. (1978) in the FeO-Fe 2O 3-SiO 2 system but 5 to 10% lower than reported by Mo et al. (1982) in multicomponent melts. The partial molar volume exhibited by Fe 3+ in this system is representative of the partial molar volume of tetrahedrally coordinated Fe 3+ in silicate melts.