![]() ![]() Furthermore, an area centred around 18 GPa and 1,400 K in the pressure–temperature diagram for carbon was attributed to a phase called ‘retrievable hexagonal-type diamond’ 10, which corresponds to the conditions where lonsdaleite has been reported 3, 11, 15. Observations and theoretical studies suggested a structural relationship among graphite, cubic diamond and lonsdaleite and an important role of the latter during the graphite-to-diamond transition 3, 9, 10, 11, 12, 13, 14. Lonsdaleite was proposed to have a wurtzite (ZnS)-type structure with space group P6 3/mmc ( a=0.251 and c=0.412 nm) and with all structural positions occupied by carbon 1, 2, 3, 8. It has since been reported from several meteorites as well as from terrestrial sediments and has been attributed to asteroidal impacts, both extraterrestrial and on Earth 4, 5, 6, 7. Its formation was attributed to shock-induced transformation of graphite within the meteorite upon impact with Earth, and its occurrence was used as an indicator of shock 1, 2, 3. Lonsdaleite was first described almost 50 years ago from the Canyon Diablo iron meteorite 1, 2. Diamond is reported to have a number of polytypes, of which lonsdaleite (also called hexagonal diamond) has received particularly intense attention. Within this diversity are materials with extraordinary properties, paramount being cubic diamond, which has the highest known hardness and thermal conductivity. Nature 267, 414–415 (1977).The allotropes of carbon display a wide diversity of structures that include the three-dimensional (3D) diamond and graphite, two-dimensional (2D) graphene and curved nanotubes and fullerenes. Handbook of Iron Meteorites (University of California Press, 1975).Īxon, H. Arizona's Meteorite Crater (American Meteorite Museum, Arizona, 1956). Here we present the metallographic and X-ray diffraction data on which this conclusion is based. ![]() The shock event that produced these high pressure phases, therefore, must have taken place on its parent body or have been associated with the disruption of that body. It seems, therefore, that the diamond and lonsdaleite were present in the meteoroid before its final ablative passage through the atmosphere and soft landing on the ground. Virtually identical diamond–lonsdaleite-containing material in ALHA77283 occurs in a meteorite specimen with a well developed heat-altered zone produced by atmospheric ablation. The suggestion 7 that formation was by high gravitational pressure has not been accepted. The Canyon Diablo, Arizona, meteorite, the excavator of Meteor Crater, is the only other iron meteorite known to contain these high-pressure minerals, and their occurrence in that meteorite has been explained as the result of shock-induced transformation of graphite, most probably at the moment of terrestrial impact and disintegration of the projectile during crater formation 3–6. One of these, ALHA77283, contains troilite(FeS)–graphite(C)–schreibersite((Fe,Ni) 3P)–cohenite(Fe 3C) inclusions rich in the carbonado-type diamond–lonsdaleite ‘nodules’ previously described from the Canyon Diablo meteorite 1,2. Of the many meteorites recovered so far from the Allan Hills, Antarctica, only nine have been irons.
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