Abstract
The concentration of diamond-bearing tagamite from the Popigai impact crater produces large amounts of graphite in addition to impact diamonds (1:100, respectively). The question arises of whether this is residual graphite not converted to diamond at the time of the Popigai impact or is a retrograde form resulting from back-conversion of impact diamond to graphite in a high-temperature tagamite melt. Experiments show that graphite from tagamite is a residual phase. Coexistence of lonsdaleite, cubic diamond, and single-crystal graphite within a limited volume may be due to different orientations of the graphite base plane relative to the impact stress direction. Thus, the diamond-bearing rocks may contain significant amounts of residual graphite, which is consistent with published evidence.
Highlights
The Popigai impact crater is located in the northern Siberian craton at the boundary between the Krasnoyarsk region and Yakutia (Figure 1)
The exceptional properties of impact diamonds are due to their microstructure, which is an aggregate of nanometer cubic diamond and hexagonal lonsdaleite phases
The second main feature of the impact diamonds of the Popigai crater is that they represent an aggregate of nanoscale grains of the cubic diamond and hexagonal lonsdaleite phases (Figures 3 and 4)
Summary
The Popigai impact crater is located in the northern Siberian craton at the boundary between the Krasnoyarsk region and Yakutia (Figure 1). Masaitis (Masaitis, 1998), a prominent Russian geologist. His later discovery of abundant diamond inclusions in the crater rocks (impactites) attracted great interest to the Popigai crater. Studies at the Popigai site in the course of 15 consecutive years provided constraints on the crater structure and the features of the discovered diamonds, and a special technology was developed for treatment of the diamondbearing rocks and extraction of diamonds. Studies of the Popigai crater and impact diamonds stopped in the mid-1980s, while the obtained results were classified as confidential. This status has recently been changed and the research may continue to gain insights into many issues that remain unclear
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