Watching How Rare, Meteoric Diamonds Form

A shock collision experiment maps their transition from graphite in real time

When a meteorite containing graphite slams into the earth, the collision’s heat and pressure can transform this form of carbon into a rare and extremely hard type of diamond. Scientists have long debated exactly how this happens at the atomic level. Now researchers can answer some questions after simulating the precise moment of impact and watching this transformation take place in real time.

In a first-of-its-kind collision chamber at Argonne National Laboratory, physicist Yogendra Gupta of Washington State University and his colleagues mimicked a meteorite impact by firing a lithium fluoride bullet at a graphite disk at 5.1 kilometers per second. Extremely bright x-rays “photographed” the event at 150 billion frames per second.

“In the meteorite world, one always asks if this transition from graphite to diamond happens during compression or as a combination of deformation and [stress release] after the shock,” Gupta says. “We show very nicely that it happens during compression.” Specifically, this rare “hexagonal diamond”—so named for its crystal structure—forms at a pressure of 500,000 atmospheres and on the timescale of billionths of a second. This finding suggests that the impact needed to form this kind of diamond might not be as violent as was previously believed.

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