It is almost 30 years since Dan Shechtman's discovery, on 8 April 1982, of an icosahedral alloy of aluminium and manganese that displays long-range order in diffraction experiments but lacks translational periodicity. The material turned out to be an example of a new phase of matter, which Don Levine and Paul Steinhardt dubbed 'quasicrystals'. Following the original observation, for which Shechtman won the 2011 Nobel Prize in Chemistry, hundreds of other quasicrystals have been synthesized and characterized.

Credit: © 2009 AAAS

However, it wasn't until 2009 that evidence of a naturally occurring quasicrystal was reported, in a rock sample (Science 324, 1306–1309; 2009). Luca Bindi, Steinhardt and collaborators have made further studies of that same sample and suggest that it is probably part of a meteorite — which would mean that the quasicrystal embedded in it is of extraterrestrial origin (Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1111115109; 2012).

The rock is held in the Museo di Storia Naturale at the Università degli Studi di Firenze. Catalogued as coming from the Koryak Mountains in far-eastern Siberia, the rock was identified as a candidate host for a natural quasicrystal after a decade-long systematic search initiated by Steinhardt and colleagues. That search had originally involved sifting through more than 80,000 data sets deposited in the International Center for Diffraction Data, but it didn't turn up any new quasicrystals, synthetic or natural. So the search was extended to include minerals having compositions similar to those of known synthetic quasicrystals, and this brought the Florentine sample onto the radar. From it came clear evidence of a natural icosahedral quasicrystal, in the form of micrometre-sized grains of an aluminum–copper–iron alloy.

Now known as icosahedrite, how this mineral formed has remained a mystery. There are several arguments against an anthropogenic origin, and now the study by Bindi et al. provides strong evidence that the sample isn't terrestrial at all. In particular, secondary-ion-mass-spectrometry experiments revealed oxygen-isotope compositions that clearly disfavour a terrestrial origin. The data are instead consistent with a scenario in which the mineral arrived on Earth in a meteorite of the so-called carbonaceous-chondrite type. That would make it about 4.5 billion years old, roughly the age of the Solar System — whereas the first lab synthesis of this particular quasicrystalline phase occurred only in 1987.