Space radiation processing of sulfides and silicates in primitive solar systems materials: Comparative insights from in situ TEM ion irradiation experiments

Abstract

Mineral grains that comprise dust particles in circumstellar, interstellar, and protostellar environments can potentially undergo amorphization and other solid-state transformations from exposure to energetic ions from space plasmas. The Fe-sulfide minerals troilite (FeS) and pyrrhotite (Fe1-xS) are important known dust components, but their potential to undergo structural changes, including amorphization, from space radiation processing in dusty space environments has not been experimentally evaluated relative to silicates. We used a transmission electron microscope (TEM) with capabilities for in situ ion irradiation to precisely follow structural changes in troilite and pyrrhotite exposed to 1.0MeV Kr++ ions selected to optimize the probability of inducing amorphization from nuclear elastic collisional processes. No indication of amorphization was found in either mineral up to an experimentally practical ion dose of 1×1016 Kr++ ionscm-2, indicating that both structures can remain crystalline up to a modeled collisional damage level of at least 26 displacements-per-atom. This behavior matches that of some of the most radiation-resistant nonmetallic phases known, and is two orders of magnitude higher than the levels at which Mg-rich olivine and enstatite become amorphous under the same irradiation conditions. Although pyrrhotite retained short-range crystalline order during irradiation, its longer range vacancy-ordered superstructure is removed at modeled damage levels equivalent to those at which olivine and enstatite become amorphous. This suggests that space radiation conditions sufficient to amorphize olivine and enstatite in circumstellar and interstellar environments would convert coexisting pyrrhotite to its disordered structural form, thereby changing magnetic and possibly other properties that determine how pyrrhotite will behave in these environments. © The Meteoritical Society, 2011.