1 Naturally occurring graphite cones John A. Jaszczak a, *, George W. Robinson b , Svetlana Dimovski c , and Yury Gogotsi c a Department of Physics and the A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan, 49931-1295, USA b Department of Geological and Mining Engineering and Sciences, and the A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan, 49931-1295, USA c Department of Materials Science and Engineering, Drexel University, 3141 Chestnut St., Philadelphia, PA 19104, USA * Corresponding author. Tel. +1-906-487-2255; fax: +1-906-487-2933. E-mail address: jaszczak@mtu.edu (J. A. Jaszczak) Abstract- Carbon, boron nitride, and other materials that form nanotubes are also able to form conical shapes. Even though potential applications of cone arrays as electron emitters and other devices are very promising, understanding of their structure and formation mechanisms is still very limited compared to nanotubes and other carbon structures. Moreover, the cones have only been synthesized in a mixture with other shapes, but never as continuous arrays. It appears, however, that we can learn from nature how to produce large carbon cone arrays. We here report the first-known natural occurrence of large arrays of conical graphite crystals. These occur on the surfaces of millimeter-sized polycrystalline spheroidal aggregates of graphite. Cone heights range from less then a micron to 40 µm, which are larger than any other carbon cones reported in literature. They are also observed to dominate sample surfaces. Surface topography of the cones and petrologic relations of the samples suggest that the cones formed from a metamorphic fluid. Unlike most laboratory- produced cones, the natural cones have a wide distribution of apex angles, which supports a disclination model for cone-helix structures. Key Words- A. Natural graphite; B. Hydrothermal treatment; C. Scanning electron microscopy, Raman spectroscopy; D. Microstructure Accepted for publication in Carbon (2003).