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We summarize the results of studies on quasicrystals (QCs) at extreme conditions over the last 4 decades with particular emphasis for compositions falling in the Al-based ternary system as the closest to those of quasicrystals discovered in nature, such as icosahedrite and decagonite. We show that, in contrast with what thought in the past, both pressure and temperature act to stabilize QCs, for which a clear phase transition to either crystalline approximants or amorphous material has been limited to very few compositions only. Such stabilization is proved by the compressibility behavior of QCs that resembles that of the pure constituent metals. Additional remarks come from the experimental observation of QC formation at high pressure and temperature in both static and dynamic experiments. These results seem, in conclusion, to suggest that the occurrence of QCs in nature might be more a rule rather than an exception. [ABSTRACT FROM AUTHOR]

We investigated the compressional behavior of i-AlCuFe quasicrystal using diamond anvil cell under quasi-hydrostatic conditions by in situ angle-dispersive X-ray powder diffraction measurements (in both compression and decompression) up to 76 GPa at ambient temperature using neon as pressure medium. These data were compared with those collected up to 104 GPa using KCl as pressure medium available in literature. In general, both sets of data indicate that individual d-spacing shows a continuous decrease with pressure with no drastic changes associated to structural phase transformations or amorphization. The d/d0, where d0 is the d-spacing at ambient pressure, showed a general isotropic compression behavior. The zero-pressure bulk modulus and its pressure derivative were calculated fitting the volume data to both the Murnaghan- and Birch-Murnaghan equation of state models. Results from this study extend our knowledge on the stability of icosahedrite at very high pressure and reinforce the evidence that natural quasicrystals formed during a shock event in asteroidal collisions and survived for eons in the history of the Solar System. [ABSTRACT FROM AUTHOR]

Two of the three natural quasiperiodic crystals found in the Khatyrka meteorite show a composition within the Al-Cu-Fe system. Icosahedrite, with formula Al 63 Cu 24 Fe 13 , coexists with the new Al 62 Cu 31 Fe 7 quasicrystal plus additional Al-metallic minerals such as stolperite (AlCu), kryachkoite [(Al,Cu) 6 (Fe,Cu)], hollisterite (AlFe 3 ), khatyrkite (Al 2 Cu) and cupalite (AlCu), associated to high-pressure phases like ringwoodite/ahrensite, coesite, and stishovite. These high-pressure minerals represent the evidence that most of the Khatyrka meteoritic fragments formed at least at 5 GPa and 1200 °C, if not at more extreme conditions. On the other hand, experimental studies on phase equilibria within the representative Al-Cu-Fe system appear mostly limited to ambient pressure conditions, yet. This makes the interpretation of the coexisting mineral phases in the meteoritic sample quite difficult. We performed experiments at 3, 5 and 21 GPa and temperatures of 800–1500 °C using the multi-anvil apparatus to investigate the phase equilibria in the Al 65 Cu 23 Fe 12 system representative of the first natural quasicrystal, icosahedrite. Our results, supported by single-crystal X-ray diffraction and analyses by scanning electron microscopy, confirm the stability of icosahedrite at high pressure and temperature along with additional coexisting Al-bearing phases representative of khatyrkite and stolperite as those found in the natural meteorite. One reversal experiment performed at 5 GPa and 1200 °C shows the formation of the icosahedral quasicrystal from a pure Al, Cu and Fe mixture, a first experimental synthesis of icosahedrite under those conditions. Pressure appears to not play a major role in the distribution of Al, Cu and Fe between the coexisting phases, icosahedrite in particular. Results from this study extend our knowledge on the stability of icosahedral AlCuFe at higher temperature and pressure than previously examined, and provide a new constraint on the stability of icosahedrite. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Icosahedrite, the first natural quasicrystal with composition Al63Cu24Fe13, was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P-T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P-T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solid-solid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures. [ABSTRACT FROM AUTHOR]

The 30-year history of quasicrystals is one in which, time after time, the conventional scientific view about what is possible has been proven wrong. First, quasicrystals were thought to be mathematically impossible; then, physically impossible; then, impossible unless synthesised in the laboratory under carefully controlled conditions. One by one, these strongly held views have been disproven, the last only recently as the result of the discovery of a natural quasicrystal found in a meteorite dating back to the formation of the solar system. This paper is a brief personal perspective on this history of misunderstanding and discovery. [ABSTRACT FROM AUTHOR]