Metastable Eutectic Condensation in a Mg‐Fe‐SiO‐H 2 ‐O 2 Vapor: Analogs to Circumstellar Dust

Abstract

Experimental studies of gas-to-solid condensation in a Fe-Mg-SiO-H2-O2vapor reveal that this process yields only solids with magnesiosilica(MgO.SiO2) and ferrosilica (Fe-oxide.SiO2) compositions that coincidewith metastable eutectics in the MgO-SiO2 and (FeO/Fe2O3)-SiO2 binaryphase diagrams plus simple metal oxides (MgO, SiO2, and FeO or Fe2O3).No solids form with mixed Mg-Fe-O compositions during condensationnor is there evidence for the formation of ferromagnesiosilica, MgO.FeyOx.SiO2solids. The experimental evidence demonstrates that condensationof multicomponent vapors yields only a limited number of metastablesolids of well-defined composition. These results have interestingconsequences for models of grain formation in circumstellar outflows,for predictions concerning the chemical and mineralogical compositionof presolar silicates, and for the composition of condensates formedin protostellar systems.