• Bulk mineralogy and three-dimensional structures of individual Stardust particles deduced from synchrotron X-ray diffraction and microtomography analysis

      Nakamura, T.; Tsuchiyama, A.; Akaki, T.; Uesugi, K.; Nakano, T.; Takeuchi, A.; Suzuki, Y.; Noguchi, T. (The Meteoritical Society, 2008-01-01)
      During preliminary examination of particles released from 81P/Wild 2 short-period comet, we analyzed 28 particles by nondestructive means, high-sensitive X-ray diffraction and high-resolution X-ray tomography, in order to characterize bulk mineralogy and three-dimensional structures of individual particles. The analyses were performed at synchrotron facilities, KEK and SPring-8 in Japan. Twenty-eight particles from 5 to 25 micrometers in size, including 25 particles from Track 35 and 3 particles from Track 44, were first analyzed by X-ray diffraction and then 4 out of 28 particles were analyzed by X-ray tomography. All particles are classified into two groups based on silicate crystallinity: crystalline type and amorphous-rich type. The abundance of the former is approximately 10% of the particles investigated. Crystalline type shows very sharp reflections of olivine and low-Ca pyroxene, while amorphous-rich type shows no or very weak silicate reflections, suggesting that silicates are mostly amorphous. Broad reflections of Fe sulfides and Fe silicides are detected from most of amorphous-rich type particles. Subsequent tomography analysis revealed that the crystalline type is non-porous material consisting of coarse silicate crystals larger than 1 micrometer in size, while the amorphous-rich type is very porous aggregates with amorphous silicates and small Fe sulfide and Fe metallic grains. All characteristics of amorphousrich type particles indicate that most of them are melted and rapidly solidified during capture in the silica aerogel. On the other hand, the crystalline type is indigenous cometary particle formed through high-temperature heating episodes that have taken place prior to formation of comet Wild 2. One of the crystalline-type particles (C2054,0,35,6,0) consists of Mg-rich olivine, pyroxene, and kamacite and exhibits porphyritic or poikilitic texture very similar to chondrules.
    • 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.