Unveiling the nanostructured nature of pyrobitumen and shungite carbons through Raman, X-ray and theoretical analyses

Abstract

X-ray powder diffraction and multi-wavelength Raman spectroscopy were employed to characterize carbonaceous geomaterials, offering the first nanostructural analysis of rare pyrobitumen (LP) samples from the Lopérec gold deposit in Brittany, France, by comparing them to Karelian shungite (KS) and carbon allotropes samples. The inter-reticular distances d₍₀₀₂₎ for LP/KS, derived from X-ray diffraction patterns, are 3.57(1)/3.48(1) Å, with crystal thickness Lc₍₀₀₂₎ and graphitization degree of 1.4/2.0 nm and 6.9/13.1, respectively. Raman band deconvolution indicates graphitic domain sizes of L_a = 6.7/8.2 nm and graphene-like flake tortuosity L_t = 9.2/11.3 nm. Extensive density functional theory calculations on various 2D nanoflakes accurately predict that the D and G Raman bands may originate from graphene quantum dots, which form part of the nanostructure of these geomaterials. LP exhibits greater structural disorder than KS, along with a lower density (1.60 vs. 1.85 g/cm³), suggesting a lower degree of graphitization, likely due to formation at a lower temperature (∼300 °C). The Lopérec pyrobitumen is believed to result from a redox process involving a CO₂-rich, oxidizing hydrothermal solution interacting with a local hydrocarbon source.