Metastable eutectic condensation in a Mg-Fe-SiO-H-2-O-2 vapor: Analogs to circumstellar dust

Experimental studies of gas-to-solid condensation in a Fe-Mg-SiO-H-2-O-2 va por reveal that this process yields only solids with magnesiosilica (MgO.Si O2) and ferrosilica (Fe-oxide.SiO2) compositions that coincide with metasta ble eutectics in the MgO-SiO2 and (FeO/Fe2O3)-SiO2 binary phase diagrams pl us simple metal oxides (MgO, SiO2, and FeO or Fe2O3). No solids form with m ixed 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 onl y a limited number of metastable solids of well-defined composition. These results have interesting consequences for models of grain formation in circ umstellar outflows, for predictions concerning the chemical and mineralogic al composition of presolar silicates, and for the composition of condensate s formed in protostellar systems.