Collisions in outer space produced an icosahedral phase in the Khatyrka meteorite never observed previously in the laboratory

Abstract

We report the first occurrence of an icosahedral quasicrystal with composition Al62.0(8)Cu31.2(8)Fe6.8(4), outside the measured equilibrium stability field at standard pressure of the previously reported Al-Cu-Fe quasicrystal (AlxCuyFez, with x between 61 and 64, y between 24 and 26, z between 12 and 13%). The new icosahedral mineral formed naturally and was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite that experienced shock metamorphism, local melting (with conditions exceeding 5 GPa and 1,200 °C in some locations), and rapid cooling, all of which likely resulted from impact-induced shock in space. This is the first example of a quasicrystal composition discovered in nature prior to being synthesized in the laboratory. The new composition was found in a grain that has a separate metal assemblage containing icosahedrite (Al63Cu24Fe13), currently the only other known naturally occurring mineral with icosahedral symmetry (though the latter composition had already been observed in the laboratory prior to its discovery in nature). The chemistry of both the icosahedral phases was characterized by electron microprobe, and the rotational symmetry was confirmed by means of electron backscatter diffraction.

Evidence of cross-cutting and redox reaction in Khatyrka meteorite reveals metallic-Al minerals formed in outer space

“…It is deposited at the Smithsonian Institution’s National Museum of Natural History, Washington DC, USA under the catalogue number USNM 7908. We also present evidence of a time sequence between the Al-Cu-Fe metal phases and silicate matrix, and document the petrological context of four new Al-Cu-Fe minerals, discovered in Grain 126A and recently reported in refs 14, 16 and 17. We note that the interpretation here of a reaction history is supported by preliminary studies on Grain 129, some results of which were reported in ref.…”

“…Among the metal grains are four new Al-Cu-Fe minerals, which were recently reported in refs 14, 16 and 17. They include stolperite (AlCu) 16, 17 , kryachkoite ((Al,Cu) 6 (Fe,Cu)) 16, 17 , hollisterite (Al 3 Fe) 16, 17 , and an as-yet-unnamed quasicrystal 14 (denoted ‘ i -II’ ) .…”

“…They include stolperite (AlCu) 16, 17 , kryachkoite ((Al,Cu) 6 (Fe,Cu)) 16, 17 , hollisterite (Al 3 Fe) 16, 17 , and an as-yet-unnamed quasicrystal 14 (denoted ‘ i -II’ ) . The quasicrystal i -II has the same icosahedral symmetry as icosahedrite 1, 10 (denoted ‘ i -I’) but a composition Al 62.0(8) Cu 31.2(8) Fe 6.8(4) , which is outside the measured equilibrium stability field at standard pressure of icosahedrite (Al x Cu y Fe z , with x between 61 and 64, y between 24 and 26, z between 12 and 13%) 29–31 .…”

Previously unknown quasicrystal periodic approximant found in space

“…1), as typically observed for other fragments of the Khatyrka meteorite 4,7,13,14 . Detailed examination by scanning electron microscopy, single-crystal X-ray diffraction, micro-computed tomography and transmission electron microscopy of fragments from Grain 126 associated to proxidecagonite revealed the presence of trevorite, diopside, forsterite, ahrensite, clinoenstatite, nepheline, coesite, stishovite, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, taenite, Al-bearing taenite, steinhardtite, decagonite, hollisterite, stolperite and kryachkoite 4,5,7,13,15–17 . The recovery of different Al-Ni-Fe crystalline (steinhardtite) and QC (decagonite) intermetallic phases, motivated a careful search for other metallic fragments, which led to the discovery of a particle with composition close to that of the known Al-Ni-Fe decagonal QC but with different diffraction characteristics.

Figure 1Micro-computed tomographic images (at different orientations) of the entire grain (labeled number 126).

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“…In the search through Khatyrka meteoritic fragments recovered from a 2011 expedition to the Koryak Mountains in far eastern Russia 3–6 , various other novel intermetallic phases have been uncovered 13–16 , including a second Al-Cu-Fe icosahedral QC phase that differs from icosahedrite and that is the first QC to be discovered in nature before being synthesized in the laboratory 17 . However to date, no natural periodic approximants to quasicrystals have been reported.…”