Impactites from Popigai Crater

Impactites (tagamites and suevites) from Popigai impact crater, whose diameter is about 100 km, are distributed over an area of 5000 sq km. The continuous sheet of suevite overlies the allogenic polymict breccia and partly authogenic breccia, and may also be observed in lenses or irregular bodies. The thickness of suevites in the central part of the crater is more than 100 m. Suevites may be distinguished by content of vitroclasts, lithoclasts, and crystalloclasts, by their dimensions, and by type of cementation, which reflects the facial settings of ejection of crushed and molten material, its sedimentation and lithification. Tagamites (impact melt rocks) are distributed on the surface predominantly in the western sector of the crater. The most characteristic are thick sheetlike bodies overlying the allogenic breccia and occurring in suevites where minor irregular bodies are widespread. The maximal thickness of separate tagamite sheets is up to 600 m. Tagamites, whose matrix is crystallized to a different degree, include fragments of minerals and gneiss blocks, among them shocked and thermally metamorphosed ones. Tagamite sheets have a complex inner structure; separate horizontal zones distinguish in crystallinity and fragment saturation. Differentiation in the impact melt in situ was not observed. The average chemical compositions of tagamites and suevites are similar, and correspond to the composition of biotite-garnet gneisses of the basement. According to the content of supplied Ir, Ni, and other siderophiles, impact melt was contaminated by 5 percent cosmic matter of collided body, probably ordinary chondrite. The total volume of remaining products of chilled impact melt is about 1750 cu km. Half this amount is represented by tagamite bodies. Though impact melt was in general well homogenized, the trend analysis showed that the concentric zonation is distribution of SiO2, MgO, and Na2O and the bandlike distribution of FeO and Al2O3 content testifies to a certain inheritance and heterogeneity in country rock composition laterally and vertically in the melting zone

Rochechouart 2017-Drilling Campaign: First Results

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CIRIR programs: drilling and research opportunities at the Rochechouart Impact Structure

International audienc

Book Review: Meteorite Hunter: The Search for Siberian Meteorite Craters, Roy A. Gallant

Book Review: Meteorite Hunter: The Search for Siberian Meteorite Craters, Roy A. Gallant. McGraw-Hill, New York, New York, USA (2001).The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

HRTEM study of Popigai impact diamond: nanodiamond in amorphous carbon matrix

Impact diamonds are formed by a rapid solid-state transformation of mainly sp2 bonded carbon (graphite or coal) under very high pressure (up to 50 GPa) on the nanosecond timescale accompanied by high temperature (up to 4000K). They are usually polyphase and can consist of cubic and hexagonal diamond, graphite, onion-like carbon and amorphous carbon [1] and exhibit unusual physical properties like extreme hardness. However, only a few (HR)TEM studies has been made on impact diamonds so far, while laboratory synthesis of ultrahard carbon allotropes and their nanostructural investigation is a current topic [2,3]. We applied FIB cutting followed by low energy Ar milling and mechanical cleavage to obtain electron transparent lamellae for HRTEM imaging. Amorphous rim of about 20 nm of width was formed around the FIB lamella, which has been reduced significantly to 5-6 nm of width by low energy Ar ion milling at 500eV (Figure 1) using Technoorg Gentle Mill. The structure of the amorphous rim has been changing rapidly during high resolution observation, while, the amorphous structure of the cleaved sample remained practically unchanged during several minutes of electron irradiation (Figure 2). EELS spectra also prove the different character of the amorphous component of the FIB-cut and cleaved samples. HRTEM study of the cleaved sample allowed imaging of nanodiamond particles embedded in the native amorphous carbon matrix of the impact grain (Figure 2). The size of the nanodiamonds ranges between 5-10 and a few hundreds of nanometres. The smaller particles are frequently nanotwinned (twin thickness is 1-2 nm), while the larger ones are either faultless or are characterized by larger twin thickness (5-10 nm). These nanostructural features are reminiscent to those published by Huang et al. [3] who fabricated ultrahard (Vickers hardness 200 GPa) nanotwinned diamond from onion-like carbon precursos and can be related to the hardness of the material. The investigation of the bonding structure of the amorphous matrix can provide a better understanding for the formation and physical properties of impact diamonds