Billions of years ago, at the dawn of our solar system, an asteroid smashed into a dwarf planet in a cataclysmic collision that blasted the insides of the planet into outer space. Over time, remnants of the dwarf planet’s mantle have fortuitously fallen to Earth as diamond-rich meteorites, called ureilites, that reveal an unprecedented glimpse into the subterranean layers of a doomed ancient world.
For years, scientists have puzzled over the fallen remains of the long-lost planet and the mysterious presence of its abundant diamonds, which include hints of lonsdaleite, an ultra-rare type of diamond named after the pioneering crystallographer Kathleen Lonsdale.
Now, scientists led by Andrew Tomkins, a professor of geosciences at Monash University, have found the largest lonsdaleite diamonds ever seen in ureilites, unambiguously confirming their existence in the meteorites. The team proposed “a unique model for ureilites that can reconcile all conflicting observations relating to diamond formation,” and which may ultimately inspire novel industrial applications, according to a study published on Monday in Proceedings of the National Academy of Sciences.
“We had known about diamonds on ureilites for some time,” Tomkins told Motherboard in an email, citing a recent study that probed the origins of these materials. “While I was doing that work, I was looking down my microscope at the diamonds in various samples and noticed that one of them contained some unusual folded diamonds. For a geologist that was completely weird—diamond is the hardest material known, so it shouldn't be possible to bend it.”
“That got me interested in following down that line of research,” he continued, prompting the involvement of his co-authors at the Commonwealth Scientific and Industrial Research Organization and the Royal Melbourne Institute of Technology. “We went from there to produce the story we have. Those folded diamonds turned out to be an unusual hexagonal form of diamond known as lonsdaleite.”
While previous studies have found evidence of lonsdaleite in ureilites, Tomkins and his colleagues used advanced electron microscopy to detect lonsdaleite crystals that are up to a micron in size, which is about 70 times smaller than the width of a human hair. While this may seem tiny, these lonsdaleite chunks are bigger than anything seen previously in the space rocks.
This level of detail helped the researchers come up with what the study calls “the only known solution” to the problem of how lonsdaleite and diamonds were forged after the ancient impact, without the presence catalyzing metals, according to the study. The team suggested that lonsdaleite sprung from hot fluid flowing in the wake of the collision, which eerily preserved the properties of the pre-existing graphite in the mantle.
Some of the lonsdaleite was replaced by conventional cubic diamonds as the rocks cooled, creating the distinct patterns seen in ureilites, according to the new model. The process, called chemical vapor deposition (CVD), is like a natural version of a manufacturing technique used to produce synthetic diamonds under laboratory conditions.
“I had thought early on that the diamonds might have possibly formed via CVD, but when we found the lonsdaleite, and then found a progression from lonsdaleite-to-diamond formation, there had to be an explanation that fitted in with the known evolution of the ureilite parent asteroid,” Tomkins said. “This is the fit that made the best sense with the observations we had.”
“It adds to the story of the complex chemistry that happened in the aftermath of the catastrophic disruption event,” he noted. “Scientists have known about this event for a long time, but nobody had realized that this is when the diamonds and lonsdaleite probably formed.”
In addition to shedding new light on these naturally forming space diamonds, the results may inspire creative approaches to making synthetic lonsdaleite and diamonds. Given that lonsdaleite is even harder than normal diamond, it has many potential applications in materials science, including cogs in miniature machines. This is far from the first time that diamonds forged in space have served as models for industrial manufacturing; scientists recently transformed plastic into diamonds by recreating the conditions inside Uranus and Neptune.
“What we've basically found is a situation where some graphite shapes—the fold shapes—were replaced with lonsdaleite that almost perfectly preserved that same shape,” Tomkins said. “So the thought is that in the lab it might be possible to replicate that natural process. It should be possible to fabricate components from very soft graphite and then turn those shapes into lonsdaleite. It's really up to engineers to find uses for such a technique—there must be many possible uses.”
“The next plan is to start conducting experiments that try to replicate the natural process that formed the lonsdaleite,” he concluded. “Who knows, maybe we'll be the first to figure out how to make these ultrahard micro-machine components.”