Melt densities in the Na20-Fe@Fez03-SiOz system and the partial molar volume of tetrahedrally-coordinated ferric iron in silicate inelts

The densities of 12 melts in the Na20-FeO-FezOrSi02 system have been determined in equilibrium with air, in the temperature range of lOOO15OO”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 oxidixed under these experimental conditions. “Fe Massbatter spectra of glasses obtained by drop quenching 80 mg melt samples from loop equilibration runs yield Fe3+/F$+ data equivalent to that for the densitometry (dip) sampies for all but the most viscous melt, and confirm that all but one melt equilibrated with air during the densitometry measunments. Melt densities range from 2.17 to 2.88 g/cm3 with a mean standard deviation (from replicate experiments) of 0.36%. Least squares regression of the density data at 1300, 1400 and 1 500°C, was calculated, both excluding and inciuding 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 Na20 and Si02 are similar to those previously repotted for melts in the Na@-AlzOrSi02 system (STEIN et al., 1986). The partial molar volumes of Fe203 obtained in these linear fits are equal to those obtained by SHIRAIS~~I et al. ( 1978) in the FeO-Fe20s-SiO2 system but 5 to 10% lower than repotted by MO et al. ( 1982) in multicomponent melts. The partial molar volume exhibited by Fe’+ in this system is representative of the partial molar volume of tetrahedrally cccrhinated Fe’+ in silicate melts. _