Measurements of vaporization coefficients of calcium alumino-silicate compositions

In this study we report vaporization coefficients for silica, alumina, and molten anorthite and gehlenite obtained by continuous weight loss measurements in a thermogravimetric system with a tungsten mesh vacuum furnace. Free surface vaporization rates are measured using 2.5 mm droplets on Ir-wire loops. Vaporization rates were converted to partial pressures with the Hertz-Knudsen-Langmuir equation. Where possible, equilibrium pressure are determined using iridium Knudsen cells in addition to model calculations. Silica and alumina exhibit no change in vaporization coefficient on melting with aSiO2 = 0.024 +/- 0.001 (all uncertainties are 1s) and aAl2O3 = 0.57 +/- 0.07. The anorthite and gehlenite composition melts exhibit vaporization coefficients (aAN = 0.033 +/- 0.003 and aGEH = 0.131 +/- 0.014) close to those of pure silica suggesting only a weak, if any, dependence of vaporization coefficients of silicates studied on the melt structure. The Knudsen cell equilibrium pressures are preferred over model calculations. Implications of experimental data to volatilization and formation of Ca-Al-rich inclusions in meteorites are considered.