Carbon Mineralogy and Crystal Chemistry

Abstract

Carbon, element 6, displays remarkable chemical flexibility and thus is unique in the diversity of its mineralogical roles. Carbon has the ability to bond to itself and to more than 80 other elements in a variety of bonding topologies, most commonly in 2-, 3-, and 4-coordination. With oxidation numbers ranging from −4 to +4, carbon is observed to behave as a cation, as an anion, and as a neutral species in phases with an astonishing range of crystal structures, chemical bonding, and physical and chemical properties. This versatile element concentrates in dozens of different Earth repositories, from the atmosphere and oceans to the crust, mantle, and core, including solids, liquids, and gases as both a major and trace element (Holland 1984; Berner 2004; Hazen et al. 2012). Therefore, any comprehensive survey of carbon in Earth must consider the broad range of carbon-bearing phases. / The objective of this chapter is to review the mineralogy and crystal chemistry of carbon, with a focus primarily on phases in which carbon is an essential element: most notably the polymorphs of carbon, the carbides, and the carbonates. The possible role of trace carbon in nominally acarbonaceous silicates and oxides, though potentially a large and undocumented reservoir of the mantle and core (Wood 1993; Jana and Walker 1997; Freund et al. 2001; McDonough 2003; Keppler et al. 2003; Shcheka et al. 2006; Dasgupta 2013; Ni and Keppler 2013; Wood et al. 2013), is not considered here. Non-mineralogical carbon-bearing phases treated elsewhere, including in this volume, include C-O-H-N aqueous fluids (Javoy 1997; Zhang and Duan 2009; Jones et al. 2013; Manning et al. 2013); silicate melts (Dasgupta et al. 2007; Dasgupta 2013; Manning et al. 2013); carbonate melts (Cox 1980; Kramers et al. 1981; Wilson and Head 2007; Walter et al. 2008; Jones et al. 2013); a rich variety of organic molecules, including methane and higher hydrocarbons (McCollom and Simoneit 1999; Kenney et al. 2001; Kutcherov et al. 2002; Sherwood-Lollar et al. 2002; Scott et al. 2004; Helgeson et al. 2009; McCollom 2013; Sephton and Hazen 2013); and subsurface microbial life (Parkes et al. 1993; Gold 1999; Chapelle et al. 2002; D’Hondt et al. 2004; Roussel et al. 2008; Colwell and D’Hondt 2013; Schrenk et al. 2013; Meersman et al. 2013; Anderson et al. 2013). / The International Mineralogical Association (IMA) recognizes more than 380 carbon-bearing minerals (http://rruff.info/ima/), including carbon polymorphs, carbides, carbonates, and a variety of minerals that incorporate organic carbon in the form of molecular crystals, organic anions, or clathrates. This chapter reviews systematically carbon mineralogy and crystal chemistry, with a focus on those phases most likely to play a role in the crust. Additional high-temperature and high-pressure carbon-bearing minerals that may play a role in the mantle and core are considered in the next chapter on deep carbon mineralogy (Oganov et al. 2013).