Micro- and nano-scale graphite cones and tubes from Hackman Valley, Kola Peninsula, Russia

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We describe several unusual forms of natural graphite from an alkaline pegmatite that cross-cuts rischorrite in the Hackman Valley, Khibiny Massif, Kola Peninsula, Russia. 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 fine-grained 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 crystals of graphite greatly elongate 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 micro- and nanoscale 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 micro- and nanoscales. The nanoscale 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.

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© 2007 Mineralogical Association of Canada. Publisher's version of record: https://dx.doi.org/10.2113/gscanmin.45.2.379

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The Canadian Mineralogist