The discovery of an icosahedral Al-Cu-Fe quasicrystal with a composition far from that of any known ideal, stable quasicrystal is notable for several reasons. It is only the third example of a natural quasicrystal to be found anywhere, all from fragments of the same Khatyrka meteorite; and it is the first documented example of the coexistence of two different Al-Cu-Fe i-phase compositions. Furthermore, it is the first example of a composition discovered in nature prior to being discovered in the laboratory.

The Al-Cu-Fe ternary phase diagram at standard pressure has been systematically studied around the icosahedral region12,13,14. Icosahedrite, Al 63 Cu 24 Fe 13 or equivalently i-phase I, lies within the stability field for the icosahedral quasicrystal phase as measured at all temperatures below melting. By contrast, i-phase II, Al 62.0(8) Cu 31.2(8) Fe 6.8(4) , has a chemical composition that lies significantly outside the stability field at standard pressure for all temperatures below melting, for example, outside the stability field at room temperature first reported by Bancel12 (dashed area in Fig. 3) as well as at elevated temperatures up to 740 °C. A composition closer to that of i-phase II described here was reported by Zhang et al.14,19 during investigations on the Al-Cu-Fe system with low Fe content starting from an alloy with composition Al 56.8 Cu 37 Fe 6.2 and annealed at 660 °C. Based on scanning electron microscopy (SEM) and X-ray powder diffraction measurements (which are not as precise as the methods reported here), they claimed an icosahedral phase with composition Al 62.3 Cu 28.6 Fe 9.1 , with significantly higher percentage of Fe and lower Cu than observed here in i-phase II. Gratias et al.20 found that at 680 °C, the stability field of the i-phase extends approximately over a triangle with vertices (62.4, 24.4, 13.2), (65, 23, 12) and (61, 28.4, 10.6), in terms of (Al, Cu, Fe) atomic %. They reported that this region splits schematically into 3 fields: (i) the perfect icosahedral phase, which is stable down to the lowest possible annealing temperature where atomic diffusion is active in a tiny region of composition close to Al 62.3 Cu 24.9 Fe 12.8 ; (ii) a well-defined periodic phase with rhombohedral structure, which transforms reversibly into the icosahedral phase near 710 °C in the lower part of the triangle; (iii) a complex region characterized by additional diffraction effects (peak broadening, line-shapes, etc.), which may correspond to various approximant structures closely related to the i-phase. However, none of these earlier studies found evidence of the i-phase II composition.

Figure 3 Subsolidus projection of the ternary Al-Cu-Fe phase diagram in the vicinity of the icosahedral phase (modified after Bancel 12). Shaded regions indicate pure phase fields and tie lines bound two-phase regions. The maximal extent of the icosahedral phase occurs within the boxed area labelled i-region. The dark shading shows the section of the i-region in which transformations have been identified. Empty red and light blue spheres correspond to data from i-phase I (icosahedrite) and i-phase II, respectively. Errors within the size of the symbols. Full size image

One possible explanation for why the i-phase II has not been found in earlier studies is that i-phase II is a kinetically stabilized composition, only preserved because of very rapid quench, and is thermodynamically unstable at any pressure and temperature. Previous investigations of Khatyrka samples15 have provided ample evidence that the meteorite experienced an impact-induced shock that generated a highly heterogeneous distribution of pressures and temperatures with conditions as high as 5 GPa and 1,200 °C, followed by rapid cooling. On this basis, the kinetic stabilization explanation is plausible. On the other hand, it is notable that fragments of 126A containing i-phase I, as illustrated in panel 1 in Fig. 1, have textures qualitatively similar to fragments with i-phase II, as shown in panels 2 and 3. The metallic phases observed with i-phase I appear to have formed according to a predictable solidification sequence along a liquid line of descent, largely consistent with the equilibrium phase diagram for Al-Cu-Fe, beginning with the formation of the λ phase and including the formation of an icosahedral phase with a composition in the equilibrium stability field at standard pressure. Based on their texture, the fragments containing i-phase II appear to have cooled at a similar rate but starting from a different initial composition, such that the solidification process began with i-phase II and skipped the λ phase. In that case, i-phase II could have a composition in a stability field for i-phase that has shifted or expanded due to the high P-T induced by the shock, and likely be a new high-pressure phase, crystallized from shock-induced melt under high pressure. This latter possibility could be explored in the laboratory by beginning with a liquid with the composition of i-phase II at high P-T and checking if the i-phase II solid forms when high pressure is maintained as the temperature decreases. Combining studies of natural quasicrystals, high-pressure diamond anvil experiments21,22, and laboratory shock experiments23, we plan to test this hypothesis and develop a better understanding of the kinetic and thermodynamic stability of quasicrystals.