- © 2014 by the Mineralogical Society of America
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.
Graphite is the stable form of elemental carbon at Earth's surface and in the crust. However, well-crystallized graphite is relatively rare. Instead, a poorly crystallized, disordered material commonly called “graphitic carbon” occurs in many geological settings. Prior to about 1950, little was known about the structure of this graphitic stuff nor about the graphitization process. However, in the late 1940s and early 1950s, while a postdoctoral fellow at the Laboratoire Central des Services Chimiques de l'État in Paris, Rosalind Franklin (later to become famous for her contribution to solving the structure of DNA) produced seminal papers on graphitization that are still widely cited (e.g. Franklin 1951). Using X-ray diffraction (XRD), she proposed a model for the microstructure of graphitic carbon in which small graphitic domains are joined by crosslinks. She also performed experimental studies of graphitization by pyrolysis at atmospheric pressure of various carbon precursors. Utilizing XRD analysis, she distinguished two classes of carbon-rich materials based on their ability to graphitize. She called them graphitizing and nongraphitizing carbon (we will call them graphitizable and nongraphitizable carbon). This distinction is still used, although analytical progress, with the advent of transmission electron microscopy (TEM) and various spectroscopic techniques applied to experimental and natural samples, has resulted in refinement of the model.
Graphitization is the name given to the progressive, irreversible transformation of disordered or partly ordered, noncrystalline carbon-bearing material into pure-carbon, end-member crystalline graphite. It occurs in a variety of terrestrial and extraterrestrial settings. Incomplete graphitization, which is common, results in materials having different degrees of crystallographic order arising from various combinations of temperature, pressure, kinetics, and, in some cases, fluid activity. The intermediate stages consist of a complex array of nanostructural and mesostructural products that can yield interesting geological information and have potentially important industrial applications. Although dominated by carbon, the immediate precursors of graphite can also contain hydrogen, oxygen, nitrogen, and sulfur impurities.
This article provides an overview of graphitization, with emphasis on the characterization of the rich structural diversity of graphitic carbon intermediates. We discuss the uses of graphitic carbon as a tracer of geological and cosmochemical processes and show that this material can be a key participant in geological processes in the crust, including metamorphism, erosion, and faulting, with important implications for global geochemical cycles.
GRAPHITIZATION IN THE LABORATORY AND INDUSTRY
The structure and chemistry of graphitic carbon that can form during the graphitization process have been studied extensively in materials science because of the potential industrial applications of graphitic carbon (Beyssac and Rumble 2014 this issue). The laboratory formation of graphite from organic molecules has been described as a two-stage process (Oberlin 1989): (1) carbonization, which eliminates most noncarbon components and initiates formation of an aromatic skeleton consisting of a network of six-membered, planar rings of carbon, followed by (2) graphitization sensu stricto, which consists mostly of polymerization and structural rearrangement of the aromatic skeleton towards the thermodynamically stable ABAB layered sequence of graphite (Fig. 1 and Beyssac and Rumble 2014).
The industrial formation of graphite requires heating of appropriate carbon-rich materials to nearly 3000 °C in an inert atmosphere (e.g. Rouzaud and Oberlin 1989). Extensive research on the effects of temperature on graphitization of various carbon precursors under different oxygen pressures and in the presence of catalysts such as iron has confirmed the graphitizable versus nongraphitizable distinction introduced by Franklin (1951).
Graphitizable carbon generally arises from hydrogen-rich precursors in which oriented polyaromatic layers develop during carbonization under pyrolysis. With heat treatment, the layers tend to grow in-plane, low-energy interactions between planes increase, and interplanar spacings decrease (Buseck et al. 1987; Beyssac and Rumble 2014). The number of stacked layers increases concurrently, and the layers rearrange until the stacking finally reaches the graphite structure.
In contrast, nongraphitizable carbonaceous material exhibits a range of compositions, including oxygen-rich precursors, and does not readily develop regular orientations of the polyaromatic layers during pyrolysis (Fig. 1). The result is a roughly isotropic microstructure that has been described as microporous or ribbon-like (e.g. Oberlin 1989; Harris 2005). Heat treatment results in curved and faceted graphitic domains, with a few stacked layers that have short in-plane dimensions and that enclose randomly shaped pores. Graphite does not form in the laboratory using nongraphitizable carbon even after heating at 2900 °C for 1½ hours (Rouzaud and Oberlin 1989). The nongraphitizability presumably results from the nature of the chemical and structural links between the small graphitic domains. These links hinder growth into well-crystallized graphite. A consequence is that nongraphitizable carbon is porous, with lower density and larger specific surface area than graphite.
Bulk physical properties such as density, electrical conductivity, and diamagnetic susceptibility of graphitizable carbon all increase with pyrolysis (Rouzaud and Oberlin 1989). Bulk reactivity is also affected as the nanoporosity is modified and the composition is simplified by release of molecules and atoms other than carbon from the developing aromatic skeleton that will end up as graphite.
Laboratory studies show that hydrostatic pressure accelerates graphitization and facilitates graphite formation at temperatures as low as ca 1000 °C for graphitizable carbon (e.g. Beyssac et al. 2003 and references therein). In such experiments, nongraphitizable carbon was locally transformed into graphite through modifications at sites with favorably oriented nanostructures, and then these modifications propagated throughout the aromatic skeleton (De Fonton et al. 1980).
High activation energies and kinetic factors limit comparisons between laboratory experiments and natural graphitization. Early studies of natural graphitization suggested that kinetic impediments to the restructuring of organic matter play an important role in the progress of graphitization (Grew 1974). More recently, Beyssac et al. (2002b, 2003) proposed that on geological timescales and under specific conditions, graphitic carbon displays ordering controlled mainly by temperature. In any case, there is room for development of a kinetic model for graphitization that reconciles laboratory data and field observations.
GRAPHITIZATION IN GEOLOGICAL ENVIRONMENTS
Many sedimentary rocks contain carbon in the form of organic compounds having a biological origin. These compounds may have been subjected to heating during their geological histories, for instance through burial or contact metamorphism, during which their structures and compositions were modified. The transformation can be visualized as a two-stage process analogous to industrial procedures, with a carbonization stage followed by graphitization. The carbonization stage, corresponding to coalification or kerogen transformation, occurs during diagenesis through burial in sedimentary basins. This stage involves cracking, a series of decomposition reactions that generate smaller molecules and form hydrocarbons, first as oil and then as gas, and leave behind a carbonaceous solid residue that may develop into coal (Vendenbroucke and Largeau 2007). During this sequence, the solid residue is progressively enriched in carbon, which becomes increasingly aromatic as the impurities are distilled off (Fig. 2).
The hydrocarbons generally migrate, whereas the solid residue is trapped in the host rock and may be involved in further burial and metamorphism, during which it can be completely graphitized.
The products of graphitization are mainly observed in metasedimentary rocks in which organic carbon was abundant (Fig. 3), although some of the most spectacular graphite occurrences result from fluid deposition in Earth's crust (Rumble 2014 this issue). Because graphitization requires metamorphism, graphitic carbon occurs mainly in rocks from orogenic belts and in metasedimentary rocks in old cratons, although it occurs in rocks of all ages. Especially interesting are Archean rocks, where the nature of the carbon precursor, biological or abiogenic, and the mechanisms of formation are the subject of intense controversy.
Graphitization also occurs during extreme events like earthquakes. Graphitic carbon has been reported in both active and fossil fault zones worldwide. It may be generated by carbon mobility during fluid–rock interactions, by in situ graphitization of organic matter in the host rock through frictional heating during coseismic slip, or both. In turn, the development of platy minerals such as graphite and sheet silicates with weak bonding between the layers can produce dramatic rheological effects by inducing strong dynamic weakening of faults and acting as lubricants (Kuo et al. 2014).
Natural carbonization and graphitization can be represented using a Van Krevelen diagram, which plots the evolution of H/C versus O/C atomic ratios; in the diagram, various starting materials exhibit different pathways (kerogens I, II, and III, as detailed by Vandenbroucke and Largeau 2007). In such a diagram, graphitic carbon is restricted to a narrow area close to the pure carbon end-member (Fig. 2).
The δ13C isotopic composition of graphitic carbon falls in the range of −35 to −20‰, largely inherited from its biological precursors. However, this composition can be modified during metamorphism by (1) progressive enrichment in 13C during graphitization and (2) exchange of carbon isotopes between coexisting carbonates and graphite (Wada et al. 1994). The latter fractionation has been studied extensively and can be used as a geothermometer above 500 °C, where exchange is facilitated by appreciable diffusion rates (Bottinga 1969; Dunn and Valley 1992).
CHARACTERIZING GRAPHITIC CARBON
The earliest work on natural graphitization was based on XRD measurements (e.g. French 1964). Subsequently, several authors characterized the structure of graphitic carbon from various regional and contact metamorphic settings using mainly the d002 spacing or the full-width half maximum (FWHM) of the 002 peak as a proxy for crystallite dimensions (Landis 1971; Grew 1974). From comparison of these parameters with metamorphic zones defined by index minerals such as chlorite, biotite, and garnet or, more rarely, by mineral assemblages or quantitative pressure–temperature conditions, it appeared that complete transformation of graphitic carbon into graphite is achieved at ~700 °C. Grew (1974) described a continuous and progressive graphitization process, whereas Landis (1971) described a discontinuous process with intermediate states of organization. In any case, there seems to be agreement that the degree of graphitization as estimated by XRD is a reliable approximate indicator of the metamorphic grade (Wada et al. 1994) and that all carbonaceous compounds, graphitizable or not, will transform to graphite at sufficiently elevated temperatures and pressures.
Starting in the 1970s, TEM was used to investigate the evolution of graphitic carbon during metamorphism. Electron diffraction was employed to characterize the crystallinity, and imaging of the 002 structure-fringes (as in Fig. 4) permitted edge-on viewing of the aromatic layers. At the submicrometer scale, natural graphitic carbon can be highly heterogeneous in structural organization, limiting the extraction of quantitative information from TEM data. As shown in Figure 4, graphitization yields graphitic carbon with a wide variety of intimately mixed structures that display microporous, onion ring, lamellar, and planar nanostructures (Buseck and Huang 1985; Beyssac et al. 2002a). The average values for the d002 interplanar spacings, in-plane (La) or along the c-axis (Lc) crystallite dimensions, and number of layers in a stack were shown to increase with metamorphic grade. These observations support the idea that the structure of graphitic carbon is controlled primarily by metamorphism and, in particular, by temperature. TEM studies were also valuable for describing the mechanisms of graphitization of nongraphitizable microporous precursors. In this case, the combined effects of pressure and temperature modify the nanostructure from microporous toward lamellar by locally promoting pore connectivity and coalescence, and graphitization then proceeds heterogeneously along the pore walls (Beyssac et al. 2002a).
Raman spectroscopy is a powerful method for studying poorly ordered materials such as graphitic carbon. It offers the advantages of both minimal sample preparation and in situ analysis, thus preserving textural information (Beyssac et al. 2002b). Although appropriate for quantifying the degree of bulk graphitization in natural graphitic carbon, Raman spectroscopy does not permit detailing the nanostructure, which is done using TEM imaging and diffraction techniques.
Tuinstra and Koenig (1970) initiated the use of Raman spectroscopy for the study of graphitic carbon and provided one of the first experimental spectra of graphite. It consists of a “G” peak (characteristic of graphitic material) that arises from the stretching vibration of aromatic carbon, plus other peaks resulting from structural and chemical defects (Fig. 5). They also established an inverse correlation between the crystallite in-plane dimension (La, estimated from XRD) and the intensity ratio of the main defect-activated peak to the graphite G peak.
Defect bands in the Raman spectra of graphitic carbon have positions and relative intensities that depend on the incident laser wavelength. These bands are unique in being dispersive (their positions and relative intensities depend on the incident laser wavelength), and they have been the subject of extensive research in materials science and physics. A theoretical explanation of these bands as arising from a double-resonance effect is given by Thomsen and Reich (2000) and Beyssac and Lazzeri (2012).
Wopenka and Pasteris (1993) showed a systematic evolution of the Raman spectrum of graphitic carbon with increasing metamorphic grade. Beyssac et al. (2002b) subsequently calibrated an empirical geothermometer based on Raman spectroscopy of carbonaceous material (RSCM thermometry) by comparing spectra with quantitative petrologic estimates of pressure–temperature conditions.
GRAPHITIC CARBON AS AN INDICATOR OF GEOLOGICAL PROCESSES
Because most graphitic carbon formed from biological matter, it potentially carries important biogeochemical information about the precursor organisms, which is of particular interest regarding the early Earth (Buseck et al. 1988; Bernard and Papineau 2014 this issue). However, problems arise because this record has commonly been altered through graphitization and fluid–rock interactions that modified the structure and composition of the graphitic material.
Graphitic carbon is common in metasediments, and its transformation offers a unique opportunity to use RSCM geothermometry to quantify metamorphic temperatures from low-grade metamorphism (~330 °C) to high-grade metamorphism (~650 °C) (Beyssac et al. 2002b). Owing to the irreversibility of graphitization, the products record peak metamorphic temperature up to the point where graphitization is achieved. Such geothermometry is especially useful for investigating low-grade metamorphism and lithologies lacking index minerals and can be a convenient complement to traditional quantitative petrology.
Graphitic carbon is also abundant in extraterrestrial material such as primitive meteorites, cometary matter, and interplanetary dust particles. In meteorites, for example, it exhibits a wide range of structures, from poorly ordered material in the matrix (generally called IOM, for insoluble organic matter) to crystalline graphite in Fe–Ni-rich grains and iron meteorites (e.g. Smith and Buseck 1981; Garvie and Buseck 2007). This structural diversity results from graphitization pathways controlled by processes such as irradiation in space and thermal metamorphism on the parent body. Combined with other geochemical proxies, the in situ characterization of IOM by Raman spectroscopy has been extensively used to assess the thermal history of meteorites and, more generally, to classify meteorites (Bonal et al. 2007; Busemann et al. 2007).
New research directions have recently been explored regarding the role of graphitization and the behavior of graphitic carbon in major geological processes. An example is the role of graphitization during erosion and weathering as part of the long-term carbon cycle. Graphitization can stabilize carbonaceous matter in rocks that have been buried and then exhumed to the surface, where graphitic carbon is exposed to erosion. Transport and reburial of graphitic carbon in marine sediments results in recycling with no net effect on atmospheric O2 and CO2 levels, whereas oxidation of graphitic carbon consumes atmospheric O2 and returns CO2 to the atmosphere. In large-scale erosional systems like the Himalayas, only well-crystallized graphite is recycled, whereas disordered graphitic carbon is oxidized during transport to the ocean (Galy et al. 2008). In the Himalayan system, CO2 released from this oxidation is of the same magnitude as CO2 consumed during silicate weathering. The extent of graphitization is an important influence on the reactivity, or resistance to oxidation, of graphitic carbon and ultimately controls its fate during erosion.
As a fundamental geological process in the long-term carbon cycle, graphitization affects significant fluxes of carbon. Graphitic carbon provides an example of a natural material that displays a range of microstructures and degrees of structural disorder. Its study will benefit from technological developments in characterization techniques, especially at the nanoscale. For instance, focused ion beam (FIB) technology now allows in situ preparation of ultrathin sections transparent to electrons and soft X-rays while preserving textural relations. This approach is promising for studying low-temperature systems in which evidence of interactions between graphitic carbon and other minerals may be restricted to the nanoscale. Also, progress in both spatial and spectral resolution of TEM-based spectroscopic techniques, such as electron energy-loss spectroscopy (EELS) and X-ray absorption near-edge structure/scanning transmission X-ray microscopy (XANES-STXM), used to perform organic geochemistry at the nanoscale, promises to open new avenues for understanding the nanostructures of graphitic carbon and its behavior in geological processes.
OB acknowledges research funding from CNRS-INSU, Ville de Paris, Paris VI University, Sorbonne Universités, and ANR. We thank Ed Grew, Doug Rumble, Sylvain Bernard, and David Bell for thoughtful reviews that helped improve the manuscript.