First synthesis of a unique icosahedral phase from the Khatyrka meteorite by shock-recovery experiment

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-phase II + 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.

Shock-Induced Chemical Reaction between Iron and Enstatite

By using a two-stage light gas gun, two experiments of shock recovery experiments with initial sample of Fe+(Mg, Fe)SiO3 (En) were conducted between 78 and 113 GPa shock pressure (the corresponding temperature is estimated as 3000~5000K). The recovered samples were analyzed by X-ray Diffraction (XRD).The XRD observation at the middle section of two recovered samples illustrates the new composition of recovered samples is (Mg, Fe)2SiO4.Comparing with the recovery experiments of MgO+SiO2 ,we can infer that iron and perovskite react to form SiO2 and (Mg, Fe)O. The experiments result indicates that reaction between liquid iron and (Mg, Fe)SiO3 perovskite may occur at the core-mantle boundary in geological history. The reaction creates a very heterogeneous zone at the base of the mantle. Si and O dissolved in liquid iron are rapidly dispersed by the flow of the liquid outer core.

Shock Wave Chemical Reactions; Synthesis of Carbon Nitrides

A series of shock recovery experiments up to ~50 GPa were performed on reactions to form carbon nitrides. Nitrogen-rich starting materials, included a C-N-O amorphous precursor, dicyandiamide, melamine, and a mixture of carbon tetrahalide and sodium dicyanoamide, were used and the recovered samples were investigated by X-ray diffraction technique, elemental analysis, transmission electron microscopy and so on. Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock pressure of 37 GPa, and production of amorphous C-N materials with a highest N/C ration of 1.26 from the reaction between carbon tetrahalide and sodium dicyanoamide. We extended to the system C3N4-Si3N4 based on the recent results on synthesis of spinel-type nitrides. Shock wave chemical reactions provide a route for synthesizing novel materials including not only high-pressure phases but also metastable, unique substances.