X-ray study of the graphite-bearing rocks from the Pestpaksha ore occurrence and structural features of graphite

Abstract

Graphite-bearing rocks and graphites from the Precambrian metamorphic sequence of the Pestpaksha ore occurrence, Murmansk district, have been studied by the X-ray method. It was determined that the rocks have a complex polycomponent composition. Graphite is associated with garnet-quartz, pyroxene-amphibole, and biotite-quartz with an admixture of pyrrhotite, kyanite, chlorite, and other minerals. In terms of mineral composition, the studied samples are subdivided into five groups. The structural characteristics of graphite—parameters a0 and c0, the degree of structural perfection (U), the content of rhombohedral phase (R), and others—were found to be different for each group. The U value varies from 1.04 to 1.56 (with predominant values from 1.4 to 1.5), while the content of rhombohedral phase in samples is no more than 18%. This fact indicates a relatively high 3D order, which is typical of graphites from metamorphic rocks. Graphites from metasomatites have the lowest U value, while those from weathering zones are characterized by a small value of the coherent scattering domain (CSD). It was established that interplanar spacing d002 of graphite correlates with depth, while the degree of ductility and flexibility of the graphite is defined by conditions of its retrograde metamorphism.