Mineralogy and Petrology of Stardust Particles Encased in the Aerogel of Track

Introduction: Stardust returned to Earth samples from comet Wild 2. Until now most of the studies have been performed on terminal particles located at the end of the deceleration tracks. However, synchrotron X-ray fluorescence studies showed that a significant fraction of the cometary material is unevenly distributed along the tracks [1-2]. Here we present results of a transmission electron microscopy (TEM) investigation of a piece of aerogel extracted from a track wall. Experiments: The piece of aerogel was extracted from the track 80. The TEM samples have been prepared at the University of Washington by the acrylic embedding method [3]. The aerogel has been compressed between two glass slides, embedded in an acrylic resin, ultramicrotomed, and then washed with chloroform to dissolve acrylic. This method avoids the biased optical selection of particles and preserves the aerogel medium in which Wild 2 particles are encased. This configuration allows the study of small particles which are hardly visible under the optical microscope and thus hardly extractable separately. TEM results have been obtained using a Philips CM30 and a FEI G2-20 both equipped with Energy Dispersive X-ray Spectroscopy (EDX) (see [4] for a full description of the analytical procedure). Result and discussion: The walls of track 80 contain abundant Wild 2 material which consists of a mixture of amorphous and crystalline materials. In accordance with the penetration depth of the particles in the aerogel, the crystalline fraction is about 0.5 at the border of the wall to 0.85 deeper in the aerogel, in good agreement with [5]. Furthermore, crystalline grain found deeper in the aerogel are bigger (typically 2µm) than those at the border (< 1µm). The amorphous components consist of a silica-rich glassy matrix embedding a large number of small Fe-Ni-S inclusions and vesicles. This is a clear evidence of thermal-induced modification due to the heating that occurred during the particle deceleration stage [6]. The average composition of these components is close to CI. Concerning the crystalline particles, various phases are detected: pigeonite and enstatite showing some microstruc-tural defects comparable to those observed in terminal particles [7], olivine with a large composition range (Fo 40-Fo 95), cris-tobalite, magnetite and apatite. The two last suggest a hydrous alteration signature. Our study suggests an initial aggregate of crystalline grains having different origins sticked together by a fine-grained material with an average composition close to CI. The latter strongly suffered thermal alteration and is found …