The graphite found in your favorite pencil could have instead been the diamond your mother always wears. What made the difference? Researchers are finding out.
How molten carbon crystallizes into either graphite or diamond is relevant to planetary science, materials manufacturing and nuclear fusion research. However, this moment of crystallization is difficult to study experimentally because it happens very rapidly and under extreme conditions.
In a new study published July 9 in Nature Communications , researchers from the University of California, Davis and George Washington University use computer simulations to study how molten carbon crystallizes into either graphite or diamond at temperatures and pressures similar to Earth's interior. The team's findings challenge conventional understanding of diamond formation and reveal why experimental results studying carbon's phase behavior have been so inconsistent.
Using leading-edge, machine learning-powered molecular simulations, the research team discovered that liquid carbon exhibits far more complex crystallization behavior than previously thought. Most surprisingly, they found that graphite — the soft, pencil-lead form of carbon — can form spontaneously even when diamond should be the stable phase, possibly "hijacking" diamond formation.
"The advantage of simulations is that you can easily realize these extreme conditions without any special machinery," said study senior author Davide Donadio , a professor in the Department of Chemistry at the College of Letters and Science at UC Davis. "Experimentally, it's very difficult to obtain such high temperature and high pressure in a controlled manner and to monitor the crystallization process."
Simulating Earth's interior
In the study, the team provided an atomistic picture of how this process goes, preparing models at various pressures from 5 to 30 gigapascals (GPa) as the molten carbon cooled from 5000 to 3500 Kelvin (K). Donadio noted that such conditions can be obtained in laser heating experiments.
While the team expected to get glassy carbon from the rapid quench of the liquid, they noticed spontaneous crystallization. At high pressures, the liquid carbon crystallized into diamond, and at lower pressures, it crystallized into graphite.
"This was a nice surprise because normally simulating crystallization is much more complicated than that," Donadio said. "You usually need to use some tricks to get the molecular dynamics simulations to crystallize. We were even more amazed to observe graphite crystallizing spontaneously at pressures up to 15 GPa — conditions where diamond should be the stable form."
The unexpected behavior follows a principle known as Ostwald's step rule, which predicts that crystallization sometimes proceeds through intermediate metastable phases rather than directly to the most stable form. The researchers found that graphite acts as a stepping stone in diamond formation because its structure more closely resembles liquid carbon's density and bonding patterns.
"The liquid carbon essentially finds it easier to become graphite first, even though diamond is ultimately more stable under these conditions," said co-author Tianshu Li, a professor of civil and environmental engineering at George Washington University. "It's nature taking the path of least resistance."
Differences in crystallization
Through the simulations, the team also revealed the molecular structures of liquid carbon as it crystallized into graphite and then, separately, as liquid carbon crystallized into diamond. Graphite crystallized in column-like patterns that eventually elongated outwards. Diamond crystallized through compact crystallites.
This research accounts for long-standing discrepancies in high-pressure carbon experiments, providing a new framework for interpreting results that seemed contradictory.
The findings have implications for a variety of areas. They help explain why natural diamond formation is rare and provide new insights into the deep carbon cycle that affects Earth's climate and geology over geological timescales. In materials manufacturing, understanding these crystallization pathways could improve industrial diamond synthesis, particularly for specialized applications like quantum computing, where precise control over crystal structure is essential.
"Crystallization is so fundamental for technology, and diamonds are extremely useful as materials," Donadio said. "The work accounts for the presence of graphite where you might not expect it."
Additional co-authors include Margaret L. Berrens, Wanyu Zhao and Shunda Chen.
The research was supported by grants from the National Science Foundation.
