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- W64876612 abstract "Introduction: Samples of three pyroxene-rich particles collected from the Wild2 comet during the Stardust mission have been investigated by transmission electron microscopy (TEM). These pyroxene grains correspond to three terminal particles from tracks 32 and 69. They are coarse-grained Ca-poor pyroxenes with compositions and structures ranging from orthorhombic enstatite to monoclinic pigeonite. Samples and experimental procedures: The TEM utramicrotomed samples, namely C2027,2,69,1,1, C2027,2,69,2,2 and C2027,3,32,2,3 originate from terminal particles of neighboring tracks. Details about extraction, manipulation and preparation for TEM by ultramicrotomy can by found in [1]. Results were acquired using LaB6 filaments Philips CM30 (300 keV) and FEI Tecnai G2-20 twin (200 kV) microscopes, equipped with Thermo-Noran and EDAX Si-detectors respectively for Energy Dispersive X-ray Spectroscopy (EDX) (see [2] for a full description of the analytical procedure). Results: C2027,2,69,2,2 and C2027,3,32,2,3. The ultramicrotomed samples consist of elongated crystalline shards. A thin and discontinuous rim of dense amorphous SiO2-rich material surrounds the particle slices. The shard-like aspect is likely due to the ultramicrotomy preparation. Despite this aspect, the crystallites present very close orientation, as deduced from diffraction patterns. They probably originate from an initial single grain prior to the sample preparation. Both samples exhibit very similar and homogeneous compositions and microstructures. The compositions correspond to enstatite within the range En94-97Wo25Fs2-5. Selected area electron diffraction patterns reveal an orthorhombic Pbca space group. In most of the shards, some planar faults parallel to (100) are observed. Lattice fringe images (figure 1) reveal that the planar faults consist in the insertion of one or more clinoenstatite lamellae (fringe spacing ~ 9 A) in the orthoenstatite matrix (fringe spacing ~ 18 A). Strain contrast is observed at the termination of the faults, suggesting the presence of dislocations at the end of the clinoenstatite lamellae. Free dislocations lying in the (100) planes are also present. They appear as long segments elongated in the [100] direction or small segments normal to it. Diffraction contrast examinations including some extinction conditions suggest a Burgers vector compatible with [010]. Figure 1: TEM bright-field image of enstatite in C2027,2,69,2,2. The clinoenstatite lamellae parallel to (100) are clearly visible by the 9 A lattice spacing, whereas the 18 A lattice spacing corresponds to orthoenstatite." @default.
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- W64876612 date "2008-03-01" @default.
- W64876612 modified "2023-09-23" @default.
- W64876612 title "TEM Investigation of Pyroxenes Microstructure in Comet 81P/Wild 2 Terminal Particles" @default.
- W64876612 hasPublicationYear "2008" @default.
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