EVIDENCE ABOUT THE FORMATION HISTORY OF NATURAL QUASICRYSTALS IN OUTER SPACE, AND THE OCCURRENCE OF STOLPERITE

A fragment of the quasicrystal-bearing CV3 carbonaceous chondrite Khatyrka shows unambiguous evidence of a reduction-oxidation reaction between Al-Cu-Fe alloys and silicate melt. This reaction occurred in outer space, before the meteorite fell to earth more than 6000 years ago. The redox reactions involve reduction of FeO and SiO₂ to Fe and Fe-Si metal, and oxidation of metallic Al to Al₂O₃. In the reaction zone, there are metallic Fe and Fe-Si beads, aluminous spinel rinds on the Al-Cu-Fe alloys, and enrichment of Al₂O₃ in the silicate melt surrounding the alloys. The silicate glass crosscuts alloys with lower melting-point temperatures, and the redox reactions occurred at the contacts between this glass and the metal alloys. The evidence of this redox reaction is an important addition to the abundant evidence that the Al-Cu-Fe alloys in the Khatyrka meteorite formed in outer space; these alloys include the three distinct quasicrystalline phases and four new crystalline minerals, which include stolperite (AlCu). In particular, it enables a detailed understanding of the formation history of the AL-Cu-Fe alloys. We conclude that Khatyrka must have experienced at least two distinct events: first, an event as early as 4.564 Ga in which the first Al-Cu-Fe alloys formed; and, second, a more recent impact-induced shock in space that led to transformations of and reactions between the alloys and the meteorite matrix.