A team of researchers says it has found in a Russian mineral sample the first natural example of a quasicrystal, an unusual material that displays some of the properties of a crystal but boasts a more intricate and complex structure. Since quasicrystals were characterized 25 years ago, numerous versions have been cooked up in the laboratory, but a natural example would indicate that nature's products are more diverse than previously thought.
Quasicrystals display ordered arrangements and symmetries but are not periodic—that is, they are not defined by a single unit cell (such as a cube) that simply repeats itself in three dimensions. The term "quasicrystal" was coined by physicists Dov Levine and Paul Steinhardt, both then at the University of Pennsylvania, to describe the class of quasiperiodic crystals in 1984, shortly after another group published observational evidence for such a material.
The new paper, published in this week's Science, was co-authored by Steinhardt, now at Princeton University,* who says he has been on the hunt for a naturally occurring quasicrystal ever since. The find could force a redefinition of minerals to include such quasicrystals.
To locate the sample, Steinhardt and his colleagues examined substances chemically similar to quasicrystals that had already been synthesized in the lab. That search led them to khatyrkite, a mineral that had reportedly been found in the Koryak Mountains of Russia. A khatyrkite-bearing sample at the University of Florence in Italy was also found to contain granules of an alloy of aluminum, copper and iron that fit the quasicrystal bill.
But the origin of such would-be minerals is a point of some contention among petrologists, scientists who study the structure and formation of rocks. Aluminum alloys do not form easily by natural processes, because the element reacts with oxygen so readily.
The possibility that the quasicrystal and its related materials, including khatyrkite, "are of man-made origin needs to be weighed very carefully before [they] are accepted uncritically as minerals," says Eric Essene, a professor emeritus of geological sciences at the University of Michigan at Ann Arbor. "The case for them as synthetic materials as opposed to natural minerals has not been considered adequately." Aluminum smelting is one human process that might produce such a substance.
The authors concede that resolving the conditions of the quasicrystal's geologic formation "remains a serious and fascinating challenge" but contend that the complex and diverse collection of minerals in the sample points to a natural origin. But Essene notes that with the benefit of high temperatures and pressures, ceramists and experimental petrologists "have no difficulty in making complex assemblages" of synthetic materials in the lab.
Steinhardt says that he and his colleagues continue to consider the various processes that could have formed the sample. "As is often the case for minerals, it is a lot easier to identify and characterize the mineral than it is to explain how it formed," he says. "But we are working very hard at it because it may prove to be interesting both for geology and materials science."
Ron Frost, a petrologist at the University of Wyoming, says that how the quasicrystal granules formed is an open question. But he notes that serpentinites such as those found in association with khatyrkite are "weird rocks" that often harbor equally weird minerals, many of which do not exist elsewhere. "I don't see anything here that makes me say 'Hell no!'" Frost says, "and I have seen enough strange stuff in serpentinites to accept this as just another example."
*This sentence was edited after publication to add Steinhardt's current affiliation.