- © 2014 by the Mineralogical Society of America
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.
Near the end of the 18th century, the Swedish chemist C. W. Scheele (1742–1786), famous for his involvement in the discovery of oxygen, demonstrated that the marking material used in his pencil was composed of carbon, not lead. “Plumbago,” the former name of graphite, reflected the belief that it was a form of lead. The name graphite was derived from the Greek word γρὰϕειν (to write) by the German mineralogist A. G. Werner (1749–1817). Graphite's crystal structure was described in the early 20th century (Bernal 1924), whereas the first structural characterization of graphitic carbon by X-ray diffraction was achieved by Rosalind Franklin (1920–1958) just before her pioneering work on the DNA structure (Franklin 1951).
Graphitic carbon exhibits a large range of structures and chemical compositions, from amorphous-like compounds (e.g. soot, low-grade coal), through a myriad of turbostratic structures (e.g. carbonaceous materials in metamorphic rocks), to crystalline graphite. The widely varying structure and chemistry of graphitic carbon control the remarkably diverse range in its physical properties. Throughout human history, graphitic carbon's properties have been exploited in many useful applications—as a raw energy source, for ornamentation of pottery, as a writing tool, and to strengthen metallurgical crucibles. Nowadays, graphitic carbon is a strategic resource for various applications and is also a source of graphene, a wonder of 21st-century technology.
Natural crystalline graphite sensu stricto is rare and, instead, graphitic carbon constitutes the main carrier of organic carbon in Earth's lithosphere. Graphitic carbon is also present in the universe as presolar grains synthesized by ancient stars before the formation of the Solar System. The occurrence of graphitic carbon in the lithosphere gives it a pivotal role in the carbon cycle at the interface between the solid Earth, the atmosphere, and the biosphere. Owing to its unique properties, graphitic carbon also plays important roles in many geological processes and may be used as tracer for such processes.
In this article, we summarize the structure and chemistry of graphitic carbon. We review the major properties of graphitic carbon, with special emphasis on graphite, and we show how these properties determine its use in countless industrial and technological applications. Then, we briefly describe the role of graphitic carbon in the long-term carbon cycle, and we discuss some geological processes where it plays an important role. Finally, we introduce the articles in this issue of Elements dedicated to graphitic carbon.
WHAT IS GRAPHITIC CARBON?
Carbon atoms can vary the configuration of their valence orbitals (2s, 2p) to form hybrid orbitals (sp1, sp2, sp3). This hybridization of carbon atoms makes possible the formation of various kinds of chemical bonds (σ or π) and is responsible for the structural and chemical diversity of graphitic carbon. Carbon with sp3 hybridization is found in diamond and in branched or linear chains (aliphatic compounds), which are present in hydrocarbons, while carbon with sp2 hybridization is characteristic of unsaturated chains and aromatic compounds. Aromatic carbon is composed of 6-membered rings of carbon atoms, each sharing one electron in the ring. These rings are observed in polyaromatic hydrocarbons (PAHs), in various organic molecules (e.g. benzene), and in graphitic carbon. In this issue, the term graphitic carbon is used broadly to describe all solid carbonaceous compounds that have a multiscale organization based on a skeleton composed of aromatic carbon atoms (Fig. 1), including coals, kerogens, and graphite, regardless of origin. In the literature, the term graphite is often used to describe compounds that are not perfectly crystalline and that should rather have been named graphitic carbon.
Graphitic carbon exhibits specific spectral features in its Raman, infrared, and XANES spectra due to aromatic carbon (Bernard et al. 2010). The nanostructure is built of units composed of aromatic layers in which aromatic rings are organized in a honeycomb structure, and these layers have various lateral extents depending on the number of carbon atoms involved. In the case of graphite, for example, its planar aromatic layers are one atom thick, extend infinitely laterally, and are called graphene. These layers may be variably stacked along the c-axis, with long-range interactions (Van der Waals type) between them, to form coherent domains that diffract X-rays and electrons. Different parameters can be used to describe this nanostructure: La, the in-plane extent of aromatic layers; n, the number of stacked layers; Lc, the length of the coherent domains along the c-axis; and d002, the distance between aromatic layers in coherent domains (Fig. 1). In three-dimensions, the respective arrangements of these aromatic layers can define a nano- to macroporosity. Graphitic carbon may be amorphous-like or turbostratic, with an imperfect stacking of the aromatic layers. The ideal crystalline structure of graphite is defined by an ABAB stacking sequence of graphene layers in a hexagonal structure (an ABCABC sequence in rhombohedral structure exists for graphite but is rare in nature). In terms of chemical composition, graphitic carbon is relatively simple as it is mostly composed of carbon and other atoms such as H and O, with possible traces of N, S, and P. During graphitization, which is the transformation of graphitic carbon into graphite, noncarbon atoms are expelled and the residual matter converges towards the pure carbon composition of graphite (see Buseck and Beyssac 2014 this issue). A comprehensive coverage of the structure and chemistry of graphitic carbon is given in Oberlin (1989).
Graphite and well-ordered graphitic carbon exhibit a huge structural anisotropy, with strong consequences for physical properties. For instance, electron delocalization in graphene layers yields a high in-plane electrical conductivity equivalent to that of metals. Perpendicular to layering, however, conductivity is so low that graphite is an insulator parallel to its c-axis. Another consequence of structural anisotropy is the high tensile strength of graphite's graphene layers compared to its weakness parallel to the c-axis. Graphite can be easily cleaved in the basal plane along which the coefficient of friction is minimal, a property that makes graphite valuable as a “dry” lubricant. Other remarkable bulk properties of graphite and some graphitic carbon include a high thermal conductivity, a high sublimation point (ca 3825 °C at atmospheric pressure), a low thermal expansion coefficient, and a relatively low density (ca 2.1–2.3). Crystalline graphite is also highly refractory and chemically inert, whereas the chemical reactivity of graphitic carbon increases with structural disorder and the abundance of chemical impurities.
Interestingly, thermodynamics predicts graphite to be the stable phase of pure carbon at Earth's surface, but also under pressure–temperature conditions prevailing in the crust or the uppermost mantle. However, graphite sensu stricto is rare, with chemically impure graphitic carbon of variably disordered structure generally more abundant.
WHERE IS GRAPHITIC CARBON?
Carbon, atomic number 12, is everywhere, in the atmosphere, biosphere, hydrosphere, and lithosphere. It is the key ingredient of living organisms, it is present as a component of trace gases in the atmosphere, it is dissolved as oxidized ionic species in the oceans, and it is prevalent throughout the lithosphere. Carbon occurs either in reduced form (graphite, diamond) or in oxidized form (carbonates) in rocks, but it is also found as molecular or ionic species in geothermal fluids and silicate melts. Reduced carbon phases occur as graphitic carbon in the lithosphere, whereas diamond is present in the deep Earth. For instance, the organic (= graphitic carbon + hydrocarbons) and inorganic (mostly carbonates) reservoirs contain approximately 20% and 80% total carbon in sedimentary rocks, respectively, and constitute a major terrestrial pool of carbon (e.g. Mackenzie et al. 2004).
Figure 2 depicts examples of graphitic carbon in various geological settings. Two main pathways generate graphitic carbon on Earth: (1) carbonization–graphitization of organic matter derived from living organisms (Buseck and Beyssac 2014), and (2) precipitation from deep fluids saturated in carbon-bearing molecular and ionic species (Rumble 2014 this issue). Both pathways have been active since the early Archean and throughout geological history. Fluid-deposited graphitic carbon is always highly crystalline (Pasteris 1999), and systematically exhibits higher crystallinity than graphitic carbon produced by graphitization at the same temperature (Galvez et al. 2013). Noticeably, human activity also generates graphitic carbon, like soot and char. The earliest occurrences of graphitic carbon include crystalline graphite from the Isua metamorphic belt in Greenland (ca 3.8 Ga; Van Zuilen et al. 2003) and disordered graphitic carbon from the Strelley Pool Formation in Australia (ca 3.35 Ga; Lepot et al. 2013). Graphitic carbon may be locally concentrated (e.g. coals, graphite veins), but in general it occurs disseminated in metamorphic rocks and more rarely in magmatic rocks. Carbon is present in sedimentary coal deposits, as well as in environmental samples (soils, rivers, air) where it is derived from biomass combustion (soots, charcoals), human activities (carbon blacks, aerosols), and the recycling of rock-derived graphitic carbon during erosion.
Carbon and graphitic carbon older than the Solar System are present throughout the universe, preserved in meteorites as minute inclusions spanning the whole range of nanostructures from amorphous carbon to graphite (Anders and Zinner 1993; Croat et al. 2014 this issue).
MINING AND PRODUCTION
Coal and graphite are symbols of mining history. Coal mining started millennia ago when the Romans exploited surface outcrops in some of the major coalfields of England, and probably elsewhere in Europe. Coal mining exploded in the 19th century, with the invention of the steam engine, and literally fueled the industrial revolution. Coal remains an important source of energy, and it is still mined on all inhabited continents. Early graphite mining is reported in the English Lake District near Borrowdale, where shepherds may have used graphite to mark sheep since the 16th century, and monks likely used it to draw lines in their books to help scribers. With the rise of metallurgical technology, large quantities of Borrowdale graphite were used in casting cannonballs. Owing to its strategic interest, the Borrowdale deposit became an exclusive property of the English crown.
Nowadays, graphite is considered a strategic material, but information on mining activity and the type of graphite exploited is limited by proprietary considerations. China is, by far, the major producer, responsible for nearly 80% of graphite produced worldwide. It is followed in production tonnage by Brazil, Canada, India, North Korea, and European countries (Barthélémy et al. 2012). Three types of graphite are mined for commercial use: (1) vein/lump, (2) flake, and (3) “amorphous/microcrystalline” graphite. Vein/lump graphite is fluid-deposited, and is generally pure and perfectly crystallized (Rumble 2014). It occurs in veins present in high-grade metamorphic or magmatic rocks, the most famous deposits being in Sri Lanka. Flake graphite occurs in high-grade metamorphic rocks worldwide (e.g. marbles, schists, gneisses), where it was generated by either fluid deposition or graphitization. Flake graphite is found as crystals generally larger than 100 μm and is disseminated in rocks with bulk carbon contents generally in the range 5–40 wt% for commercial viability. “Amorphous/microcrystalline” graphite also occurs in metamorphic rocks, with bulk carbon contents in the range 15–80 wt% for commercial viability. This type is made of small graphite grains, generally below 1 μm in size. The grains are not necessarily crystalline. In all cases, purification for commercial use consists of milling in an aqueous slurry, followed by flotation to separate graphite from its mineral matrix. Further treatment may involve acid treatment (HCl and/or HF), sometimes followed by high-temperature heating.
Alternatively, synthetic graphite can be produced industrially. Large volumes of pure, polycrystalline pyrolytic graphite are obtained by high-temperature heating (up to 3000 °C) of a graphitizable carbonaceous precursor, for example, tar or petroleum cokes. Pyrolytic graphite may also be obtained by chemical vapor deposition at temperatures above 2500 °C, yielding high-crystallinity graphite. Further working of pyrolytic graphite by annealing under compressive stress at high-temperature (ca 3000 °C) yields highly oriented pyrolytic graphite of the highest crystallinity and purity.
PROCESSING AND USES
Natural and synthetic graphitic carbon may be used as raw material or be further processed for specific applications (Fig. 3). Discussion here is focused on graphite as an uncommonly versatile substance. However, one should keep in mind that the largest volume of graphitic carbon used by human society is in the form of coal for energy purposes, with its many environmental consequences. In the case of graphite, it is possible to intercalate various chemical species, such as alkalis, alkaline-earth metals, lanthanides, metallic alloys, and ionic compounds, between the graphene layers. Graphite intercalation is based on oxidation–reduction reactions between the graphene layers and the intercalated material, and it improves the electrical conductivity of graphite (Fig. 4); some phases are even superconducting. Intercalated graphite is widely used in batteries, for example, lithium batteries. It is also possible to expand graphite and generate exfoliated graphite. There are various protocols to do this, but in general the method consists in overcoming Van Der Waals interactions by mechanical action or thermal expansion of an intercalated agent. This yields a low-density material used alone (e.g. in gaskets and seals) or mixed with other compounds (e.g. in thermal insulators and electrodes).
Graphitic carbon is used for many purposes, as listed in Table 1, which highlights its main uses relative to its specific properties. The main use of graphite is in heavy industry (steelworks and foundries) to make crucibles and other refractory components (Fig. 4). Graphite is mixed in variable amounts with other compounds, like oxides (e.g. MgO, Al2O3) and silicon, to strengthen ceramics, taking advantage of graphite's strength in tension and its refractory nature at high temperature (Table 1). Another major use of graphite is in energy and electrical applications. The key property in this case is the high electrical conductivity of graphite, which is needed in electrical storage batteries and in the brushes of electric motors. Graphite finds itself at the forefront of modern technological applications in storage batteries, as market demands for higher storage capacity versus smaller size and weight make necessary the utilization of graphite's many unique properties. As a result of its low coefficient of friction, graphite is commonly used as a lubricant dispersed in oil and fat, as colloidal graphite suspensions, and as an additive in metallic powders and resins. Since the 1990s, graphite has been used to progressively replace asbestos in the manufacture of brakes and clutch linings in cars, as it dissipates heat efficiently owing to its high thermal conductivity, is a good lubricant, and resists corrosion. One of the oldest uses of graphite is in pencils, where it was initially used in the form of pure, hand-carved rods clasped between cedar-wood holders. Currently, it is mixed with clays, mostly bentonites and/or kaolins, to control the softness of the marking medium. There are many other applications of graphite, including the coloring of plastics and paints, as a refractory in the glass industry, and as antifire components. In the nuclear industry, for instance, graphite is used as a moderator of neutrons and because of its high resistance to temperature and corrosion.
The recycling of graphite is limited. Electrodes may be recycled to make new electrodes or refractory compounds. In some applications (e.g. brakes), graphite becomes dispersed in the environment and may constitute a source of pollution: considering its indestructibility and the nanosize of abraded particles, graphite poses a difficult problem for remediation.
GRAPHITIC CARBON IN THE LONG-TERM CARBON CYCLE
Graphitic carbon constitutes one of the main terrestrial carbon reservoirs, occupying a key position at the interface between the lithosphere, the biosphere, the hydrosphere, and the atmosphere (Berner 2003) (Fig. 5). Although some graphitic carbon (e.g. coal, graphite) has been extensively investigated for economic purposes, few studies are dedicated to the behavior of graphitic carbon in interactions among the biosphere, lithosphere, hydrosphere, atmosphere, and deep Earth; more generally, its role in global models of the long-term carbon cycle is poorly constrained. The deficit in knowledge about graphitic carbon is readily apparent when compared to knowledge of carbon cycling in carbonate deposits. However, minimizing fluxes involving fossil organic carbon in the global carbon cycle has important implications for the long-term evolution of atmospheric chemistry (Hayes and Waldbauer 2006), as evidenced by the increasing atmospheric CO2 caused by human use of hydrocarbons and coals on short timescales. In contrast, graphitic carbon with highest crystallinity and/or purest chemistry (e.g. graphite) is highly refractory and stable during geological processes, and thus constitutes a carbon sink.
Graphitic carbon is primarily formed by incorporation of dead biomass into rocks through the graphitization process, and secondarily by fluid deposition. It is evident that during some intervals of geologic time, deposition of organic carbon increased and this carbon was preserved in sedimentary basins. Examples are the Carboniferous period, when huge coal deposits were formed, and the Jurassic, when thick sequences of black shale were deposited during sustained anoxic conditions. Once carbon is incorporated into rocks, two main processes significantly impact the cycle of graphitic carbon on geological time scales and hence global carbon cycling: (1) subduction, during which carbon is exchanged between the surface and the deep Earth, and (2) erosion, where carbon is exchanged between the solid Earth and the atmosphere. In subduction, the fate of graphitic carbon contained in sediments is virtually unknown. It may be oxidized and dissolved in fluids and then contribute to degassing at volcanic arcs, or it may be preserved in rocks, due to its refractory nature and thermal stability, and subducted into the deep Earth. Carbon cycling might be even more complicated as there are exchanges between the inorganic and organic carbon pools during subduction, for example, by the reduction of calcite to graphite as observed recently in high-pressure metamorphic rocks (Galvez et al. 2013). The same questions arise when considering the fate of graphitic carbon in rocks exposed to erosion. There is evidence that graphite can resist in situ weathering and be transported long distances to eventually be recycled in marine sediments; in contrast, disordered graphitic carbon is oxidized (Bouchez et al. 2010). This may have strong implications for the CO2 and O2 composition of the atmosphere over long timescales. In any case, there is still research to be done to identify the biogeochemical processes involving graphitic carbon and to assess quantitatively their global contribution to the long-term carbon cycle.
GEOLOGICAL ROLES OF GRAPHITIC CARBON
Most graphitic carbon derives from the conversion of biomass during carbonization and graphitization. There is a growing effort to use the latest technological developments in electron microscopy (FIB + TEM), spectroscopy (Raman, XANES), and mass spectrometry (e.g. nanoSIMS) to thoroughly characterize the multiscale structure of graphitic carbon in situ in its host-mineral matrix (Bernard and Papineau 2014 this issue). Preserving textural information down to the finest scale is important as it reveals that graphitic carbon is not inert in rocks and that it intimately interacts with other minerals. Interestingly, graphitic carbon may carry potential biosignatures in its own structure and/or chemistry, including its carbon isotope composition. So, graphitic carbon actively participates in the fossilization process as a putative descendant of living biomass. It also interacts with minerals like sulfides, carbonates, and clays, and contributes to the control of fluid composition and redox conditions.
As described in previous sections, graphitic carbon has physical and chemical properties that make it a unique geomaterial. These properties are also critical for determining the role and fate of graphitic carbon in geological systems (Fig. 5). For instance, it has high electrical conductivity, and only fluids or metals can rival it in this property. Deep fluids and graphitic carbon are the best candidates for explaining zones in the Earth's crust with anomalously high electrical conductivity (e.g. Raab et al. 1998). It also has a low coefficient of friction perpendicular to the c-axis, making it a potential lubricant in seismogenic zones, where it is commonly observed (Oohashi et al. 2012). Last, carbon, together with iron, is one of the main buffers of redox conditions in the Earth, with graphitic carbon exerting a key control on the C–O–H composition of deep fluids (Connolly and Cesare 1993) and melts (Cesare 1995), which is critical in controlling their internal oxygen fugacity.
IN THIS ISSUE
In this issue, graphitic carbon is described in a variety of environments over a wide range of length scales and time intervals. Buseck and Beyssac review how graphitization forms graphitic carbon and graphite through a series of transformations of organic matter exposed to increasing temperatures during diagenesis and metamorphism. They explore the structural diversity of graphitic carbon, and they discuss how graphitization is a key participant in geological processes and how it can be used as an indicator for these processes. Rumble describes terrestrial hydrothermal graphite and the range of geological settings in which crustal fluids can deposit graphite. Hydrothermal processes yield the most spectacular textural expressions of graphite, and they have profound implications for carbon mobility in the lithosphere. Bernard and Papineau discuss the biological legacy of graphitic carbon, from the earliest time in Earth history. Considering both experimental and natural studies, they describe how biological molecules are transformed into graphitic carbon during burial, and they propose a series of criteria that should be considered when investigating ancient graphitic carbon for a potential biological origin. Croat, Bernatowicz, and Daulton investigate the structure and isotopic composition of interstellar carbonaceous dust, in particular presolar graphitic carbon. This material provides unique information on nucleosynthesis and star formation prior to the formation of the Solar System. Finally, Lazzeri and Barreiro present the basic physics and nanotechnological applications of recently discovered carbon nanomaterials, including nanotubes, fullerenes, and graphene.
Warm thanks are due to the Elements editorial board, including John Valley, Georges Calas, and Pierrette Tremblay, for their help, guidance, and patience. Reviews of this paper by John Valley and Jay Ague, as well as a careful reading by Peter Buseck, Karim Benzerara, and Sylvain Bernard are acknowledged. Thanks are also due to all the reviewers who did a great job with the papers in this issue, sometimes with very short notice. OB acknowledges funding from the CNRS-INSU, the City of Paris (Emergence Program), UPMC, and Sorbonne Universités. DR is grateful for continuing support from the Geophysical Laboratory and the Deep Carbon Observatory.