Calorimetry of liquids in the system Na2O-Fe2O3-SiO2

Abstract

Transposed-temperature, drop-calorimetry measurements were performed in the systems Na2SiO3-Fe2O3-SiO2 and Na2SiO3-NaFeSi2O6 at 1373 K to investigate thermodynamic properties of Fe3+-bearing silicate liquids. The results confirm previously measured enthalpy of Na2SiO3 and SiO2 melts. No significant heats of mixing were observed in the Na2SiO3-SiO2 liquids or in peralkaline liquids in the system Na2SiO3-Fe2O3. The enthalpy of mixing of peralkaline liquids in the system Na2SiO3-Fe2O3-SiO2 was less than the standard deviation of drop calorimetry (approx. ±10 kJ/mol). From calorimetric data of Na2SiO3-Fe2O3 liquids extrapolated to hematite composition and published heat capacity, the enthalpy of fusion of hematite is estimated to be 83.3 ± 10.5 kJ/mol at 1373 K and 133.4 ± 10.5 kJ/mol at its melting point 1895 K. The Fe3+/Fe2+ ratio of the liquid was estimated from thermodynamic calculations using the fusion enthalpy of hematite and published data of FeO and O2, and reproduced measured Fe3+/Fe2+ ratios in natural silicate liquids and in CaO-Al2O3-Fe2O3-FeO-SiO2 liquids. No significant heats of mixing were observed in the Na2SiO3-NaFeSi2O6 liquids. The enthalpy of formation of NaFeSi2O6 (acmite) at 1bar and 298 K calculated from calorimetric data is −2546.9 ± 17 kJ/mol. The enthalpy and entropy of fusion of acmite are estimated to be 70.5 ± 9.4 kJ/mol and 51.3 ± 6.8 J/K-mol, respectively, at metastable congruent melting point, 1373 K. The entropy of fusion of acmite is similar to that of NaAlSi2O6 (jadeite), indicating that Fe3+ and Al3+ have analogous structural roles in pyroxene melts.