Unusual diamonds in meteorites may point the way towards manufacture of ultra-hard microscopic machine components
It has been speculated that lonsdaleite, a rare hexagonal form of diamond, is even harder and stronger than diamond itself.
Examples of lonsdaleite that were generated by the partial replacement of folded graphite crystals have been discovered in ureilite meteorites.
Monash University scientists, in collaboration with researchers at RMIT University, CSIRO, the Australian Synchrotron and Plymouth University, have now revealed a novel process for the creation of diamond and lonsdaleite in the natural world.
The research is described today in an international study published in the Proceedings of the National Academy of Sciences (PNAS).
“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,” said lead study author Professor Andy Tomkins, an ARC Future Fellow at Monash University’s School of Earth, Atmosphere and Environment.
“We propose that lonsdaleite in the meteorites formed from a supercritical fluid at high temperature and moderate pressures, almost perfectly preserving the textures of the pre-existing graphite. Later, lonsdaleite was partially replaced by diamond as the environment cooled and the pressure decreased,” he said.
“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 replacement of pre-shaped graphite parts by lonsdaleite.”
In this work, the researchers used cutting-edge electron microscopy and synchrotron techniques to create maps of lonsdaleite, diamond, and graphite found in the meteorites.
Typically containing larger abundances of diamond than any known rock, ureilite meteorites are arguably the only major suite of samples available 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.
“These findings help address a long-standing mystery regarding the formation of the carbon phases in ureilites that has been the subject of much speculation,” Professor Tomkins said.
“And, they offer a novel model for diamond formation in ureilites that settles contradictions in the existing concepts.”