STRUCTURAL, CHEMICAL, AND GEOLOGIC CHARACTERIZATION OF CRYSTALLINE GRAPHITE MINERALIZATION

Natural crystalline graphite is an industrial mineral of high criticality for the U.S. economy, driven by its use as the primary anode material in Li-ion batteries— a technology crucial for the energy transition away from fossil fuels. In response to graphite’s criticality, the U.S. Geological Survey is currently conducting a national graphite resource assessment and exploring research topics regarding the geologic occurrence of graphite deposits and the associated physicochemical properties. Crystalline graphite can form in a range of geologic environments, from high-temperature metamorphic terranes to moderate-temperature hydrothermal vein systems. Thus, despite its simple chemistry and easily conceptualized sheet-structure, natural graphite can have a wide range of characteristics including variations in particle size, crystallinity, micro- to nanoscale mineral inclusions, and structurally bound trace elements, all of which may have deleterious or beneficial effects on battery anode performance.

This study presents results from a global suite of graphite samples, including vein and flake samples from both current (e.g., Balama Mine, Mozambique) and former (e.g., Franklin Pierce Mine, New Hampshire, U.S.) producing mines. Samples were characterized using an array of analytical tools including carbon isotope analysis, laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS) trace element analysis, scanning electron microscopy (SEM) imaging, Raman spectroscopy, and X-ray diffraction. Combined, these results provide a holistic evaluation of graphite properties, and are evaluated in the context of the various geologic processes leading to graphite formation in different types of deposits. Preliminary LA-ICP-MS results reveal differences between samples in trace element concentrations (not detected to ~1000 ppm), including uranium, sulfur, vanadium, and REEs. SEM observations in some graphite samples enriched in trace elements reveal micron- to nano-scale mineral inclusions and others have no observable mineral inclusions indicating structurally bound trace elements, highlighting the complexity of graphite characteristics. This is the first step in cross-functional efforts to bridge geologic-focused research of natural graphite deposits to the ultimate use of graphite in energy technologies.