Diamond, with its tough-to-break carbon lattice of interlocking cubes, is traditionally considered the hardest material on Earth. Yet a rare form of diamond known as lonsdaleite—a crystal with carbon atoms arranged in flexing three-dimensional hexagons—may be even harder than its cubic cousin.
To date, natural lonsdaleite has been found only in impact craters, where it has formed by the intense pressure of meteorites crashing to Earth. But now researchers say that they’ve identified lonsdaleite crystals that formed billions of years before the meteorites carrying them ever reached the planet. If the team’s theory of the crystals’ formation is correct, its findings could offer scientists a better way to manufacture the ultra hard substance on Earth.
For a study published this month in the Proceedings of the National Academy of Sciences USA, the research team, primarily based in Australia, examined 18 different meteorite samples from a family known as “ureilites.” Because ureilites are relatively homogeneous in their chemical composition—which is uniquely rich in carbon—scientists theorize they originate from the same parent body.
“There was this dwarf planet just after the start of our solar system—so 4.5 billion years ago—and the planet got hit by an asteroid,” says Alan Salek, a graduate student in applied physics at the Royal Melbourne Institute of Technology in Australia and co-author of the new study. This cataclysmic impact tore the dwarf planet apart, sparking a chemical reaction that could have turned pieces of the planet’s graphite into lonsdaleite, he adds.
Graphite is made up of flat layers of carbon atoms bonded together as hexagons. These stacked layers are weakly attracted to each other and relatively easy to pull apart. On Earth, high heat and pressure can rearrange these carbon atoms into a 3-D lattice of cubes, thereby creating the traditional kind of diamond. But a brief period of extremely intense pressure—such as that of a meteorite impact—can preserve graphite’s original hexagonal arrangement while its layers bond into the strong 3-D lattice of lonsdaleite.
The researchers propose that rather than the rapid impact pressure known to produce tiny lonsdaleite crystals on Earth, these samples instead formed through a rapid release of pressure. They claim that a fluid mix of carbon, hydrogen, oxygen and sulfur was heated and pressurized in the dwarf planet’s mantle until an asteroid impact smashed that mantle into pieces. Study co-author Andrew Tomkins, a geologist at Monash University in Australia, says that the rapidly depressurizing mix of chemicals could have interacted with the dwarf planet’s graphite to transform it into lonsdaleite.
In this particular reaction, graphite crystals would have been essentially torn apart and rebuilt into lonsdaleite. “It’s called ‘coupled dissolution-reprecipitation’ because it’s kind of dissolving this thing and replacing it at the same time,” Tomkins says. This fluid-driven reaction took place in chunks of the dwarf planet as they went flying into space. And much like escape vessels from the planet Krypton, those chunks eventually carried their precious cargo all the way to Earth.
The researchers landed on this lonsdaleite origin story through painstaking analysis of their 18 ureilite samples. Tomkins explains that the structure of these meteorites’ minerals indicates a rapid cooling process that points to a dramatic collision. Looking at particular radioactive signatures of the minerals, the researchers estimated a date for this collision—roughly 4.5 billion years ago. Plus, the samples contain interlocking layers of lonsdaleite, cubic diamond and graphite in a pattern that points to the fluid-driven transformation Tomkins’s team describes.
Outside researchers note that this is only one potential explanation for the presence of lonsdaleite in these meteorites. “I think the formation method presented here is logical and may be one possible pathway to form this material, but I admit I am not 100 percent convinced,” says Jodie Bradby, who researches high-pressure physics at the Australian National University but was not involved in the study. “I hope this paper prompts more theoretical and modeling studies in this area,” she adds.
Dominik Kraus, a high-energy-density physicist at the University of Rostock in Germany, who was also not involved in the study, feels similarly. “To me, it looks kind of like a Goldilocks scenario at the moment—everything has to be just right,” he says.
To verify the formation method, Kraus says, researchers will need to replicate it: a key next step is “to mimic these conditions ... and see if we can really grow these hexagonal diamond crystals somewhat effectively on this regime.”
Kraus and Bradby have both been involved in research projects that successfully created synthetic lonsdaleite by subjecting other forms of carbon to intense pressure, much like what occurs during a meteorite impact. And yet, like the lonsdaleite found in meteorite craters, these synthetic lonsdaleite crystals tend to be tiny—on the scale of nanometers. (That’s one billionth of a meter.)
The specks of lonsdaleite that Salek and Tomkins’s research group identified were up to a micron in size—still extremely small but roughly 1,000 times larger than any lonsdaleite crystals previously known. This suggests that a fluid-driven transformation of graphite into lonsdaleite might produce bigger crystals than the impact method.
If Salek and Tomkins’ team can re-create their theoretical transformation process in a lab, it wouldn’t just be a win for their theory but a win for materials science at large. Lonsdaleite has never been obtained in samples large enough to test its true hardness (or to evaluate its true color, which remains uncertain). But mathematical models of lonsdaleite’s structure indicate that the substance could be up to 58 percent harder than cubic diamond.
“A material like this, that could practically be undamageable by anything on Earth, would be pretty handy to have,” Salek says. It could replace diamond on saw blades and drill bits, for instance, or function as tiny, extremely durable electronic components. Bradby adds that lonsdaleite could help cut regular diamonds with relative ease.
Still, re-creating the exact conditions described in this paper will be a challenge. Researchers will have to produce extreme temperatures, elevated pressures and a precise mix of the chemicals that supposedly catalyzed the transformation of graphite into lonsdaleite. It’s doable, at least in theory. But even if such a process successfully produces the crystal, it’s unclear how difficult or expensive it would be to scale up to industrial-level output.
“It’s a long road ahead before we get a lonsdaleite ring on a finger,” Salek says.