Study unlocks hidden secrets of how ultra-hard diamonds formed in rare meteorites

NWA 4231 meteorite fragment classified as ureilite (Credit: James St John via Wikimedia Commons)

It has been speculated that lonsdaleite, a rare hexagonal form of diamond, is even harder and stronger than diamond itself.
Now an international team of scientists has discovered how the substance forms in ureilite meteorites, and suggested how that process could be replicated on Earth.
By using cutting-edge electron microscopy and synchrotron techniques they were able to create maps of the lonsdaleite, diamond, and graphite found in ureilite – a rare type of stony meteorite.
This showed the lonsdaleite formed from a supercritical fluid at high temperature and moderate pressures, which almost perfectly preserved the textures of the pre-existing graphite, but it was then partially replaced by diamond as the environment cooled and the pressure decreased.
Writing in Proceedings of the National Academy of Sciences (PNAS), researchers say the current method for producing industrial diamonds involves chemical vapour deposition, in which diamonds are formed onto a substrate from a gas mix at low pressures.
However, they say nature has now provided a process of creating lonsdaleite and, if it can successfully replicated in industry, the substance could be used to make tiny, ultra-hard machine parts and replace the pre-shaped graphite parts used across the world today.
The research was led by scientists at Monash University in Australia, in collaboration with researchers at the University of Plymouth, RMIT University, CSIRO, and the Australian Synchrotron.
Professor Andy Tomkins, an ARC Future Fellow at Monash University’s School of Earth, Atmosphere and Environment, is the study’s lead author. He said:
“These findings help address a long-standing mystery regarding the formation of carbon phases in ureilites that has been the subject of much speculation. They offer a novel model for diamond formation in ureilites that settles contradictions in the existing concepts. Typically containing larger abundances of diamond than any known rock, ureilite meteorites are arguably the only major suite of samples we have from the mantle of a dwarf planet.
"The parent asteroid was catastrophically disrupted by a giant impact while the mantle was still very hot, creating the ideal conditions for lonsdaleite then diamond growth as the pressure and temperature decreased in a fluid and gas-rich environment.”

Co-author Dr Natasha Stephen, Director of the Plymouth Electron Microscopy Centre at the University of Plymouth, was involved in the analytical side of the research using electron microscopes to analyse meteorite samples. She said:
“Ureilite meteorites are one of the most abundant sources of diamond in the Solar System. However, any occurrences are typically at the micro – and even nano – scale, so they can be hard to see. Using advanced analytical microscopy, we can not only identify these tiny crystals in their host rocks but also probe their precise chemistry and structure, therefore unravelling their formation and evolution. This led to the identification of lonsdaleite and a new method of formation, which could potentially be replicated on Earth in order to synthetically produce this industrial mineral on a commercial scale.”

<p>Dr Natasha Stephen</p>

Dr Natasha Stephen

<p>A fragment of the Winchcombe meteorite (Credit: Trustees of the Natural History Museum)<br></p>

A fragment of the Winchcombe meteorite (Credit: Trustees of the Natural History Museum)

Last year, the Plymouth Electron Microscopy Centre was used to analyse the origins and structure of the Winchcombe meteorite, which fell in and around the Gloucestershire town in early 2021. Dr Stephen added:
“The use of electron microscopy within meteorite studies is unparalleled in analytical science. The non-destructive nature of most EM techniques means we can study these extra-terrestrial objects in unprecedented detail (up to several million times magnification), revealing what they are made of and how they formed. However, we can also do so without damaging these rare samples, ensuring they are preserved for future science with new technological advances, and for future generations to enjoy.”

  • The full study – Tomkins et al: Sequential lonsdaleite to diamond formation in ureilite meteorites via in situ chemical fluid/vapour deposition – is published in PNAS, DOI: 10.1073/pnas.2208814119.

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