Earth’s natural quasicrystals came from outer space

Earth’s natural quasicrystals came from outer space. Last year’s Nobel Prize in Chemistry honored Dan Shechtman for his 1982 synthesis of quasicrystals, materials lacking translational symmetry but displaying rotational symmetries not possible in crystals (see Physics Today, December 2011, page 17 ). Nature, however, had long before pulled off that trick; in 2009 theorist Paul Steinhardt, mineralogist Luca Bindi, and colleagues announced that a museum in Florence, Italy, housed a mineral that included a quasicrystal, since named icosahedrite, alleged to have been found in Russia’s Koryak Mountains (see Physics Today, August 2009, page 14 ). Various researchers speculated on the source of the quasicrystal, but a team led by Steinhardt solved the mystery: Icosahedrite was brought to Earth by a 4.5-billion-year-old carbonaceous chondrite meteorite. The key clue was that the icosahedrite was surrounded by a second mineral, stishovite, which forms only at high temperature and extreme pressure. To distinguish between the plausible possibilities of formation deep within Earth and formation in a meteorite collision, the researchers studied the icosahedrite-containing mineral’s oxygen-17 and oxygen-18 content; the relative abundances of the two isotopes are different for materials of terrestrial and extraterrestrial origin. Subsequent to that analysis, Steinhardt organized an expedition to the Koryak Mountains, where the group found other icosahedrite-containing minerals (see the figure) whose study confirmed the extraterrestrial origin of the natural quasicrystal. From a condensed-matter perspective, say the authors, the analyses (and other evidence not yet published) suggest that quasicrystals are not necessarily delicate metastable oddities; rather, like crystals, they can be energetically stable. (P. J. Steinhardt, L. Bindi, Rep. Prog. Phys. 75, 092601, 2012; L. Bindi et al., Proc. Natl. Acad. Sci. 109, 1346, 2012.)