Graphite deposits may form alternatively by metamorphism of sedimentary rocks and from fluids. Both types occur in supracrustal successions within the Lewisian Complex of Northwest Scotland, and similarly in Palaeoproterozoic supracrustal rocks across the North Atlantic region in Canada, Greenland and Scandinavia. Carbon isotope compositions show that the graphite in Scotland had a mixed origin from metamorphism of sedimentary organic matter (schists) and the decarbonation of limestones (marbles). Raman spectroscopy shows that most of the graphite in Scotland exhibits some structural disorder, unlike the complete order in graphite vein ore deposits across the region. Exceptionally, where graphite was precipitated from fluid, in albitized rock in Tiree and Scardroy, it is fully ordered. While organic matter may survive granulite facies metamorphism without being transformed to fully ordered graphite, it can yield commercially more valuable ordered graphite when mobilized in a fluid.
Graphitic samples in Scotland were collected with the help of A. Wright, J. Armstrong and M. Duffy. Samples beyond Scotland were kindly supplied by the Canadian Museum of Nature (Saglek Bay sample 31445, Kimmirut samples 31236, 31274), the Smithsonian Museum (Akuliaruseq sample 127248), the British Museum (Skaland sample BM 1996,149), Leading Edge Materials Corp. (Woxna) and the National Museum of Scotland (Pargas sample G.2007.72.2). This work was partly supported by NERC grant NE/M010953/1. Electron Microscopy was performed with the help of J. Still in the ACEMAC Facility at the University of Aberdeen, and Raman spectroscopy was aided by D. Muirhead. The manuscript benefitted from constructive review by H. Dill and the Terra Nova editorial staff. The authors have no conflicts of interest.
Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.
- carbon isotopes
- Raman spectroscopy