Asteroids make bizarre diamonds on Earth

Washington: Asteroid impacts on Earth may give rise to rare, structurally bizarre diamonds on our planet, a new study suggests.

Scientists have settled a longstanding controversy over a purported rare form of diamond called lonsdaleite – formed by impact shock, but which lacks the three-dimensional regularity of ordinary diamond.

A group of scientists has now shown that what has been called lonsdaleite is in fact 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 Peter Nemeth, a former Arizona State University (ASU) visiting researcher.

These can occur, he said, due to shock metamorphism, plastic deformation or unequilibrated crystal growth.

Scientists said that a large meteorite, called Canyon Diablo after the crater it formed on impact in northern Arizona, contained a new form of diamond with a hexagonal structure.

They described it as an impact-related mineral and called it lonsdaleite, after Dame Kathleen Lonsdale, a famous crystallographer.

Since then, “lonsdaleite” has been widely used by scientists as an indicator of ancient asteroidal impacts on Earth, including those linked to mass extinctions.

In addition, it has been thought to have mechanical properties superior to ordinary diamond, giving it high potential industrial significance.

All this focused much interest on the mineral, although pure crystals of it, even tiny ones, have never been found or synthesised.

Scientists re-examined Canyon Diablo diamonds and investigated laboratory samples prepared under conditions in which lonsdaleite has been reported.

Using the advanced electron microscopes, the team found, both in the Canyon Diablo and the synthetic samples, new types of diamond twins and nanometre-scale structural complexity.

These 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, from the University of Bayreuth in Germany.

However, interruptions can occur in the regularity, and these are called defects.

“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,” said Buseck.

The outcome of the new work is that so-called lonsdaleite is the same as the regular cubic form of diamond, but it has been subjected to shock or pressure that caused defects within the crystal structure.

The study was published in the journal Nature Communications.