Template:Short description {{#invoke:other uses|otheruses}} Template:Infobox terrestrial impact site
The Popigai impact structure is the eroded remnant of an impact crater in northern Siberia, Russia. It is tied with the Acraman impact structure as the fourth largest verified impact structure on Earth.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A large bolide impact created the Template:Convert crater approximately 35 million years ago during the late Eocene epoch (Priabonian stage).<ref name="Alexander">Template:Cite journal</ref><ref name=Armstrong>Template:Cite book</ref> It might be linked to the Eocene–Oligocene extinction event.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The structure is Template:Convert east from the outpost of Khatanga and Template:Convert 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.<ref name="Alexander2">Template:Cite journal</ref> 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.<ref name="Alexander"/>
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.<ref name="Alexander2"/> 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 kilometres in diameter.<ref>Template:Cite journal</ref>
The shock pressures from the impact instantaneously transformed graphite in the ground into diamonds within a Template:Convert radius of the impact point. These diamonds are usually Template:Convert in diameter, though a few exceptional specimens are Template:Convert in size. The diamonds inherited the tabular shape of the original graphite grains and also the original crystals' delicate striations.<ref name="Alexander2"/>
Diamond depositsEdit
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.<ref name=geonews>Template:Cite news</ref> Many of the diamonds at Popigai contain crystalline lonsdaleite, an allotrope of carbon that has a hexagonal lattice.<ref name=itartass>Template:Cite news</ref> Pure, laboratory-created lonsdaleite is up to 58% harder than ordinary diamonds.<ref name=theory>Template:Cite journal
- Template:Cite news</ref><ref name=geonews /> 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.<ref name=itartass/> They may have industrial uses but are unsuitable as gems.<ref>Pros and cons of extraterrestrial diamonds Template:Webarchive, from "Rough&Polished–information and analytics on diamond and jewellery markets."</ref>
Additionally, carbon polymorphs, a combination of diamond and lonsdaleite even harder than pure lonsdaleite, have been discovered in the crater.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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Template:Impact cratering on Earth Template:Authority control