HRTEM study of Popigai impact diamond: heterogeneous diamond nanostructures in native amorphous carbon matrix

Diamond-Graphene Composite Nanostructures

“…This material has tetrahedrally bonded carbon atoms that are covalently linked to form six-membered rings in a “chair” conformation, and the atoms are stacked into layers according to a cubic ( c ) close-packing arrangement or the 3C stacking polytype . A metastable sp 3 -bonded carbon allotrope identified within natural impact diamonds and laboratory samples created under static HP and HT or shock conditions is “lonsdaleite”, a dense crystalline form displaying diffraction features consistent with hexagonal symmetry. − Observations of such hexagonal diffraction features provide an important mineralogical marker for the P and T conditions recorded in diamonds from bolide impact sites. ,, Lonsdaleite was interpreted as a 2H polytype with carbon layers stacked in a hexagonal fashion ( h ), although recent analyses suggest that natural and experimentally produced lonsdaleite samples are more accurately described as nanotwinning and stacking-disordered sequences of c and h units. − Density functional theory (DFT) calculations as well as experimental measurements suggest that the hardness and other mechanical properties of lonsdaleite may be superior to that of cubic diamond, thus motivating the search for identifying the sp 3 -bonded hexagonally stacked polytype among natural materials, or preparing it in the laboratory. , Computational studies suggest a wide range of novel carbon polymorphs that might be produced in the laboratory or discovered in natural samples. − In particular, several metastable carbon phases have been identified as being associated with the transition between sp 2 - and sp 3 -bonded structures. − …”

Formation of diamond and lonsdaleite in ureilites by impact shock processing of graphite

“…Based on texture, it is most likely that the parent graphite in region 1 (Figures 4b and 12a) of MS‐MU 045 and region 1 (Figures 4e and 9) of MS‐MU 027 was fragmented into many subdomains during a shock event, followed by transformation to cubic diamond/lonsdaleite (e.g., Figure 9c), similar to diamond in the Popigai impact structure. Diamonds formed in the Popigai impact structure is nanopolycrystalline (5–100 nm) and formed by direct conversion of graphite at high shock pressures and temperatures (Kis et al., 2016; Ohfuji et al., 2015, 2017). These diamond grains also show high concentrations of defects and Moiré fringes similar to the diamond grains analyzed in our study.…”

“…Kis et al. (2016) studied diamonds from Popigai and found that nanodiamond crystals are embedded in an amorphous carbon matrix. Also, they found (111) twinning within diamond crystals similar to the {111} stacking faults within the diamond grains studied here (Figure 14).…”

“…These diamond grains also show high concentrations of defects and Moiré fringes similar to the diamond grains analyzed in our study. Kis et al (2016) studied diamonds from Popigai and found that nanodiamond crystals are embedded in an amorphous carbon matrix. Also, they found (111) twinning within diamond crystals similar to the {111} stacking faults within the diamond grains studied here (Figure 14).…”

“…(2020) found large blade‐shaped graphite–diamond assemblages (up to 1 mm in length), in which diamonds are embedded along (0001) planes of graphite, suggesting that diamonds formed crystallographically controlled by a rapid shock process. Similarly, diamonds in the Popigai impact crater formed through shock‐induced transformation of graphite (Kis et al., 2016; Koeberl et al., 1997; Ohfuji et al., 2015, 2017; Shumilova et al., 2014) and are structurally and texturally similar to diamonds found in ureilites. Also CL studies show that impact diamonds from the Ries crater have similar CL properties as ureilitic diamonds but differ from unshocked terrestrial and synthetic diamonds (Grund & Bischoff, 1999).…”