The study of phase equilibria in the Al-Cu-Fe system at high pressures is important for understanding the processes occurring in the interiors of planets and meteorites. Of particular interest is the composition of Al65Cu23Fe12, close to the composition of quasicrystals found in the Khatyrka meteorite. This paper presents the results of experimental studies of phase equilibria in the Al65Cu23Fe12 system at pressures of 3, 5 and 21 GPa.
Three natural quasiperiodic crystals have been identified in the Khatyrka meteorite, two of which belong to the Al-Cu-Fe system. Icosahedrite with the formula AlCu24Fe13 is found alongside a new quasicrystal, Al62Cu31Fe7, and other aluminometallic minerals: stolperite (AlCu), kryachkoite [(Al, Cu)6(Fe, Cu)], hollisterite (AlFe3), khatyrkite (Al2Cu), and cupalite (AlCu). All of these phases are associated with high-pressure conditions, as evidenced by the presence of ringwoodite/arensite, coesite, and stishovite. The presence of these high-pressure minerals suggests that most of the Khatyrka meteorite fragments formed at pressures of at least 5 GPa and temperatures of 1200 °C, or possibly under more extreme conditions.
In contrast, experimental studies of phase equilibria in the Al-Cu-Fe system are mainly carried out at normal atmospheric pressure, which complicates the interpretation of coexisting mineral phases found in the meteorite.
To investigate the phase equilibria in the Al65Cu23Fe12 system, which is a natural icosahedrite, we carried out experiments at pressures of 3, 5 and 21 GPa and temperatures of 800–1500 °C using a multiple-anvil apparatus. The results, supported by single-crystal X-ray diffraction and scanning electron microscopy data, prove the stability of icosahedrite at high pressures and temperatures and indicate the presence of other aluminum-bearing phases such as khatyrkite and stolperite found in the meteorite. An experiment carried out at 5 GPa and 1200 °C demonstrated the formation of an icosahedral quasicrystal from a mixture of pure Al, Cu and Fe, which is the first synthesis of icosahedrite under such conditions. Pressure does not seem to have a significant effect on the distribution of Al, Cu and Fe among the coexisting phases, including icosahedrite. These results expand our understanding of the stability of icosahedral AlCuFe at higher temperatures and pressures than previously thought and provide new data on the stability of icosahedrite.
The experiments were carried out using a high-pressure chamber of the “anvil with a dimple” type. Al65Cu23Fe12 samples were synthesized by arc melting in an argon atmosphere. The resulting samples were placed in the high-pressure chamber and annealed at specified temperatures and pressures. After annealing, the samples were quenched and examined using X-ray diffraction and electron microscopy.
The results showed that at a pressure of 3 GPa, the stable phase is the icosahedral quasicrystal (i-phase). When the pressure increases to 5 GPa, the i-phase coexists with other phases, including β-Al(Cu,Fe) and γ-Cu9Al4. At a pressure of 21 GPa, the i-phase becomes unstable and new phases are formed, the structure of which is currently being clarified.
The obtained results allow us to better understand the conditions of formation of quasicrystals in the Khatyrka meteorite. It is assumed that the Khatyrka meteorite was formed as a result of the collision of two bodies in space. The high pressure that arose during the collision could contribute to the formation of the i-phase. Further studies of phase equilibria in the Al-Cu-Fe system at high pressures will help to clarify the model of formation of the Khatyrka meteorite and other bodies containing quasicrystals.
Author: Vincenzo Stagno, Luca Bindi, Paul J. Steinhardt, Yingwei Fei
Institute: Department of Earth Sciences, Sapienza University of Rome, Via Aldo Moro, 5, 00185 Rome, Italy, Department of Geosciences, University of Florence, Via La Pira, 4, I-50121 Florence, Italy, Department of Physics and the Princeton Center for Theoretical Science, Princeton University, Princeton, NJ 08544, USA, Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA