Phase equilibria in the nominally Al₆₅Cu₂₃Fe₁₂ system at 3, 5 and 21 GPa: Implications for the quasicrystal-bearing Khatyrka meteorite

Two of the three natural quasiperiodic crystals found in the Khatyrka meteorite show a composition within the Al-Cu-Fe system. Icosahedrite, with formula Al₆₃Cu₂₄Fe₁₃, coexists with the new Al₆₂Cu₃₁Fe₇ quasicrystal plus additional Al-metallic minerals such as stolperite (AlCu), kryachkoite [(Al,Cu)₆(Fe,Cu)], hollisterite (AlFe₃), khatyrkite (Al₂Cu) 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 Al₆₅Cu₂₃Fe₁₂ 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.