The quasicrystal that fell to Earth

The 2011 Nobel Prize in chemistry was awarded to Dan Schechtman for his discovery of quasicrystals, materials that do not have the regular lattice structure of crystalline solids. Schechtman produced quasicrystals in the laboratory in 1982, but until 2008 nobody had found a naturally occurring quasicrystal. Now researchers in Italy and the United States have examined the rock that contained these natural quasicrystals and determined it may actually be part of a meteorite.

Normal crystalline solids have atoms or molecules arranged in cubes, hexagons, or other regular repeating patterns. Quasicrystals exhibit different symmetries that never precisely repeat: pentagons, icosahedrons, and so forth. Schechtman and researchers after him produced these quasi-periodic lattices by melting materials under high pressure, then cooling them quickly in a process known as quenching.

In 2008, Luca Bindi of the Museo di Storia Naturale in Firenze, Italy approached Paul Steinhardt at Princeton University to investigate a curious rock collected in eastern Russia during the late 1970s. The researchers (including Bindi, Steinhardt, Nan Yao, and Peter Lu) found it contained naturally occurring quasicrystal grains—the first ever identified.

The rock sample consists of grains of more ordinary metallic and silicon compounds interspersed with the quasicrystal grains, so it's not wholly a quasicrystal. Some structures in the rock are only formed under high shocks (unlike sedimentary or volcanic rocks), and one particular silicate structure, known as stishovite, is most strongly associated with meteorites. To confirm this suspicion, the researchers investigated the ratios of various oxygen isotopes, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O, and compared them to the ratios found on Earth in analogous minerals. Because of the differences in formation and environment, meteorites have a distinctive isotope signature compared to their chemically similar terrestrial cousins. The scientific team found the sample containing the quasicrystals looked like it had an extraterrestrial source.

The quasicrystal within the rock is a known type, Al₆₃Cu₂₄Fe₁₃, first synthesized in a lab in 1987. However, if the Russian rock is as similar to a chondrite meteorite as its composition suggests, that places it around 4.5 billion years old, meaning quasicrystals evidently were present (at least in small amounts) at the start of our Solar System's history. The great age also speaks to the long-term stability of quasicrystals, at least under some conditions.

How this particular meteorite formed is still a mystery, though. The metallic aluminum present in the rock usually requires a very different set of processes to form, and it has not been found in any other meteorites. In other words, while the isotope ratios indicate an extraterrestrial origin for the rock, its composition marks it as a new type of meteorite, one with uncertain origins. The authors suggest a high-velocity impact may have broken this rock sample off a larger parent body, and some combination of the conditions before impact and the collision itself may have made the strange combination of metals seen. However, they admit that's speculation; one thing that is certain is this rock has a lot to tell us beyond containing the first quasicrystals observed in nature.

PNAS, 2012. DOI: 10.1073/pnas.1111115109  (About DOIs).

Listing image by Photograph by stanford.edu