Laboratory condensation of refractory dust in protosolar and circumstellar conditions

Abstract

In order to better understand condensation processes that took place in the solar nebula and to evaluate the effect of kinetics on the condensed matter, we have built an experimental apparatus for studying condensation of multi-elemental refractory gases at high-temperature and low-pressure. The condensation of a Mg–Si-rich gas, with solar interelement ratios of Ca, Al, Mg and Si, and of a Ca–Al-rich gas under a total pressure of ∼4 × 10 −3 bar at temperatures from 1045 to 1285 °C and for run times of 4–60 min results in direct formation of crystalline oxides or silicates such as corundum, spinel, anorthite, melilite, Al-diopside, forsterite and enstatite. The mineralogy of the condensates, close to that predicted at equilibrium, varies with the duration of an experiment and the temperature of condensation. The chemical reactions between gas and condensates are rapid enough to attain a steady state in less than one hour. The condensation results in chemical fractionation of the gas, i.e. a depletion of the gas in refractory elements at high temperature. Finally, besides revealing the textures of refractory crystals, which condense from a gas of complex chemical composition, this study shows that certain phases, such as spinel, have favored kinetics of condensation. Our experimental results confirm that refractory inclusions in primitive meteorites could have formed by condensation from a hot nebular gas. Similarly, we confirm that crystalline grains can condense at high temperature in the outflows of evolved stars. In both cases, our results indicate that kinetic processes certainly influence grain mineralogy. Kinetic processes must thus be taken into account in modeling the pressure-temperature conditions of circumstellar environments.