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. 2021 Jan 28;12:641. doi: 10.1038/s41467-021-20975-1

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 7 — a, b TLM (a) and reflected light microscopy (RLM) (b) photomicrographs of the same Type A filament. c TLM photomicrograph of Type B filament. d, e Raman spectra of Type A and Type B filaments acquired on locations marked by circular dots in a and c, respectively, showing characteristic bands for low-grade carbonaceous material, including D1-band at ~1350 cm^–1 and G-band at ~1580 cm^–1 in the first order region (1100–1800 cm^–1), and bands at ~2700 and ~2900 cm^–1 in the second-order region⁸⁴. Raman bands at ~340 and ~380 cm^–1 are characteristic of pyrite (FeS₂)⁸⁵. Both analyzed filaments are from Datang (sample 17DT-A-9).