The beautiful and bizarre mosaic-like structures called quasicrystals, whose 1982 discovery won Dan Shechtman the 2011 Nobel Prize in Chemistry, are almost all made in laboratories. Just one natural sample has ever been found, in a millimetre-sized rock fragment at the Museum of Natural History in Florence, Italy. Until now.
In January this year, theoretical physicist Paul Steinhardt, of Princeton University in New Jersey, reported how he had traced the Florence rock back to its source — the Listvenitovyi Stream in the Koryak mountains in Chukotka, a remote gold-mining region in northeastern Russia. It turned out that a man named Valery Kryachko had dug up the stuff in 1979 while searching for platinum, and it was eventually smuggled to Florence.
Together with nine other scientists from the United States, Russia and Italy (and two drivers and a cook), Steinhardt embarked on an expedition across the tundra last July to look for more samples, as I wrote about at the beginning of the year.
In a review published today in the journal Reports on Progress in Physics, Steinhardt and Luca Bindi (a mineralogist at the Florence museum) say that their expedition did find new rock fragments containing grains of quasicrystals: solids with a mosaic-like atomic structure that appears to show long-range order but never quite repeats its arrangement. And the evidence further backs up their January report that these quasicrystals appear to come from a meteorite some 4.5 billion years old. Rocks found around the site suggest the meteorite, a carbonaceous chondrite, landed on Earth before or during the last ice age, 15,000 years ago.
The natural quasicrystal, an aluminium–copper–iron alloy, looks very much like a material created in a laboratory in 1987 by An-Pang Tsai at Tohoku University in Japan. These lab samples are typically made by slowly cooling the molten alloy, followed by 10 days of annealing at more than 800 °C — a lot of hard and careful work. The natural samples, however, were found embedded in other minerals (including metallic aluminium and silica), and probably formed at high temperatures and pressures — such as, for example, a collision with a meteorite.
This all invites the question of whether we can find other quasicrystals in nature — especially as we don’t understand how atoms come to arrange themselves in this form. “Hopefully, the discovery will now trigger the re-examination of other terrestrial and extraterrestrial minerals,” Steinhardt and Bindi write.
As an aside, Steinhardt notes that it’s not clear even today that the structure Dan Shechtman created in 1982 is actually a quasicrystal. The available data still aren’t good enough to prove that, he says, suggesting that some of the heated debate around Shechtman’s finding was entirely justified. It was only after Tsai’s 1987 structure — with the benefit of X-ray microscopy data — that crystallographers accepted that quasicrystals could exist.