Previously unknown quasicrystal periodic approximant found in space

Luca Bindi,Joyce Pham,Paul J Steinhardt

doi:10.1038/s41598-018-34375-x

Abstract

We report the discovery of Al34Ni9Fe2, the first natural known periodic crystalline approximant to decagonite (Al71Ni24Fe5), a natural quasicrystal composed of a periodic stack of planes with quasiperiodic atomic order and ten-fold symmetry. The new mineral has been approved by the International Mineralogical Association (IMA 2018-038) and officially named proxidecagonite, which derives from its identity to periodic approximant of decagonite. Both decagonite and proxidecagonite were found in fragments from the Khatyrka meteorite. Proxidecagonite is the first natural quasicrystal approximant to be found in the Al-Ni-Fe system. Within this system, the decagonal quasicrystal phase has been reported to transform at ~940 °C to Al13(Fe,Ni)4, Al3(Fe,Ni)2 and the liquid phase, and between 800 and 850 °C to Al13(Fe,Ni)4, Al3(Fe,Ni) and Al3(Fe,Ni)2. The fact that proxidecagonite has not been observed in the laboratory before and formed in a meteorite exposed to high pressures and temperatures during impact-induced shocks suggests that it might be a thermodynamically stable compound at high pressure. The most prominent structural motifs are pseudo-pentagonal symmetry subunits, such as pentagonal bipyramids, that share edges and corners with trigonal bipyramids and which maximize shortest Ni–Al over Ni–Ni contacts.

Highlights

The first decagonal quasicrystalline (QC) phase found in nature, decagonite Al71Ni24Fe51,2, was discovered in the Khatyrka meteorite, a CV3 carbonaceous chondrite[3,4,5,6,7,8]
In the search through Khatyrka meteoritic fragments recovered from a 2011 expedition to the Koryak Mountains in far eastern Russia[3,4,5,6], various other novel intermetallic phases have been uncovered[13,14,15,16], 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]
Periodic approximant is an accepted technical term that refers to a crystalline solid with similar chemical composition to a QC, but whose atomic arrangement is slightly distorted so that the symmetry conforms to the conventional laws of three-dimensional crystallography

Summary

Introduction

The first decagonal quasicrystalline (QC) phase found in nature, decagonite Al71Ni24Fe51,2, was discovered in the Khatyrka meteorite, a CV3 carbonaceous chondrite[3,4,5,6,7,8]. The finding followed the discovery of icosahedrite Al63Cu24Fe139,10, the first quasicrystal discovered in nature, which displays three-dimensional icosahedral symmetry and is quasiperiodic in all directions. The chemical compositions of both decagonite and icosahedrite match those of the QC phases previously synthesized in a laboratory setting at standard pressure[11,12]. Lin et al.[7] have shown that some of the metallic Al-alloys in Khatyrka pre-dated the shock

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Conclusion