NATURAL QUASICRYSTALS: APPLICATION OF PETROLOGY, MINERALOGY, AND STRATIGRAPHY TOWARD UNRAVELING THE MYSTERY OF FORMATION OF METALLIC AL COMPOUNDS IN OUTER SPACE

The search for the conditions of formation of the only known natural quasicrystals involves multiple disciplines that include physics, laws of superposition (geology), mineralogy, crystallography, and petrology. The assemblage of metallic Al-bearing minerals, some of which are quasiperiodic (hence called quasicrystals) and have crystallographically forbidden rotational symmetries, require an exceptional combination of circumstances for their formation. The Al-Cu metals occur in a carbonaceous chondrite meteorite, called Khatyrka, that fell in far eastern Russia more than 8000 years ago. The presence of quasicrystals, which are exotic states of matter, may offer clues for conditions and physical processes during the formation of the solar system. The meteorite containing the quasicrystals may be the oldest accreted terrane!

Pieces of Khatyrka were collected by our team in 2011, at the same location where a Russian group discovered two metallic Al and Cu minerals in 1979. We have since identified and described six more new metallic Al-bearing minerals from the meteorite. Khatyrka shows evidence of a heterogeneous distribution of temperatures and pressures that were generated by shock. We propose that a projectile made of an assemblage of Al-Cu metal alloys, possibly also including several FeO-free silicates (forsterite, diopside), impacted a porous, primitive carbonaceous chondrite. The impact produced the high-pressure phases ahrensite (Fe-ringwoodite) and a new, unnamed Fe-bearing spinelloid. In places, temperatures exceeded 1200°C, with localized melting. The meteorite cooled exceedingly rapidly following the impact. The combination of shock and quench melted the metal and created new metal phases in some places; in others, it preserved metal phases that formed prior to the collision. Shock was clearly involved in the complex history of formation of the metals of Khatyrka, but we cannot yet say that shock was necessary for the formation of the quasicrystals. We are still trying to understand the original source of metallic Al and Cu and the processes that caused them to alloy in the first place.