Synchrotron X-rays reveal the secrets of meteoritic AlCuFe quasicrystals

Quasicrystals are part of the broader class of aperiodic crystals, which exhibit long-range order without translational symmetry [1]. They appear in various material systems, including intermetallic compounds, soft condensed matter, and oxide surfaces. Their diffraction patterns are characterized by sharp Bragg peaks and symmetries that defy periodic lattice structures, such as icosahedral symmetry with five-fold rotational axes.

In 2009, an icosahedral quasicrystal, later named icosahedrite, was identified in the Khatyrka meteorite [2]. Its composition, Al₆₃Cu₂₄Fe₁₃, closely resembles that of the first high-quality quasicrystal reported in 1984 [1]. Subsequent studies, supported by laboratory shock-recovery experiments, suggested that icosahedrite formed under extreme impact conditions in space, with temperature exceeding 1400 K, and pressures above 5 GPa. 

Despite these advances, the phase diagram near the i-AlCuFe regions remains complex, featuring temperature-dependent structural variations that obscure the precise formation mechanism of icosahedrite. While transmission electron diffraction had previously confirmed the long-range icosahedral symmetry in a separate grain [2], high-resolution X-ray diffraction, capable of detecting subtle diffuse scattering, had not yet been applied to this natural material.  

In this study, a single prismatic grain of icosahedrite (dimensions 40 x 40 x 60 mm in size) was selected from the holotype material held at the Natural History Museum, University of Florence, Italy (sample no. 46407/G). The grain was mounted on a glass capillary and examined using high-resolution X-ray diffraction at beamline ID28, which is dedicated to the detection of diffuse scattering and weak signals. 

The collected diffraction data were indexed using CrysalisPro software, and reciprocal space sections were visualized using a dedicated tool developed at ID28. Indexing employed a superspace formalism [1], which is required to describe icosahedral symmetry and involves six integer indices. The quaiscrystal symmetry was assigned to the  Fm-35 space group, with a six-dimensional lattice parameter of 2 x 0.628 nm. Use of a small collimator enabled probing of the different regions along the sample’s z-axis, confirming the presence of a single dominant quasicrystal grain.

Figure 1 shows reconstructed diffraction patterns along the two-fold and five-fold axes. The overall icosahedral symmetry is well preserved; however, additional streaks are clearly visible along directions parallel to the five-fold axis, particularly in the enlarged inset of the two-fold plane. 

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Fig. 1: Reconstructed two-fold and five-fold diffraction planes of the icosahedrite grain, based on orientation matrix (UB matrix) determination and visualized using custom software developed at ID28. Data were collected with a sample-to-detector distance of 244 mm. Red dashed lines indicate the main two-, three-, and five-fold symmetry directions. In the two-fold plane, diffuse streaks aligned to the two five-fold axes are clearly visible. The inset displays an enlarged region of this plane, highlighting these features. A logarithmic intensity scale and colour mapping were applied to enhance the visibility of weak reflections.  

The diffraction pattern of this icosahedrite grain turned out to be significantly more complex than that of an ideal icosahedral quasicrystal. Each Bragg reflection is surrounded by 12 satellite peaks, arranged along directions parallel to the five-fold axes, and forming an icosahedral distribution.

This is illustrated in Figures 2a and 2b, which display cross-sections perpendicular to a five-fold axis: the characteristic ring of five satellites and its mirror image is clearly visible at positions + and -D. These satellite peaks are located at a distance of approximately 0.3 nm-1 from the main Bragg peaks and are broader than the latter, as shown in Figure 2c. The appearance of these satellite reflections is a hallmark phason modulation, indicating a structural distortion of the ideal quasicrystal phase with a long modulation wavelength of 19.5 nm. 

Click figure to enlarge

Fig. 2: Reciprocal space slices around main Bragg peaks along the five-fold symmetry directions of the icosahedrite grain. (a) and (b) Reciprocal sections at -D and +D (D = 0.358 nm⁻¹), respectively, near the 18/29 Bragg peak along a five-fold axis. The pentagonal arrangement and its mirror image of satellite reflections reveal the characteristic icosahedral modulation. Each panel spans 1.84 nm^-1 x 1.84 nm^-1 in reciprocal space. c) Intensity profile along the five-fold axis around the 7/11 Bragg peaks, showing two satellite reflections at +-0.03 Å^-1. The satellite peaks display Lorentzian profiles and are significantly broader than the main Bragg peak. d) and (e) Intensity distributions of satellite reflections around the two-fold 8/12 Bragg peak and at a reciprocal-space height just above D = 0.298 nm^-1, respectively. f) Schematic representation of the satellite reflection geometry, highlighting expected extinction along directions d and g. Weak and strong satellite intensities are observed along directions (ε, ζ) and (β, α), respectively. The signs + and – indicate the position of the satellite reflections relative to the main Bragg peak along the five-fold axis. Icosahedral reflections are indexed using a shorthand N/M notation in place of the full six-integer indices. 

The modulation observed in this study is consistent with that previously reported for the synthetic i-Al_63.5Cu₂₄Fe_12.5 quasicrystal, which is stable only above 675°C. Upon cooling, this synthetic phase transforms into a large-period rhombohedral approximant via intermediates states, including a modulated icosahedral phase. In this transitional state, satellite reflections appear along the 12 five-fold symmetry directions surrounding each Bragg peak [3].

As in earlier synchrotron studies performed at LURE, the modulation detected here is attributed to phason fluctuations, a type of distortion unique to quasiperiodic order. Specifically, the modulation wave is polarized in phason space along the five-fold axes, resulting in a distinctive satellite intensity distribution, as shown in Figures 2d–f. A quantitative analysis of the satellite spacing and intensity yields a phason wave amplitude of approximately 0.05 nm, in good agreement with values reported for the synthetic analogue. 

Although the high-pressure phase diagram governing the formation of natural icosahedrite likely differs from that at ambient-pressure, and despite the possibility that different grains experienced different pressure-temperature (P-T) histories, the detection of a modulated phase in this grain may serve as a potential temperature tracer. This opens the possibility of constraining the thermal conditions under which this quasicrystal formed.

More broadly, the identification of phason wave modulation in a natural quasicrystal provides a crucial link between synthetic and meteoritic systems. It offers a new structural marker for tracing extreme formation environments in planetary materials and paves the way for further studies of quasicrystal stability and transformation under astrophysical conditions.
 

Principal publication and authors
High resolution synchrotron X-ray study of icosahedrite, an icosahedral AlCuFe quasicrystal from the Khatyrka meteorite, H. Takakura (a), K. Mizunuma (b), T. Yamada (c), A. Bosak (d), F. Formisano (e), L. Paolasini (d), M. de Boissieu (f), P. J. Steinhardt (g), L. Bindi (h), IUCrJ (2025); https://doi.org/10.1107/S2052252525004130
(a) Faculty of Engineering, Division of Applied Physics, Hokkaido University, Hokkaido (Japan)
(b) Graduate School of Engineering, Division of Applied Physics, Hokkaido University, Hokkaido (Japan)
(c) Faculty of Science, Department of Applied Physics, Tokyo University of Science, Tokyo (Japan)
(d) ESRF
(e) CNR-IOM & INSIDE@ILL c/o Operative Group in Grenoble (OGG) and Institut Laue Langevin (ILL), Grenoble (France)
(f) Université Grenoble Alpes, CNRS, Grenoble INP, SIMaP, Grenoble (France)
(g) Department of Physics, Princeton University, Princeton, NJ (USA)
(h) Department of Earth Sciences, University of Florence, Florence (Italy)  

References
[1] T. Janssen, G. Chapuis, and M. de Boissieu, Aperiodic Crystals. From modulated phases to quasicrystals. 2nd ed., IUCr Monographs on Crystallography (Oxford University Press, Oxford), 2018.
[2] L. Bindi et al., Science 324, 1306 (2009).
[3] N. Menguy et al., J. Phys. France 3, 1953 (1993).

Top image: The image shows a picture of the Khatyrka meteorite from which the quasicrystal was extracted. The diffraction pattern in the centre is from the quasicrystal and taken at beamline ID28; the icosahedron is an illustration of the symmetry.