Amorphous carbon is the name used for carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed, but there is no long-range pattern of atomic positions.

While entirely amorphous carbon can be made, most of the material described as "amorphous" actually contains crystallites of graphite [1] or diamond [2] with varying amounts of amorphous carbon holding them together, making them technically polycrystalline or nanocrystalline materials. Commercial carbon also usually contains significant quantities of other elements, which may form crystalline impurities.

In technical terms, true amorphous carbon has localized π electrons (as opposed to the aromatic π bonds in graphite), and its bonds form with lengths and distances that are inconsistent with any other allotrope of carbon. It also contains a high concentration of dangling bonds , which cause deviations in interatomic spacing (as measured using diffraction) of more than 5%, and noticeable variation in bond angle [3].

Coal and soot are both informally called amorphous carbon. However, both are products of pyrolysis, which does not produce true amorphous carbon under normal conditions. The coal industry divides coal up into various grades depending on the amount of carbon present in the sample compared to the amount of impurities. The highest grade, anthracite, is about 90 percent carbon and 10% other elements. Bituminous coal is about 75-90 percent carbon, and lignite is the name for coal that is around 55 percent carbon.

See also: Diamond-like carbon

Sources

1. Nasa TEM photos showing graphite crystals in soot
2. Discussion of hard "amorphous" carbon films
3. Definition, from International Union of Pure and Applied Chemistry