Natural quasicrystal with decagonal symmetry

Abstract

We report the first occurrence of a natural quasicrystal with decagonal symmetry. The quasicrystal, with composition Al71Ni24Fe5, was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite. Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal to be identified, was found in the same meteorite. The new quasicrystal was found associated with steinhardtite (Al38Ni32Fe30), Fe-poor steinhardtite (Al50Ni40Fe10), Al-bearing trevorite (NiFe2O4) and Al-bearing taenite (FeNi). Laboratory studies of decagonal Al71Ni24Fe5 have shown that it is stable over a narrow range of temperatures, 1120 K to 1200 K at standard pressure, providing support for our earlier conclusion that the Khatyrka meteorite reached heterogeneous high temperatures [1100 < T(K) ≤ 1500] and then rapidly cooled after being heated during an impact-induced shock that occurred in outer space 4.5 Gya. The occurrences of metallic Al alloyed with Cu, Ni, and Fe raises new questions regarding conditions that can be achieved in the early solar nebula.

Figure 1. The top panel shows micro CT-SCAN 3D-images (at different rotations) of the whole Grain 126.

The brighter and the darker regions are Cu-Al metals and meteoritic silicates, respectively. The bottom panel shows a SEM-BSE image of Al₇₁Ni₂₄Fe₅ quasicrystal (QC) in apparent growth contact with “olivine” (“Ol”). See text for discussion of the “olivine” composition. The surface of the quasicrystal appears to exhibit growth steps. The image also contains sodalite (Sod).

Figure 2

Reconstructed precession images along the ten-fold symmetry axis (b) and perpendicular to the ten-fold direction (c, d) obtained using the collected single-crystal X-ray data set (MoKα radiation) from the fragment of Grain 126 shown in (a).

Figure 3. X-ray powder diffraction pattern for natural Al₇₁Ni₂₄Fe₅ (CuKα radiation).

Figure 4. Selected area (a) and convergent beam (b) electron diffraction patterns collected with a TEM along the ten-fold axis.

These patterns, consisting of sharp peaks (or Kikuchi lines) arranged with ten-fold symmetry, are the characteristic signature of a decagonal quasicrystal.

Figure 5. High-resolution transmission electron microscopy (HRTEM) image showing that the real space structure consists of a homogeneous, quasiperiodic and ten-fold symmetric pattern.

The diffraction patterns given in Fig. 4 and the HRTEM image were obtained from the thin region of the granule in the inset indicated by the red (dashed) circle, an area 0.2 μm across.