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

Abstract

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 (Fe 1)x S) 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 1.0 MeV 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 · 10 16 Kr ++ ions cm )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 radiationresistant 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.

Atomic‐scale simulation of space weathering in olivine and orthopyroxene

“…Experiments reported in Wang et al [] indicate that olivine is more resistant than orthopyroxene to amorphization due to radiation damage. The same trend was reported in Christoffersen and Keller [], although the results were accurate within the experimental error. The results presented here suggest an explanation for these observations.…”

“…The particular focus is on comparison to existing radiation damage experiments performed on enstatite and olivine samples [ Christoffersen and Keller , ; Wang et al , ; Carrez et al , ]. Irradiation experiments using heavy ions by Christoffersen and Keller [] showed a similar critical radiation dose for amorphization in olivine and enstatite, although olivine was found to be slightly more resistant to amorphization. On the other hand, Wang et al [] found that olivine required about 4 times higher radiation dose than enstatite to get amorphized, suggesting olivine might be more radiation resistant.…”

Microchemical and structural evidence for space weathering in soils from asteroid Itokawa

“…The ion processing behavior of pyroxene relative to a wide range of other silicates has, however, been investigated under irradiation similar to the regime expected for the solar wind (Wang and Ewing 1992;Wang et al 1998a,b;Christoffersen and Keller 2011). These studies suggest that structures in which the Si-O tetrahedral sites are more polymerized, as in plagioclase, with a greater variety of interstitial sites, are generally much more susceptible to radiation-induced amorphization (Wang and Ewing 1992).…”

Rock-Forming Minerals Radiation-Induced Volumetric Expansion – Revisiting Literature Data

“…Hence, the provided dataset seems only relevant for magnesian olivines, and not for all types of olivines. In particular, it should be noted that ion-beam irradiation data indicate that the irradiation resistance of forsterite (Wang et al 1998;Carrez et al 2002;Christoffersen and Keller 2012) is much higher than that of fayalite .…”