• A TEM study of thermally modified comet 81P/Wild 2 dust particles by interactions with the aerogel matrix during the Stardust capture process

      Leroux, H.; Rietmeijer, F. J. M.; Velbel, M. A.; Brearley, A. J.; Jacob, D.; Langenhorst, F.; Bridges, J. C.; Zega, T. J.; Stroud, R. M.; Cordier, P.; et al. (The Meteoritical Society, 2008-01-01)
      We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous dust-rich patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
    • Origin and formation of iron silicide phases in the aerogel of the Stardust mission

      Rietmeijer, F. J. M.; Nakamura, T.; Tsuchiyama, A.; Uesugi, K.; Nakano, T.; Leroux, H. (The Meteoritical Society, 2008-01-01)
      Suessite along with hapkeite and more Fe-rich iron-silicides up to Fe7Si2 formed near the entrance of aerogel track #44. These phases are ~100 nm, quenched-melt spheres, but the post-impact cooling regime was such that melt vitrification produced a polycrystalline mixture of Fe silicides and kamacite. The compositional similarities of the impact-produced Fe-Si phases and the Fe-Ni-S phases scattered throughout the aerogel capture medium strongly supports the idea that Fe silicides resulted from a reaction between molten Fe-Ni-S phases and aerogel at very high heating and cooling rates. Temperatures of around 1500 degrees C are inferred from the observed compositions had the silicide spheres formed at thermodynamic equilibrium, which seems unlikely. When the conditions were kinetically controlled, they could have been similar to those leading to the formation of solids with predictable deep metastable eutectic compositions in laboratory condensation experiments.