Kinetic Evaporation of Troilite (FeS) under Protoplanetary Disk Conditions

Abstract

The bulk sulfur abundance in chondrites, excluding CI chondrites, is lower than the solar composition, suggesting that elemental fractionation of sulfur occurred in the Sun's protoplanetary disk. One possible cause of this fractionation is incongruent evaporation of troilite (FeS). A previous study on evaporation of troilite was conducted at much higher temperatures than the plausible temperature range for evaporation of FeS in protoplanetary disks. In this study, we conducted incongruent evaporation of FeS, from Canyon Diablo iron meteorite, at 1123-973 K, which is lower than in previous studies (>1173 K), at hydrogen pressures of 0.2-5 Pa. We found that the evaporation residues were metallic iron, indicating selective evaporation of sulfur, and that the evaporation rate is controlled by the surface reaction. The obtained evaporation rates at 1123-973 K were consistent with those extrapolated from the previous study. The dependence of the evaporation rate on hydrogen pressure was smaller than the thermodynamical predictions, implying that S₂, in addition to H₂S, contributes to evaporation as suggested previously.