Condensation Experiments of Mg-rich Crystalline and Amorphous Silicates in Vacuum

Introduction: Magnesian silicates such as for-sterite (Mg 2 SiO 4) and enstatite (MgSiO 3) are major minerals in universe including disks around young stars [e.g., 1], shells around evolved stars, in particular, oxygen rich stars [e.g., 2, 3] and so on. It is thus important to understand evaporation and condensation behaviors of magnesian silicates, which are major phase-transformation reactions at low pressures, in order to understand evolution of solid in space. We have been carrying out condensation experiments of silicates under non-equilibrium conditions in vacuum. In this study, we will present experimental study on condensation of silicates to understand formation of silicate dusts in space. We describe the concept of our experiments and experimental techniques, and experimental results, and discuss the interpretation of the results for dust formation in universe, especially around AGB stars. Experimental: Condensation of dusts proceeds through nucleation and growth. Nucleation is hardly observed and hardly controlled in experiments, and we perform condensation experiments focusing on a growth process of Mg-silicates to know the condensing phases under highly kinetic conditions and critical conditions for condensation, that is, the relationship between condensation coefficient and the supersatura-tion ratio. Two types of experiments have been carried out: one is " quench " experiments, where gas generated at high temperature collides with a substrate at much lower temperature [4]. No further surface process takes place, and condensing phases as a function of pressure or temperature are examined. The second type of experiment is condensation in the environment where gas atoms and molecules interact in a cooling condition, which we call " cooling " experiments [5]. The gas source is synthesized single crystal of forsterite, which was heated to generate Mg, SiO, and O gas molecules with the ratio of 2: 1: 3. Condensation took place on a substrate. Results: Quench experiments: Equilibrium condensation temperature of forsterite in the quench experiments is about 1200°C, and it should be the only stable phase at lower temperature as far as the bulk gas composition is kept stoichiometry of forsterite. No condensation occurred on the substrate at 1145 and 1040°C except for small amounts of platinum, which was from the thermocouple, as Mo-Pt alloy. No sili-cate condensed at 860°C, either, but silicon condensed as Mo-Si alloy. Amorphous silicates condensed at