High-resolution synchrotron X-ray study of icosahedrite, an icosahedral AlCuFe quasicrystal from the Khatyrka meteorite

A single grain of the icosahedral AlCuFe quasicrystal extracted from the Khatyrka meteorite has been studied by means of high-resolution synchrotron X-ray diffraction at the ESRF. We found that the mineral is a phason-wave modulated icosahedral quasicrystal, a feature already observed for synthetic quasicrystals. This result might be used as a tracer to shed light on the thermal history of the meteorite.

Icosahedrite, natural icosahedral Al₆₃Cu₂₄Fe₁₃, was discovered in a meteorite about 15 years ago. We have carried out a high-resolution X-ray diffraction study on a sample of this meteoritic mineral at the ESRF. The diffraction pattern turned out to be identical to an intermediate phase observed in synthetic i-AlCuFe during the transformation from the quasicrystalline state to a periodic rhombohedral phase. This particular natural Al₆₃Cu₂₄Fe₁₃ grain is an icosahedral quasicrystal on which a modulation by six cosine waves propagating along the fivefold axes is superimposed, with a wavelength of about 20 nm and a polarization in the phason/perpendicular space. By examining the thermodynamic conditions for producing this modulated icosahedral phase at high pressure in the laboratory, we may gain insights into the formation process of the Khatyrka meteorite.