Thematic Articles

Fullerenes, carbon nanotubes, and graphene are nanometer-sized forms of carbon with the properties of almost ideal low-dimensional systems. These systems have been at the center of exceptionally intense scientific interest. They have been considered not only as objects of fundamental research but also as components in a wide range of possible applications. In popular science, their names are synonymous with nanotechnology.

Graphitic carbon spherules found in primitive meteorites have large carbon isotope anomalies, indicating that they are carbonaceous stardust (also known as presolar grains) expelled from dying stars prior to the formation of the Sun. Presolar spherules show varying degrees of graphitization, ranging from poorly graphitic, turbostratic layers in low-density spherules to well-crystallized graphitic outer shells in high-density ones, and some spherules also contain a polycrystalline phase in their core. Within the spherules, grains of other refractory phases (including carbides and metals) are common, and these assemblages can be studied as one would study a rock. The isotopic and microstructural information available from these presolar graphitic assemblages gives insights into nucleosynthesis and grain condensation in late-stage carbon-rich stars.

The unambiguous identification of graphitic carbons as remains of life in ancient rocks is challenging because fossilized biogenic molecules are inevitably altered and degraded during diagenesis and metamorphism of the host rocks. Yet, recent studies have highlighted the possible preservation of biosignatures carried by some of the oldest graphitic carbons. Laboratory simulations are increasingly being used to better constrain the transformations of organic molecules into graphitic carbons induced by sedimentation and burial processes. These recent research advances justify a reevaluation of the putative biogenicity of numerous ancient graphitic carbons, including the presumed oldest traces of life on Earth.

Graphitic carbon deposited from hydrothermal fluids occurs globally, in rocks from all depths in Earth’s crust and ranging in age from Precambrian to Tertiary. The varieties of deposits include graphitic cones and “artichokes” filling rock pores, explosively injected veins, graphitic pegmatites with platinum-bearing ores, and isochemical–“iso-isotopic” reactions of calcite + quartz to form graphite + wollastonite. In many deposits, carbon’s structure attains well-ordered, nearly perfect graphite crystallinity. The carbon isotope composition of hydrothermal graphitic material ranges widely, from that of biogenic organic debris to that of biogenic carbonate minerals, and overlaps the isotopic composition of mantle carbon as measured in diamonds.

Organic compounds, which on Earth originate mainly through biological activity, are transformed under the physical conditions of Earth’s crust, with the end product being graphite. In this graphitization process, they pass progressively and irreversibly through a wide variety of intermediate macrostructures and nanostructures before finally attaining the stable graphite structure. Characterizing this rich array of carbon structures, which are also of industrial interest, provides valuable information on the geological processes affecting carbon-bearing rocks. These processes impact global energy supplies, the geophysical behavior of the crust, and the habitability of the surface environment.

Graphitic carbon, with its diverse structures and unique properties, is everywhere at Earth’s surface. Strategically located at the interface between the lithosphere, biosphere, hydrosphere, and atmosphere, graphitic carbon constitutes a major terrestrial carbon reservoir. Natural and synthetic graphitic carbon is also used in a broad range of applications, and graphitic carbon, so widely varied in its physical properties, has proven to be adaptable to many uses in society. Graphitic carbon has played an important role in human history (for example, coal mining) and is now a building block of nanotechnology, but this remarkable material is also an active player in geological processes.