THERMODYNAMICS OF CAO-SIO2 AND MNO-SIO2 MELTS .1. EXPERIMENTAL-STUDY

The thermodynamic properties of MnO-SiO2 and CaO-SiO2 melts were studi ed over the entire composition range at temperatures between 1400 and 1820 K by Knudsen effusion measurements combined with mass spectrometr y of vapor species. In activity measurements, we induced and studied r eactions involving volatile products by reducing components of silicat e melts directly in the effusion cell. As a result, volatile suboxides were present in the saturated vapor. The mass spectra of saturated va por over the mixed oxides studied showed the presence of Mn+, SiO+, Nb O2+, NbO+, MoO+, MoO2+, and MoO3+ ionic species formed through ionizat ion of Mn, SiO, NbO2, NbO, MoO, MoO2, and MoO3, respectively. For MnO- SiO2 melts, component activities were calculated by two independent pr ocedures. The results coincide within the experimental error (2-3%) an d are in reasonable agreement with the data reported in the literature . The calculated activities of components in CaO-SiO2 melts agree with in the experiment error with the values obtained by extrapolating the thermodynamic properties of CaF2-CaO-SiO2 ternary melts to the corresp onding side of the composition triangle and with the data available in the literature.