Comparison of carbon in shungite rocks to other natural carbons: An X-ray and TEM study

The shungites of Karelia (Russia) form a large, diverse group of black Precambrian rocks, all of which contain an intriguing type of poorly crystalline carbon. Wide differences of opinion exist about its structural state and its relation to carbon from other geological environments and origins. We used a variety of measurement techniques to determine the structural features of the carbon in shungite samples and to relate them to other natural sources of carbon. Although there is a wide range of types of shungite rocks, it appears as if the structure of their carbon is similar throughout in respect to high-resolution transmission electron microscopy (HRTEM) images, and electron and X-ray diffraction patterns. Other samples whose carbon is indistinguishable using these techniques include those from the Erickson gold mine (Canada), the Sovetskaya gold mine (Russia), and the Sudbury impact structure (Ontario). Carbon samples from different localities of the Shunga district are characterized by containing curved layers, similar to samples from natural and synthetic cokes. The HRTEM images and nanodiffraction patterns of shungites suggest that some 3-dimensional closed shells occur but, more commonly, there are fractions of such shells or regions of structure that are highly disordered into bent stacks of graphene layers.