Popigai impact structure

The Popigai impact structure is the eroded remnant of an impact crater in northern Siberia, Russia. It is tied with the Manicouagan structure as the fourth largest verified impact structure on Earth. A large bolide impact created the 100 km diameter crater approximately 35 million years ago during the late Eocene epoch (Priabonian stage). It might be linked to the Eocene–Oligocene extinction event.

The structure is 300 km east from the outpost of Khatanga and 880 km northeast of the city of Norilsk, NNE of the Anabar Plateau. It is designated by UNESCO as a Geopark, a site of special geological heritage. There is a small possibility that the Popigai impact crater may have formed simultaneously with the approximately 35-million-year-old Chesapeake Bay and Toms Canyon impact craters. For decades, the Popigai impact structure has fascinated paleontologists and geologists, but the entire area was completely off limits because of the diamonds found there. However, a major investigatory expedition was undertaken in 1997, which greatly advanced understanding of the structure. The impactor is suggested to have been a H chondrite asteroid based on ejecta layers from Italy, with the impactor thought to have been several kilometeres in diameter.

The shock pressures from the impact instantaneously transformed graphite in the ground into diamonds within a 13.6 km radius of the impact point. These diamonds are usually 0.5 to 2 mm in diameter, though a few exceptional specimens are 10 mm in size. The diamonds inherited the tabular shape of the original graphite grains and also the original crystals' delicate striations.

Diamond deposits
Most modern industrial diamonds are produced synthetically. The diamond deposits at Popigai have not been mined because of the remote location and lack of infrastructure, and are unlikely to be competitive with synthetic diamonds. Many of the diamonds at Popigai contain crystalline lonsdaleite, an allotrope of carbon that has a hexagonal lattice. Pure, laboratory-created lonsdaleite is up to 58% harder than ordinary diamonds. These types of diamonds are known as "impact diamonds" because they are thought to be produced when a meteorite strikes a graphite deposit at high velocity. They may have industrial uses but are unsuitable as gems.

Additionally, carbon polymorphs, a combination of diamond and lonsdaleite even harder than pure lonsdaleite, have been discovered in the crater.