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.
AFANASIEV, VALENTIN; GROMILOV, SERGEY; SONIN, VALERI; ZHIMULEV, EGOR; and CHEPUROV, ALEKSEI
"Graphite in rocks of the Popigai impact crater: residual or retrograde?,"
Turkish Journal of Earth Sciences: Vol. 28:
3, Article 8.
Available at: https://journals.tubitak.gov.tr/earth/vol28/iss3/8