Shock synthesis of quasicrystals with implications for their origin in asteroid collisions
Paul D. Asimow, Chaney Lin, Luca Bindi, Chi Ma, Oliver Tschauner, Lincoln S. Hollister, and Paul J. Steinhardt
PNAS2016 ; published ahead of print June 13, 2016, doi:10.1073/pnas.1600321113
The singular occurrence, to date, of natural quasicrystals requires an explanation both of the possibility and of the rarity of their formation outside of the laboratory. Successful synthesis by an experimental shock, with starting materials similar to the exotic intermetallic alloys in the Khatyrka meteorite, demonstrates a mechanism that is feasible in space but not in any natural setting on Earth. The previously unrecognized composition of the synthesized quasicrystal, the first, to our knowledge, to be created by shocking discrete bulk starting materials, demonstrates a method for discovery of previously unknown quasicrystal compositions.
We designed a plate impact shock recovery experiment to simulate the starting materials and shock conditions associated with the only known natural quasicrystals, in the Khatyrka meteorite. At the boundaries among CuAl5, (Mg0.75Fe2+0.25)2SiO4 olivine, and the stainless steel chamber walls, the recovered specimen contains numerous micron-scale grains of a quasicrystalline phase displaying face-centered icosahedral symmetry and low phason strain. The compositional range of the icosahedral phase is Al68–73Fe11–16Cu10–12Cr1–4Ni1–2 and extends toward higher Al/(Cu+Fe) and Fe/Cu ratios than those reported for natural icosahedrite or for any previously known synthetic quasicrystal in the Al-Cu-Fe system. The shock-induced synthesis demonstrated in this experiment reinforces the evidence that natural quasicrystals formed during a shock event but leaves open the question of whether this synthesis pathway is attributable to the expanded thermodynamic stability range of the quasicrystalline phase at high pressure, to a favorable kinetic pathway that exists under shock conditions, or to both thermodynamic and kinetic factors.