Mineral-templated growth of natural graphite films

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: van Zuilen, Mark A., Fliegel, Daniel, Wirth, Richard, Lepland, Aivo, Qu, Yuangao, Schreiber, Anja, Romashkin, Alexander E., Philippot, Pascal
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2012
• Subjects: [SDU]Sciences of the Universe [physics]; karelia*
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-03583355; https://doi.org/10.1016/j.gca.2011.12.030

International audience Organic material in sediments is progressively altered during diagenesis and metamorphism, leading to the formation of kerogen and ultimately crystalline graphite. Bulk carbonaceous material in metamorphic terrains typically has attained an overall degree of structural order that is in line with peak metamorphic temperature. On a micron- to nano-scale, however, carbonaceous material can display strong structural variation. The main factor that drives this variation is the chemical and molecular heterogeneity of the precursor biologic material. Specific conditions during metamorphism, however, can also play a role in shaping the microstructure of carbonaceous material. Here we describe the structural variation of carbonaceous material in rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia. Raman spectroscopy indicates that bulk carbonaceous matter in these rocks has experienced peak temperatures between 350 and 400 °C consistent with greenschist-facies metamorphism. On a nano-scale, however, a strong structural heterogeneity is observed. Transmission electron microscopy (TEM) reveals the occurrence of thin films of highly ordered graphitic carbon at mineral surfaces. These graphite films - consisting of 20-100 individual layers - completely envelop quartz crystals and occur on specific crystal surfaces of chlorite. It is proposed that minerals can act as templates for the parallel ordering of carbon crystallites causing enhanced graphitization within narrow zones at mineral surfaces. Alternatively, oriented organic precursor molecules could have been adsorbed onto charged mineral surfaces, leading to thin graphitic films during later metamorphic heating episodes. Overall the presented observations demonstrate that mineral surfaces can initiate and accelerate localized graphitization of sedimentary organic material during metamorphism, and therefore cause distinct nano-scale variation in structural order.