Good to Know Series – 07 – Natural Graphite – Part 1: Geological Origin and Formation
After a few blogs focused on coal, it's time to take a look at other carbon materials. Let’s start with graphite—arguably the mother of all carbons. In this first of three short blogs, we’ll briefly explore the geological origin and formation of natural graphite. Later, we’ll look at how it’s characterised and where it’s used.
Graphite is one of the most stable and ordered forms of carbon. Its structure—layers of carbon atoms arranged in a perfect hexagonal lattice—represents an “ideal state” for carbon under certain natural conditions: high temperature and moderate pressure.
Where does it form?
Graphite typically occurs in metamorphic rocks (e.g., marble, gneiss, schist), especially those of sedimentary protoliths (schists), and more rarely in magmatic rocks.
There are two main geological pathways for graphite formation:
Biogenic transformation – conversion of organic matter through regional (amphibolite or granulite facies) or contact metamorphism.
Abiogenic precipitation – crystallisation from carbon-rich fluids (CO₂, CO, and/or CH₄) circulating in the crust.
(See Beyssac and Rumble, 2014 and Luque et al., 1998 for more details).
These processes have occurred throughout geological history, even as far back as the Archean. For example, some of the oldest known graphite, found in Greenland’s Isua belt, is more than 3.8 billion years old (Zulein et al., 2003).
How can we tell the difference?
One way to distinguish between biogenic and abiogenic graphite is through carbon isotope analysis (δ13C) (Luque et al., 1998).
Graphite formed from organic matter (biogenic) usually has lighter isotopic values around –25‰.
Graphite formed from deep carbon-rich fluids (abiogenic) tends to have heavier values—often > –10‰ and sometimes close to 0‰.
That said, some graphite samples can have mixed signatures, especially where fluids circulate through organic-rich sediments. Isotope data must always be interpreted in combination with the rock’s geological context.
Biogenic origin (Metamorphic graphite)
As we’ve seen in earlier coal-related posts, organic matter evolves through coalification. At the end of diagenesis—also called metagenesis—the process of graphitisation begins. Many researchers suggest this starts at the anthracite stage, when the basic structural units (BSUs) begin to grow and align.
The boundary between diagenesis and metamorphism is gradual, but many cite ~200°C and 300 MPa as a rough dividing line. Under increasing pressure-temperature (P–T) conditions, anthracite progresses to meta-anthracite, then semi-graphite, and eventually graphite—the final product of organic metamorphism. This typically forms in medium- to high-grade metamorphic rocks.
Studies have shown that carbon-rich material can begin to crystallise at temperatures as low as 350–420°C if pressure is high enough (0.5–3.0 GPa over 10 million years). Fully ordered graphite typically forms at 450–600°C depending on peak pressure (Nakamura et al., 2020).
Abiogenic origin (Fluid-deposited graphite)
Carbon-bearing fluids—mainly composed of CO₂, CH₄, and H₂O—can originate from:
The breakdown of organic matter,
The devolatilisation of carbonates,
Or from mantle-derived fluids.
Graphite precipitates from these fluids when carbon solubility drops, or when carbon concentration increases. This can happen:
If the fluid cools or enters a more reducing environment (lower solubility), or
If CO₂-rich and CH₄-rich fluids mix, or water is removed from the fluid phase (higher carbon content).
(See Luque et al., 1998; Touret et al., 2019.)
In summary
Graphite forms in different geological settings, but is most common in orogenic belts and metamorphosed sediments—especially in old cratons. Although, it can occur in rocks of any age.
And don’t forget: graphite is not just terrestrial. It’s also been found in meteorites like ureilites and enstatite chondrites. Even more astonishingly, graphite grains with an interstellar origin have been discovered in primitive chondrites (Amari et al., 1990; Storz et al., 2021).
But that’s truly out of this world.
References:
Amari et al., 1990. Interstellar graphite in meteorites. Nature 345, 238-240.
Beyssac and Rumble, 2014. Graphitic Carbon: A ubiquitous, diverse and useful geomaterial. Elements 10, 415-420.
Luque et al., 1998. Natural fluid deposited graphite: mineralogical characteristics and mechanisms of formation. American Journal of Science 298, 471-498.
Nakamura et al., 2020. Pressure dependence of graphitization: implications for rapid recrystallization of carbonaceous material in a subduction zone. Contributions to Mineralogy and Petrology 175:32, 1-14.
Storz et al., 2021. Graphite in ureilites, enstatite chondrites, and unique clasts in ordinary chondrites – Insights from the carbon-isotope composition. Geochimica et Cosmochimica Acta 307, 86-104.
Touret et al., 2019. Vein-type graphite deposits in Sri Lanka: The ultimate fate of granulite fluids. Chemical Geology 508, 167-181.
Zuilen et al., 2003. Graphite and carbonates in the 3.8 Ga old Isua Supracrustal Belt, southern West Greenland. Precambrian Research 126, 331-348.
