Metastable Eutectic Condensation in a Mg-Fe-SiO-H2-O2 Vapor: Analogs to Circumstellar Dust

摘要

Experimental studies of gas-to-solid condensation in a Fe-Mg-SiO-H2-O2 vapor reveal that this process yields only solids with magnesiosilica (MgO.SiO2) and ferrosilica (Fe-oxide.SiO2) compositions that coincide with metastable eutectics in the MgO-SiO2 and (FeO/Fe2O3)-SiO2 binary phase diagrams plus simple metal oxides (MgO, SiO2, and FeO or Fe2O3). No solids form with mixed Mg-Fe-O compositions during condensation nor is there evidence for the formation of ferromagnesiosilica, MgO.FeyOx.SiO2 solids. The experimental evidence demonstrates that condensation of multicomponent vapors yields only a limited number of metastable solids of well-defined composition. These results have interesting consequences for models of grain formation in circumstellar outflows, for predictions concerning the chemical and mineralogical composition of presolar silicates, and for the composition of condensates formed in protostellar systems.