Micro- and Nano-scale Graphite Cones and Tubes from Hackman Valley, Kola Peninsula, Russia

Several unusual forms of natural graphite from an alkaline pegmatite that crosscuts rischorrite in the Hackman Valley, Khibiny Massif, Kola Peninsula, Russia are described. The graphite occurs macroscopically in two forms: as spherical aggregates up to 2 cm in diameter of friable, radially-aligned fibers ~20 μm in cross section, and as finegrained surface coatings in cavities covering aegirine, strontian fluorapatite and K-feldspar. Optical microscopy and field emission scanning electron microscopy (FESEM) show that the fibers are actually hollow channels whose walls are composed of tabular graphite crystals greatly elongated in the direction of the fiber axis and with their basal planes oriented parallel to the channel walls. Inside and among the channels occur rolled graphitic structures (RGS): scrolls, tubes, and cones, up to 2 μm in diameter and up to 15 μm in length. The fine-grained graphite coatings on the surfaces of cavities, on the other hand, consist almost solely of microand nano-scale RGS. The largest of the RGS are hollow scrolls, with the c-axis predominantly perpendicular to the scroll axis. These are usually cigar shaped but can also be more tubular. Conical RGS occur at the microand nano-scales. The nano-scale cones tend not to be hollow and may have a cone-helix structure. Transmission electron microscopy (TEM), Raman spectroscopy, and FESEM indicate that the RGS are composed of well-ordered graphitic layers but are commonly coated by amorphous carbon. The morphologies and paragenesis of these unusual graphite forms suggest a possible hydrothermal origin.