title: Natural occurrence of pure nano-polycrystalline diamond from impact crater.
abstract: Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds. Here we identified natural pure NPD, which was produced by a large meteoritic impact about 35 Ma ago in Russia. The impact diamonds consist of well-sintered equigranular nanocrystals (5-50 nm), similar to synthetic NPD, but with distinct [111] preferred orientation. They formed through the martensitic transformation from single-crystal graphite. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD. A huge amount of natural NPD is expected to be present in the Popigai crater, which is potentially important for applications as novel ultra-hard material.
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Ohfuji
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.|Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan.
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Irifune
authors.affiliation: V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, RAS, Novosibirsk, 630090, Russia.|Novosibirsk State University, Novosibirsk, 630090, Russia.
authors.forename: Konstantin D
authors.initials: KD
authors.lastname: Litasov
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Tomoharu
authors.initials: T
authors.lastname: Yamashita
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Futoshi
authors.initials: F
authors.lastname: Isobe
authors.affiliation: V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, RAS, Novosibirsk, 630090, Russia.
authors.forename: Valentin P
authors.initials: VP
authors.lastname: Afanasiev
authors.affiliation: V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, RAS, Novosibirsk, 630090, Russia.
authors.forename: Nikolai P
authors.initials: NP
authors.lastname: Pokhilenko