Abstract
We report the first occurrence of an icosahedral quasicrystal with composition Al62.0(8)Cu31.2(8)Fe6.8(4), outside the measured equilibrium stability field at standard pressure of the previously reported Al-Cu-Fe quasicrystal (AlxCuyFez, with x between 61 and 64, y between 24 and 26, z between 12 and 13%). The new icosahedral mineral formed naturally and was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite that experienced shock metamorphism, local melting (with conditions exceeding 5 GPa and 1,200 °C in some locations), and rapid cooling, all of which likely resulted from impact-induced shock in space. This is the first example of a quasicrystal composition discovered in nature prior to being synthesized in the laboratory. The new composition was found in a grain that has a separate metal assemblage containing icosahedrite (Al63Cu24Fe13), currently the only other known naturally occurring mineral with icosahedral symmetry (though the latter composition had already been observed in the laboratory prior to its discovery in nature). The chemistry of both the icosahedral phases was characterized by electron microprobe, and the rotational symmetry was confirmed by means of electron backscatter diffraction.
Highlights
The Al-Cu-Fe ternary phase diagram at standard pressure has been systematically studied around the icosahedral region[12,13,14]
I-phase II, Al62.0(8) Cu31.2(8)Fe6.8(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 Bancel[12] 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 Al56.8Cu37Fe6.2 and annealed at 660 °C
Summary
All recovered fragments of Khatyrka including Grain 126 have been shown to have CV3-like oxygen isotopic compositions[6,15,16], confirming their common meteoritic origin. Most of the Khatyrka meteoritic fragments display evidence of an impact shock that generated a heterogeneous distribution of pressures and temperatures in which some portions of the meteorite exceeded 5 GPa and 1200 °C15. Other fragments of Grain 126 have previously led to the discovery of novel phases, including the new polymorph of Al, steinhardtite[17], as well as other new crystalline Al-Cu-Fe alloys[18]. Other phases found include ringwoodite, coesite, stishovite, magnetite, diopside, forsterite, clinoenstatite, sodalite, nepheline, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite (CuAl2), cupalite (CuAl), taenite, Al-bearing trevorite, and Al-bearing taenite[13]
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