¹Luca Bindi et al. (>10)*
¹Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
*Find the extensive, full author and affiliation list on the publishers website

Decagonite is the second natural quasicrystal, after icosahedrite (Al63Cu24Fe13), and the first to exhibit the crystallographically forbidden decagonal symmetry. It was found as rare fragments up to ~60 μm across in one of the grains (labeled number 126) of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The meteoritic grain contains evidence of a heterogeneous distribution of pressures and temperatures that occurred during impact shock, in which some portions of the meteorite reached at least 5 GPa and 1200 °C. Decagonite is associated with Al-bearing trevorite, diopside, forsterite, ahrensite, clinoenstatite, nepheline, coesite, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, taenite, Al-bearing taenite, and steinhardtite. Given the exceedingly small size of decagonite, it was not possible to determine most of the physical properties for the mineral. A mean of seven electron microprobe analyses (obtained from three different fragments) gave the formula Al70.2(3)Ni24.5(4)Fe5.3(2), on the basis of 100 atoms. A combined TEM and single-crystal X-ray diffraction study revealed the unmistakable signature of a decagonal quasicrystal: a pattern of sharp peaks arranged in straight lines with 10-fold symmetry together with periodic patterns taken perpendicular to the 10-fold direction. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. The likely space group is P105/mmc, as is the case for synthetic Al71Ni24Fe5. The five strongest powder-diffraction lines [d in Å (I/I0)] are: 2.024 (100), 3.765 (50), 2.051 (45), 3.405 (40), 1.9799 (40). The new mineral has been approved by the IMA-NMNC Commission (IMA2015-017) and named decagonite for the 10-fold symmetry of its structure. The finding of a second natural quasicrystal informs the longstanding debate about the stability and robustness of quasicrystals among condensed matter physicists and demonstrates that mineralogy can continue to surprise us and have a strong impact on other disciplines.

Reference
Bindi L. et al. (2015) Decagonite, Al71Ni24Fe5, a quasicrystal with decagonal symmetry from the Khatyrka CV3 carbonaceous chondrite. American Mineralogist 100, 2340-2343
Link to Article [doi:10.2138/am-2015-5423]
Copyright: The Mineralogical Society of America

¹T. Michikami, ²A. Hagermann, ³T. Kadokawa, ¹A. Yoshida, ³A. Shimada, ⁴S. Hasegawa, ³A.Tsuchiyama
¹Faculty of Engineering, Kinki University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
²Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
³Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kiashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8052, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-8510, Japan

Laboratory impact experiments have found that impact fragments tend to be elongated. Their shapes, as defined by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a ⩾ b ⩾ c), are distributed around mean values of the axial ratios b/a ∼0.7 and c/a ∼0.5. This corresponds to a: b: c in the simple proportion 2: √2: 1. The shape distributions of some boulders on asteroid Eros, the small- and fast-rotating asteroids (diameter < 200 m and rotation period < 1 h), and asteroids in young families, are similar to those of laboratory fragments created in catastrophic disruptions. Catastrophic disruption is, however, a process that is different from impact cratering. In order to systematically investigate the shapes of fragments in the range from impact cratering to catastrophic disruption, impact experiments for basalt targets 5 to 15 cm in size were performed. A total of 28 impact experiments were carried out by firing a spherical nylon projectile (diameter 7.14 mm) perpendicularly into the target surface at velocities of 1.60 to 7.13 km/s. More than 12,700 fragments with b ⩾ 4 mm generated in the impact experiments were measured. We found that the mean value of c/a in each impact decreases with decreasing impact energy per unit target mass. For instance, the mean value of c/a in an impact cratering event is nearly 0.2, which is considerably smaller than c/a in a catastrophic disruption (∼0.5). The data presented here can provide important evidence to interpret the shapes of asteroids and boulders on asteroid surfaces, and can constrain current interpretations of asteroid formation. As an example, by applying our experimental results to the boulder shapes on asteroid Itokawa’s surface, we can infer that Itokawa’s parent body must have experienced a catastrophic disruption.

Reference
Michikami T, Hagermann A, Kadokawa T, Yoshida A, Shimada A, Hasegawa S, Tsuchiyama A (2015) Fragment shapes in impact experiments ranging from cratering to catastrophic disruption. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.09.038]

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