Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal

Icosahedrite, ideally Al63Cu24Fe13, is a new mineral from the Khatryka River, southeastern Chukhotka, Russia. It represents the first definitive case of a naturally-occurring quasicrystal with a crystallographically-forbidden icosahedral symmetry. It occurs as dark grey-black anhedral to subhedral grains up to 100 μm across, closely associated with spinel, diopside, forsterite, nepheline, sodalite, corundum, stishovite, khatyrkite, cupalite and an unnamed phase of composition AlCuFe. Icosahedrite is opaque with a metallic luster, possesses a gray streak, and is brittle with an uneven fracture. The density could not be determined. 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. In plane-polarized incident light, icosahedrite exhibits neither bireflectance nor pleochroism. Between crossed polars, it is isotropic. Reflectance percentages (Rmin= Rmax) for the four standard COM wavelengths are 62.3 (471.1 nm), 60.6 (548.3 nm), 58.1 (586.6 nm), and 56.0 (652.3 nm), respectively. The X-ray powder pattern was indexed on the basis of six integer indices, as conventionally used with quasicrystals, where the lattice parameter (in six dimensional notation) is measured to be a6D = 12.64 Å, with probable space group Fm . The four strongest X-ray powder-diffraction lines [d in Å (I/I0) (hklmno)] are: 2.006 (100) (4 0042), 2.108 (90) (42 22), 1.238 (30) (60 064), and 3.41 (25) (31 11). Average results of 34 electron-microprobe analyses gave, on the basis of total atoms = 100, the formula Al63.11Cu24.02Fe12.78Si0.03Co0.01Ca0.01Zn0.01Cr0.02Cl0.01. The simplified formula is Al63Cu24Fe13, which requires the mass fractions Al 43.02, Cu 38.60, Fe 18.38, total 100.00 wt%. The new mineral is named for the icosahedral symmetry of its internal atomic structure, as observed in its diffraction pattern. Both the new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2010-042).