Strange diamonds from an ancient dwarf planet could lead to ultra-hard materials

A research team from Monash University, RMIT University, CSIRO, Australian Synchrotron, and Plymouth University found evidence that a particular type of diamond may have formed shortly after an ancient dwarf planet had collided with a large asteroid about 4.5 billion years ago.

The research team has confirmed that they have found lonsdaleite, a rare hexagonal form of diamond in ureilite meteorites from the mantle of the dwarf planet. Lonsdaleite is named after a famous British pioneering crystallographer, Dame Kathleen Lonsdale, who was the first woman elected as a fellow to the Royal Society.

The research team published their findings in the Proceedings of the National Academy of Sciences (PINAS). This study was led by Andy Tomkins, professor of geology at Monash University.

One of the team, Dougal McCulloch, from RMIT University, said the team had thought lonsdaleite was a harder diamond than the diamonds we see here on Earth because of its hexagonal shape. Our diamonds have a cubic shape.

McCulloch, Director of the RMIT Microscopy and Microanalysis Facility, said in a statement, “This study proves categorically that lonsdaleite exists in nature. We have discovered the largest lonsdaleite crystals known to date that are up to a micron in size. Much, much thinner than a single human hair.”

The team also stated that the differing structure of the lonsdaleite diamond could be a way to enhance manufacturing techniques for ultra-hard materials in mining applications.

Origins of these mysterious diamonds

McCulloch and the RMIT team made up of Dr. Mathew Field, and Ph.D. scholar Alan Salek used advanced electron microscopy to take intact slices and solid samples from the ureilite meteorites. They create snapshots of how lonsdaleite and Earth’s diamonds form.

McCulloch reported “There’s strong evidence that there’s a newly discovered formation process for the lonsdaleite and regular diamonds, which is like a supercritical chemical vapor deposition process that has taken place in this space rocks, probably in the dwarf planet shortly after a catastrophic collision. Chemical vapor deposition is one of the ways that people make diamonds in the lab, essentially by growing them in a specialized chamber.”

The team proposed that the shape of the texture of the preexisting graphite was preserved when the lonsdaleite diamonds formed from a supercritical fluid at high temperature and moderate pressure.

Tomkins, an ARC Future Fellow at Monash University’s School of Earth, Atmosphere, and Environment, went on to state, “Later, lonsdaleite was partially replaced by diamond, as the environment cooled, and pressure decreased. Nature has thus provided us with a process to try and replicate in industry. We think that lonsdaleite could be used to make tiny, ultra-hard machine parts if we can develop an industrial process that promotes the replacement of the pre-shaped graphite parts by lonsdaleite.”

Tomkins said the study addresses a mystery regarding the formation of the carbon phases in ureilites, which had been a long-standing issue under study.

The study was published in the journal Proceedings of the National Academy of Sciences.

Abstract:

Ureilite meteorites are arguably our only large suite of samples from the mantle of a dwarf planet and typically contain greater abundances of diamond than any known rock. Some also contain lonsdaleite, which may be harder than diamond. Here, we use electron microscopy to map the relative distribution of coexisting lonsdaleite, diamond, and graphite in ureilites. These maps show that lonsdaleite tends to occur as polycrystalline grains, sometimes with distinctive fold morphologies, partially replaced by diamond + graphite in rims and cross-cutting veins. These observations provide strong evidence for how the carbon phases formed in ureilites, which, despite much conjecture and seemingly conflicting observations, has not been resolved. We suggest that lonsdaleite formed by pseudomorphic replacement of primary graphite shapes, facilitated by a supercritical C-H-O-S fluid during rapid decompression and cooling. Diamond + graphite formed after lonsdaleite via ongoing reaction with C-H-O-S gas. This graphite > lonsdaleite > diamond + graphite formation process is akin to industrial chemical vapor deposition but operates at higher pressure (∼1–100 bar) and provides a pathway toward manufacture of shaped lonsdaleite for industrial application. It also provides a unique model for ureilites that can reconcile all conflicting observations relating to diamond formation.