Melt densities in the CaO-FeO-Fez03-SiOz system and the compositional partial molar volume of ferric iron in silicate melts dependence of the

The densities of 10 melts in the CaO-FeO-Fe20,-Si02 system were determined in equilibrium with air, in the temperature range of I200 to 1550°C. using the double-bob Archimedean technique. Melt compositions range from 6 lo 58 wt% SK&, 14 lo 76 wt% Fe20j and 10 lo 46 wt% CaO. The ferric-ferrous tatios of glasses drop-quenched from loop fusion equilibration experiments were determined by “Fe Mhbauer spectroscopy. Melt densities range from 2.689 lo 3.618 gm/cm’ with a mean standard deviation from replicate experiments of 0.15%. Least-squares regressions of molar volume verms molar composition have been performed and the root mean squared deviation shows that a linear combination of partial molar volumes for the oxide components (CaO, FeO, Fe203 and SiQ) cannot describe the data ti within experimental error. instead, the inclusion of excess terms in CaFe” and CaSi (product terms using the oxides) is requirwl to yield a ht that describes the experimental data within error. The nonlinear compositionaldependence of the molar volumes of melts in this system can be explained by structural cdnsiderations of the roles of Ca and Fe’+. The volume behavior of melts in this system is significantly different from that in the Na#-FeO-Fe203Si02 system, consistent with the droposal that a proportion of Fe” in melts in the CaO-FeO-Fe203-SiOz system is not tetrahedrallycoordinated by oxygen, which is supported by differences in “Fe Miissbauer spectra of glasses. Specifically, this study confirms that the’5’Fe M&sbauer spectra exhibit an area asymmetry and higher values of isomer shift of the ferric doublet that vary systematically with composition and temperature (this study; DINGWELL and VIRGO, 1987, 1988). These observations are consistent with a number of other lines of evidence (e.g.. homogeneous redox equilibria, DICKENSON and HESS, 1986; viscosity, DINGWELL and VIRGO, 1987, 1988). Two species offerric iron, varying in proportions with temperature, composition and redox state, are sufficient lo describe the above observations. The presence of more than one coordination geometry for Fe’+ in low pressure silicate melts has several implications for igneous petrogenesis. The possible effects on compressibility, the pressure dependence of the redox ratio, and redox enthalpy are briefly noted.