Scientists discover how rare diamond shapes are created in outer space. Found in four meteorites in northwestern Africa, the hexagonal-shaped diamond does not appear to have formed naturally on Earth. They came from an unknown ancient planet that was long dead.
Billions of years ago, there was a dwarf planet in the inner Solar System that had carbon in its mantle. After a catastrophic collision with a large asteroid, that carbon was compressed into lonsdaleite, where the carbon atoms are arranged in a hexagonal lattice instead of the usual cubic diamond structure.
These lonsdaleite crystals are trapped inside ureilite meteorites, a rare form of rocky space rock rich in carbon, usually graphite and nanodiamonds.
In a new paper in the Proceedings of the National Academy of Sciences, researchers make connections between all these different carbon-based minerals that suggest that diamonds formed from the mantles of long-dead dwarf planets.
They assumed graphite turned into diamond and lonsdaleite. Graphite is made of layers of carbon arranged in a hexagonal lattice. The research team believes in the impact of creating a supercritical fluid made of carbon, hydrogen, oxygen and sulfur.
These impacts interact with graphite at high temperatures and moderate pressures allowing the carbon to maintain a hexagonal distribution of graphite, but in 3D space, not in 2D layers.
"Then, lonsdaleite was partially replaced by diamond as the environment cooled and the pressure decreased," said lead author Professor Andy Tomkins of Monash University, quoted by IFL Science.
The team used advanced electron microscopy techniques to study the meteorite piece by piece. This allowed them to create a series of photographs that tell the story of how lonsdaleite was formed and how it was partially replaced by diamond and graphite.
"We have also found the largest lonsdaleite crystal known to date measuring down to one micron, much thinner than a human hair," said senior author Professor Dougal McCulloch of RMIT University.
Lonsdaleite is believed to be much harder than diamond due to its structure. But it was difficult to test it until now because naturally occurring examples are so few.
However, as McCulloch points out, four African meteorites feature crystals down to one micrometer.
Nature has given us processes to try and replicate in industry. "We think lonsdaleite can be used to make small, ultra-hard machine parts if we can develop industrial processes that encourage replacement of graphite parts before they are formed by lonsdaleite," explains Tomkins.
The name Lonsdaleite is used in honor of the pioneer of British crystallography Kathleen Lonsdale. Together with biochemist Marjory Stephenson, they were the first two women to be elected to the Royal Society in 1945.