Evidence for the extraterrestrial origin of a natural quasicrystal

Abstract

We present evidence that a rock sample found in the Koryak Mountains in Russia and containing icosahedrite, an icosahedral quasicrystalline phase with composition Al 63 Cu 24 Fe 13 , is part of a meteorite, likely formed in the early solar system about 4.5 Gya. The quasicrystal grains are intergrown with diopside, forsterite, stishovite, and additional metallic phases [khatyrkite (CuAl 2 ), cupalite (CuAl), and β-phase (AlCuFe)]. This assemblage, in turn, is enclosed in a white rind consisting of diopside, hedenbergite, spinel (MgAl 2 O 4 ), nepheline, and forsterite. Particularly notable is a grain of stishovite (from the interior), a tetragonal polymorph of silica that only occurs at ultrahigh pressures (≥10 Gpa), that contains an inclusion of quasicrystal. An extraterrestrial origin is inferred from secondary ion mass spectrometry 18 O∕ 16 O and 17 O∕ 16 O measurements of the pyroxene and olivine intergrown with the metal that show them to have isotopic compositions unlike any terrestrial minerals and instead overlap those of anhydrous phases in carbonaceous chondrite meteorites. The spinel from the white rind has an isotopic composition suggesting that it was part of a calciumaluminum-rich inclusion similar to those found in CV3 chondrites. The mechanism that produced this exotic assemblage is not yet understood. The assemblage (metallic copper-aluminum alloy) is extremely reduced, and the close association of aluminum (high temperature refractory lithophile) with copper (low temperature chalcophile) is unexpected. Nevertheless, our evidence indicates that quasicrystals can form naturally under astrophysical conditions and remain stable over cosmic timescales, giving unique insights on their existence in nature and stability. I cosahedrite, a naturally occurring quasicrystal, with composition Al 63 Cu 24 Fe 13 and exhibiting Bragg peak diffraction with icosahedral symmetry, was recently discovered in a rock sample found in the Museo di Storia Naturale of the Università degli Studi di Firenze (catalog number 46407/G), labeled as khatyrkite and identified as coming from the Khatyrka ultramafic zone in the Koryak Mountains in the Chukotka Autonomous Okrug of Far Eastern Russia (1, 2). Investigations over the past year have determined that the sample studied here, with size 0.27 × 0.30× 0.32 cm, is one of two rocks containing khatyrkite (CuAl 2 ) and cupalite (CuAl) to have been found in a claybed at the Listvenovyi stream in 1979 by V. V. Kryachko, the other being the holotype sample (3) deposited in the St. Petersburg Mining Institute. The holotype is an aggregate of metallic crystals, approximately 1 mm across, with no sign of surrounding host rock-forming minerals visible on the exterior. The interior has not been examined to date. By contrast, the sample studied here shows a remarkably complex set of assemblages (Fig. 1, Fig. 2), with a core containing quasicrystalline icosahedrite, crystalline metallic phases khatyrkite, cupalite, and β-phase (AlCuFe), diopside (Fig. S1), and forsterite (Fig. S2), as well ...

Impact-induced shock and the formation of natural quasicrystals in the early solar system

“…The correlation is based on O-isotope measurements of an olivine grain ( Fig. 1), and the presence of the same types of minerals and rare Cu-Al phases found in the other Khatyrka samples [5][6][7] . Also, the shocked portion of no.…”

“…1 in Bindi et al 5 ). The local, transient ultra-high pressures led to the nucleation and growth of ahrensite and spinelloid V. Icosahedrite could have grown from melted metals, and stishovite could have grown around it while the pressure was still high, thus explaining the stishovite grains with icosahedrite inclusions found in the original museum sample 5 .…”

“…It was discovered in the Khatyrka meteorite [5][6][7] , an oxidized subgroup (Allende-like) CV3 carbonaceous chondrite (CV3 CC) that formed at least 4.5 billion years ago. The preservation of a quasicrystal in a meteorite that formed at the early stages of the solar system indicates that it can be stable over cosmic time scales, giving new insights on the existence and stability of these materials in nature 5,6 . However, the processes that produced this unusual assemblage have been a mystery, as the highly reducing conditions implied by the presence of metallic Al are unknown for natural terrestrial materials and, before our studies, for meteorites.…”

In search of natural quasicrystals

“…Its presence in our sample points strongly to formation in an extreme environment, as might occur in a hypervelocity impact or in the deep mantle. The stishovite contains inclusions of quasicrystal (figure 10), an indication that the quasicrystal formed before or during an extremely high-pressure event (Bindi et al 2012). The image encapsulates everything that seems puzzling about natural icosahedrite-a quasicrystal grain with metallic aluminum surrounded by silicon dioxide with no reaction rim in between-combined with clear evidence that it formed Figure 11.…”

“…Despite extremely high Al/Mg ratios, no detectable radiogenic 26 Mg excesses were found in the quasicrystal. This implies either that the quasicrystal never contained live 26 Al, or that it formed more than 3 million years later than most CAIs, or that its Mg isotopic composition re-equilibrated with co-existing Mg-rich phases after the decay of 26 Al (Bindi et al 2012).…”