Iron-nickel sulfides in anhydrous interplanetary dust particles

Fe,Ni-sulfide grains in nine anhydrous chondritic, porous (CP) interplaneta ry dust particles (IDPs) and one hydrated chondritic smooth (CS) IDP were e xamined using 200 and 400 keV transmission electron microscopy. Crystal structures of the grains were investigated using selected area elec tron diffraction, electron microdiffraction. and high-resolution lattice fr inge imaging. Grain compositions were measured using quantitative energy-di spersive x-ray spectroscopy. Three types of sulfide grains were examined: 1 0 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 g rains, and large 0.5 to 5 mum diameter grains dispersed throughout the fine -grained matrices of IDPs. Most sulfide nanocrystals within and on the surfaces of GEMS are low-Ni pyr rhotite (0 less than or equal to Ni less than or equal to5.5 atomic %) with a hexagonal unit cell where a(o)= 0.34 nm and c(o) = 0.57 nm. Small 100 to 500 nm mafic grains dispersed throughout the matrices of the IDPs include hexagonal pyrrhotite (0 less than or equal to Ni greater than or equal to 2 0 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 (similar to2 X 5 mum) grain within o ne anhydrous IDP is a polycrystalline mixture of hexagonal pyrrhotite and c ubic sulfide. Electron diffraction and lattice fringe imaging show that hex agonal and cubic sulfide are coherently intergrown on a unit cell scale. When heated in the electron beam the cubic sulfide transforms into hexagona l pyrrhotite. Therefore. it is possible that most of the pyrrhotite, the do minant sulfide in anhydrous chondritic IDPs, is a secondary thermal alterat ion product of frictional heating and partial sulfur loss during atmospheri c entry. Neither troilite (FeS) nor pentlandite was identified in any of th e nine anhydrous IDPs. Pentlandite was identified in the single hydrated ID P, in accordance with a previous study of sulfides in chondritic IDPs. Copy right (C) 2001 Elsevier Science Ltd.