Iron-nickel sulfides in anhydrous interplanetary dust particles

Abstract

Fe,Ni-sulfide grains in nine anhydrous chondritic porous (CP) interplanetary dust particles (IDPs) and one hydrated chondritic smooth (CS) IDP were examined using 200 and 400 keV transmission electron microscopy.Crystal structures of the grains were investigated using selected area electron diffraction, electron microdiffraction, and high-resolution lattice fringe imaging. Grain compositions were measured using quantitative energy-dispersive x-ray spectroscopy. Three types of sulfide grains were examined: 10 to 100 nm diameter nanocrystals within and on the surfaces of GEMS (glass with embedded metal and sulfides), small 100 to 500 nm diameter isolated grains, and large 0.5 to 5 μm diameter grains dispersed throughout the fine-grained matrices of IDPs.Most sulfide nanocrystals within and on the surfaces of GEMS are low-Ni pyrrhotite (0 ≤ Ni ≤ 5.5 atomic %) with a hexagonal unit cell where ao = 0.34 nm and co = 0.57 nm. Small 100 to 500 nm mafic grains dispersed throughout the matrices of the IDPs include hexagonal pyrrhotite (0 ≤ Ni ≥ 20 atomic %), ordered hexagonal pyrrhotite exhibiting prominent superlattice reflections, and a cubic sulfide that appears to have a sulfur-deficient “spinel-like” (Fd3m) structure. A large (∼2 × 5 μm) grain within one anhydrous IDP is a polycrystalline mixture of hexagonal pyrrhotite and cubic sulfide. Electron diffraction and lattice fringe imaging show that hexagonal and cubic sulfide are coherently intergrown on a unit cell scale.When heated in the electron beam the cubic sulfide transforms into hexagonal pyrrhotite. Therefore, it is possible that most of the pyrrhotite, the dominant sulfide in anhydrous chondritic IDPs, is a secondary thermal alteration product of frictional heating and partial sulfur loss during atmospheric entry. Neither troilite (FeS) nor pentlandite was identified in any of the nine anhydrous IDPs. Pentlandite was identified in the single hydrated IDP, in accordance with a previous study of sulfides in chondritic IDPs.