Mass-Spectrometric Investigation of Evaporation Processes and Thermodynamic Properties of Wollastonite

Evaporation of crystalline and liquid wollastonite from a molybdenum cell was studied by Knudsen effusion mass spectrometry in the temperature range 1751–2087 K. Molecular species typical of simple oxides and gaseous CaSiO3 were identified in the gas phase over wollastonite. Partial pressures of vapor species over wollastonite were determined for the first time. The determination of oxide activities in wollastonite made it possible to calculate the enthalpy and entropy of mixing: –38.92 ± 0.52 kJ/mol and 2.33 ± 0.29 J/(mol K), as well as the enthalpy of CaSiO3 melting at 1818 ± 7 K: 27.64 ± 0.57 kJ/mol. Based on this information, the mixing enthalpy and entropy in rankinite (Ca3Si2O7) were estimated in the temperature range 1600–1737 K: –42.63 ± 0.63 kJ/mol and 2.39 ± 0.35 J/(mol K), which are similar to the available thermochemical data. It was shown that redox interaction with the effusion cell material was mainly due to molecular oxygen, the amount of which decreased significantly compared with atomic oxygen in the absence of interaction. The behavior of wollastonite melt follows the patterns typical of other melts of compounds found in the refractory calcium–aluminum-rich inclusions in chondrites.