CARBON MINERALOGY

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, or 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. Therefore, any comprehensive survey of carbon in Earth must consider the broad range of carbon-bearing phases. The International Mineralogical Association recognizes more than 380 carbon-bearing minerals (http://rruff.info/ima/), including carbon allotropes, carbides, carbonates, and a variety of minerals that incorporate organic carbon in the form of molecular crystals, organic anions, or clathrates. The objective of this presentation is to review the mineralogy and crystal chemistry of carbon—its known mineralogical reservoirs, its potential high-pressure phases that might play a role in the core and lower mantle, and its mineral evolution since before the formation of Earth.