Abstract
Icosahedral quasicrystals (i-phases) in the Al-Cu-Fe system are of great interest because of their perfect quasicrystalline structure and natural occurrences in the Khatyrka meteorite. The natural quasicrystal of composition Al62Cu31Fe7, referred to as i-phase II, is unique because it deviates significantly from the stability field of i-phase and has not been synthesized in a laboratory setting to date. Synthetic i-phases formed in shock-recovery experiments present a novel strategy for exploring the stability of new quasicrystal compositions and prove the impact origin of natural quasicrystals. In this study, an Al-Cu-W graded density impactor (GDI, originally manufactured as a ramp-generating impactor but here used as a target) disk was shocked to sample a full range of Al/Cu starting ratios in an Fe-bearing 304 stainless-steel target chamber. In a strongly deformed region of the recovered sample, reactions between the GDI and the steel produced an assemblage of co-existing Al61.5Cu30.3Fe6.8Cr1.4 i-phaseII + stolperite (β, AlCu) + khatyrkite (θ, Al2Cu), an exact match to the natural i-phase II assemblage in the meteorite. In a second experiment, the continuous interface between the GDI and steel formed another more Fe-rich quinary i-phase (Al68.6Fe14.5Cu11.2Cr4Ni1.8), together with stolperite and hollisterite (λ, Al13Fe4), which is the expected assemblage at phase equilibrium. This study is the first laboratory reproduction of i-phase II with its natural assemblage. It suggests that the field of thermodynamically stable icosahedrite (Al63Cu24Fe13) could separate into two disconnected fields under shock pressure above 20 GPa, leading to the co-existence of Fe-rich and Fe-poor i-phases like the case in Khatyrka. In light of this, shock-recovery experiments do indeed offer an efficient method of constraining the impact conditions recorded by quasicrystal-bearing meteorite, and exploring formation conditions and mechanisms leading to quasicrystals.
Highlights
Quasicrystals (QCs) are a unique type of solid characterized by quasiperiodic translational order (Lifshitz, 2003)
We report two new shock-recovery experiments that used Al–Cu–W graded density impactors (GDIs), initially designed for quasi-isentropic impact loading, as starting materials in the targets
The Al–Cu–W GDI disk that we used has graded composition from aluminium on top through the full range of Al–Cu alloys to copper in the middle, through the range of Cu–W alloys to tungsten at the bottom (Fig. 1)
Summary
Quasicrystals (QCs) are a unique type of solid characterized by quasiperiodic translational order (Lifshitz, 2003). The second is decagonite (d-phase, Al71Ni24Fe5), named after its decagonal symmetry (Bindi et al, 2015a,b) Both icosahedrite and decagonite are known to be thermodynamically stable at subsolidus temperatures and were produced by Al-alloy quenching experiments before their discovery in nature (Tsai et al, 1987; Lemmerz et al, 1994). The third phase is another icosahedral QC but with composition Al62Cu31Fe7 (Bindi et al, 2016), known as i-phase II This composition is outside the stability field of icosahedral QCs in the Al–Cu–Fe system and has not previously been produced in any experimental study.
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