An Antarctic iron meteorite contains preterrestrial impact-produced diamond and lonsdaleite

Abstract

Of the many meteorites recovered so far from the Allan Hills, Antarctica, only nine have been irons. One of these, ALHA77283, contains troilite(FeS)–graphite(C)–schreibersite((Fe,Ni)₃P)–cohenite(Fe₃C) inclusions rich in the carbonado-type diamond–lonsdaleite ‘nodules’ previously described from the Canyon Diablo meteorite^1,2. 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. The suggestion⁷ that formation was by high gravitational pressure has not been accepted. Virtually identical diamond–lonsdaleite-containing material in ALHA77283 occurs in a meteorite specimen with a well developed heat-altered zone produced by atmospheric ablation. 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. 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. Here we present the metallographic and X-ray diffraction data on which this conclusion is based.