Diamond grains from the Canyon Diablo meteorite
November 25, 2014

Diamonds From Canyon Diablo Meteorite Are A Structurally Disordered Form Of Ordinary Diamond

April Flowers for redOrbit.com - Your Universe Online

Approximately 50 years ago, a large meteorite, named Canyon Diablo after the crater formed by the meteorite's impact in northern Arizona, was found to contain a new form of diamond with a hexagonal structure. The scientists who found the diamond characterized it as an impact-related mineral and named it lonsdaleite for famed crystallographer Dame Kathleen Lonsdale.

Since the discovery, debate has raged in the scientific community over whether lonsdaleite is, in fact, a new type of diamond. A new study led by Arizona State University (ASU) and published in Nature Communications reveals that it is, instead, a structurally disordered form of ordinary diamond.

"So-called lonsdaleite is actually the long-familiar cubic form of diamond, but it's full of defects," said Péter Németh, a former ASU visiting researcher (now with the Research Centre of Natural Sciences of the Hungarian Academy of Sciences), in a recent statement. Such defects can occur due to shock metamorphism, plastic deformation or unequilibrated crystal growth. Németh collaborated with ASU's Laurence Garvie, Toshihiro Aoki and Peter Buseck, plus Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth in Germany.

In the fifty years since the initial identification, lonsdaleite has been used as an indicator of ancient astroidal impacts on Earth, including those linked to mass extinctions. Scientists have also believed that the hexagonal diamond had mechanical properties superior to an ordinary diamond. This would give the mineral a very high potential industrial significance. The problem is that pure examples of the crystal have never been found, or synthesized, creating a mystery.

The ASU-led team re-examined diamonds from Canyon Diablo and laboratory samples prepared under conditions in which the mineral has been reported. The team used advanced electron microscopes in ASU's Center for Solid State Science to discover that both types of samples — natural and synthesized — contained new types of diamond twins and nanometer-scale structural complexity that give rise to features attributed to lonsdaleite.

"Most crystals have regular repeating structures, much like the bricks in a well-built wall," said Peter Buseck. Interruptions in regularity, called defects, can occur, however. "Defects are intermixed with the normal diamond structure, just as if the wall had an occasional half-brick or longer brick or row of bricks that's slightly displaced to one side or another."

The researchers found that lonsdaleite is actually the same as the regular cubic form of diamond, but it has been subjected to shock or pressure that resulted in defects in the crystal structure. This finding means that many past studies based on the idea that lonsdaleite is a new, separate type of diamond need to be re-examined.

The findings also reveal that both shock and static compression can produce defective diamond structures, which can sometimes, as in the case of the Canyon Diablo diamonds, have interesting mechanical properties that are candidates for products with exceptional hardness.

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