Tem Investigation of a “mysterite” Inclusion from the Krymka Ll-chondrite:

Introduction: In only two meteorites, Krymka and Supuhee, a very rare, fine-grained mysterious material has been discovered so far. Laul et al. [1] detected in these two ordinary chondrites an enrichment in Ag, Tl, and Bi. They concluded that this enrichment has its origin in a late condensate introduced during a brecciation event. Since a petrographic carrier of the volatiles could not be specified, they named this fine-grained areas " mysterite ". Higuchi et al. [2] studied by radiochemical neutron activation four mysterite fragments from the H6 chondrite Supuhee, whereas Grossman et al. [3] investigated with electron micro-probe a mysterite-bearing inclusion from the Krymka LL3.1 chondrite. Although they used very different analytical techniques, both groups concluded that mys-terite belongs to an unknown kind of carbonaceous material condensed in a special nebular region. Another carbonaceous clast (K1) from Krymka with similar mineralogy was studied by Semenenko et al. [4, 5]. The presence of organic compounds, micro-crystals of graphite and an anomalous enrichment in Ag are distinctive features of this clast. It was supposed that the graphite was formed from the organic compounds as a result of metamorphism [5]. Due to this features mysterite was interpreted as the most likely material with a cometary origin [6]. In our study we carried out an additional detailed investigation of the Krymka carbonaceous clast K1 by using reflected light microscopy, scanning electron microscopy, secondary ion mass spectrometry, and transmission electron microscopy (TEM). The aims of the study were to investigate structural features and nature of graphite and the other minerals. Here we present preliminary results of TEM analyses, which we made on a thin section prepared by focussed ion beam (FIB) technique. Method: A ~1.5 ×1.5 mm² and 200 µm thick polished section of mysterite was prepared for the investigations. To get a 100 nm thin slice for TEM analyses the very new FIB technique was used. In this method a TEM specimen can be cut out of a thick section by an ion beam generated from a gallium liquid metal ion source. The ions irradiate a predetermined sample position ~15 µm in length and ~10 µm in depth from both sides until a ~100 nm thin section is generated. For TEM investigations, which were performed with a JEOL JEM 3010 instrument (300 kV), the thin section will be positioned on a carbon coated copper grid.