Graphite’s Unexpected Rise: Diamond Formation Rethought

Imagine the carbon in your everyday pencil lead deciding to become something far more precious—a diamond. New research suggests this scenario is not as far-fetched as it seems, challenging long-held beliefs about how diamonds form.

The process by which molten carbon morphs into either graphite or diamond has implications that stretch across fields, from understanding planetary formation to refining industrial processes and even advancing nuclear fusion research. Yet, observing this pivotal moment of crystallization remains a significant hurdle due to its incredibly rapid nature and the extreme conditions required.

A groundbreaking study, published in Nature Communications, by researchers at the University of California, Davis, and George Washington University, sheds new light on this phenomenon. By employing advanced computer simulations, the team explored the crystallization pathways of molten carbon under conditions mimicking those found deep within Earth. Their findings present a novel perspective on diamond formation, potentially resolving inconsistencies in previous experimental results.

Fueled by state-of-the-art, machine learning-enhanced molecular simulations, the research team uncovered a surprisingly complex crystallization behavior in liquid carbon. In what they describe as an Unexpected Anomaly, they observed the spontaneous formation of graphite—the soft, familiar form of carbon in pencils—even under conditions where diamond, the much harder and more stable form, was expected. The team suggests graphite could be “hijacking” the diamond formation process.

The researchers highlight this pivotal insight:

“The conventional wisdom said diamond formation should be straightforward under certain conditions, but our simulations show that’s not always the case.”

Simulating Earth’s interior

The research team crafted detailed atomistic models simulating molten carbon’s cooling process under varying pressures, ranging from 5 to 30 gigapascals (GPa), and temperatures from 5,000 to 3,500 Kelvin (K). Davide Donadio of UC Davis noted these conditions can be reproduced in laser heating experiments, enhancing the study’s real-world applicability.

Initially, the researchers anticipated the formation of glassy carbon from the rapid cooling. However, they witnessed spontanous crystallization instead. Under high pressure, liquid carbon solidified into diamond, while lower pressures resulted in graphite crystallization.

“This was a nice surprize 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.”

This unexpected behavior aligns with Ostwald’s step rule, a principle predicting that crystallization can proceed through intermediate, metastable phases before reaching the most stable form. In this context, the researchers propose that graphite acts as a stepping stone in diamond formation because its structure more closely resembles the density and bonding patterns of liquid carbon.

“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 resistence.”

One local gemologist, speaking anonymously, commented on the findings, stating it might explain some of the imperfections seen in lab-grown diamonds, often attributed to other causes.

Immediate Reaction to these findings has been widespread within the materials science community, triggering debates and inspiring follow-up experiments.

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1. Graphite as a Stepping Stone: Graphite forms as an intermediate phase during diamond crystallization due to structural similarities with liquid carbon.
2. Pressure-Dependent Crystallization: High pressures favor diamond formation, while lower pressures lead to graphite crystallization.
3. Machine Learning Simulations: Cutting-edge simulations provided a detailed atomistic picture of the crystallization process.
4. Reconciling Experimental Discrepancies: The findings offer a framework for interpreting previously contradictory high-pressure carbon experiments.

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Differences in crystallization

The simulations unveiled distinct molecular structures as liquid carbon crystallized into graphite and diamond, respectively. Graphite displayed column-like patterns that progressively elongated outwards, whereas diamond formed through compact crystallites.

These new insights could resolve persistent discrepancies in high-pressure carbon experiments. The findings provide a novel framework for understanding previously contradictory results, potentially leading to more precise interpretations of carbon’s behavior under extreme conditions. For those working in the field, this provides an interesting challenge.

The implications of this research extend to diverse fields. It offers insight into why natural diamond formation is relatively rare, contributing to a deeper understanding of the deep carbon cycle and its influence on Earth’s climate and geology over geological timescales. In the realm of materials manufacturing, a clearer understanding of these crystallization pathways may optimize industrial diamond synthesis, particularly for specialized applications such as quantum computing, where precise control over crystal structure is crucial. “Life would never be quite the same,” said a local physicist, reflecting on the potentially broad impacts of the study.

“Crystallization is so fundemental 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.

One commentator on X.com said, “If graphite can ‘hijack’ diamond formation, what other surprising transformations are hidden in the materials around us?”

Lingering Question: Could manipulating the crystallization pathway of carbon revolutionize materials science?