Steinhardt and Bindi reported in 2012 that the concept of quasicrystals was first introduced twenty-eight years ago and, since then, over a hundred types have been discovered in the laboratory under precisely controlled physical conditions designed to avoid crystallization. Yet the original theory suggested that quasicrystals can potentially be as robust and stable as crystals, perhaps even forming naturally. These considerations motivated a decade-long search for a natural quasicrystal culminating in the discovery of icosahedrite (Al63Cu24Fe13), an icosahedral quasicrystal found in a rock sample composed mainly of khatyrkite (crystalline (Cu,Zn)Al2) labeled as coming from the Koryak Mountains of far eastern Russia. In this paper, the authors reviewed the search and discovery, the analysis showing the sample to be of extraterrestrial origin and the initial results of an extraordinary geological expedition to the Koryak Mountains to seek further evidence.
The authors subsequently concluded that natural icosahedrite has the same structure and stoichiometry as the first sample of icosahedral Al63Cu24Fe13 synthesized and characterized in the laboratory in 1987 (Tsai et al 1987). The laboratory samples were prepared under controlled conditions: casting from the melt in an argon atmosphere; solidifying under vacuum at a cooling rate of 1 °C min−1 until room temperature; then annealing for 10 days at 825 °C, for example (Audier et al 1990, Bancel 1991). The stability of the phase was examined in a series of studies (Bancel 1991). The range of stable compositions of the quasicrystal phase as a function of temperature narrows as the temperature decreases down to 550 °C, below which isothermal sections are difficult to obtain because of prohibitively long equilibration times (Bancel 1991). At 550 °C (in vacuum), there remains a small stability zone centered around the stoichiometry measured in the natural icosahedrite sample (to within measurement uncertainties).
The discovery of icosahedrite pushes the age of the oldest known example of this phase and of quasicrystals generally back to ca. 4.5 Gya, the age of all known unequilibrated chondrites. The occurrence inside the meteorite demonstrates that a quasicrystal phase can form naturally within a complex, inhomogeneous medium. This sample formed under astrophysical conditions; whether a quasicrystal of some type can form in the course of planetary evolution or under terrestrial conditions remains an open question, but the likely answer is yes, now that a first example has been found.
Hopefully, the discovery will now trigger the re-examination of other terrestrial and extraterrestrial minerals in search of different quasicrystals. From the perspective of condensed matter physics, the observations constitute significant new support for the original proposal that icosahedral quasicrystals can be energetically stable states of matter, on the same footing as crystals (Levine and Steinhardt 1984). This proposal was based on the notion that short-range atomic interactions can impose energetic
constraints analogous to the face-to-face matching rules shown in figure 1(b), and thereby ensure that the icosahedral quasicrystal is energetically preferred. The alternative view rejects the matching rule idea as being too complicated and proposes instead that icosahedral quasicrystals can only form because of entropic effects. An analogy is having units with no matching rules (random tilings (Henley 1991)), in which case on-average icosahedral symmetry has the maximum entropy.
The entropic picture, which assumes that the atoms have enough time to explore many different random configurations, is hard to reconcile with the extreme conditions under which icosahedrite formed and annealed. The finding of natural quasicrystal enclosed within stishovite and other evidence not presented here indicates that formation likely occurred under shock conditions and rapid cooling, where entropic effects were unimportant compared with energetics and kinetics in forming the phase. Furthermore, all the icosahedrite grains that have been studied thus far have the same composition, to within better than one per cent, even though the phases surrounding them are different. This observation is expected if the composition corresponds to an energetically preferred structure, whereas the entropic model generally allows a continuous range of composition (corresponding to crystal approximant structures). Further laboratory studies could shed light on this issue. For example, investigations of Al63.5Cu23Fe12.5 in diamond-anvil cells (Gupta et al 2004, Sadoc et al 1994) at high pressures up to 35 GPa have not shown evidence of a phase transformation, but there have not yet been systematic studies of the phase diagram of quasicrystals at high pressure.
In terms of geoscience, one of the puzzles presented by our sample is the presence of metallic aluminum, which requires highly reducing conditions to form even in alloys. Metallic aluminum has never been detected even as a trace component in metal within enstatite chondrites and achondrites (aubrites). Furthermore, the association of aluminum, a lithophile that condenses at very high temperatures, with copper, a chalcophile that condenses at much lower temperatures, is not yet understood. Resolving these and other mysteries posed by this sample will not only further clarify the origin of the quasicrystal phase but also provide insights on previously unobserved early solar-system processes.
Last but not least, the opportunity remains for mineralogy to play another important role: namely, through further searches, the discovery of quasicrystalline phases not yet observed in the laboratory.
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