Spectroscopic and X-ray diffraction studies on fluid deposited rhombohedral graphite from the Eastern Ghats Mobile Belt, India

We report here the first experimental results on the laser Raman spectroscopic, powder X-ray diffraction (XRD) and X-ray photoemission spectroscopic and high-temperature differential thermal analysis studies on graphite from the Burugubanda-Tapasikonda shear zone in the Proterozoic Eastern Ghats Mobile Belt (EGB) of Andhra Pradesh, India. Crystallinity, interplanar spacing and unit-cell parameters were determined by powder XRD. Powder XRD studies on the graphite samples from the shear zone show all the strong XRD peaks corresponding to syngenetic (hexagonal) graphite and additional peaks at 0.208 and 0.197 nm corresponding to the presence of epigenetic (rhombohedral) polytype of graphite. On the basis of the intensity ratio of the (101) diffraction peaks for both syngenetic and epigenetic phases, the content of epigenetic phases within this sample is estimated to be up to 20-25%. The presence of epigenetic phase indicates that the Burugubanda shear zone contains fluid-deposited graphite, a solid carbon from natural carbon-bearing fluids such as those containing CO2, CO and. CH4. The fluid-deposited nature of the graphite has also been confirmed independently by X-ray photoelectron spectroscopic studies. Laser Raman spectroscopic studies on all the samples show Raman bands at 1581 to 1585 cm(-1) (ordered peak), 1351 to 1355 cm(-1) (disordered peak) and second order peak at 2724 cm(-1). The intensity ratio of ordered to disordered peaks (0.09 to 0.10) indicates the peak metamorphic temperature of the host rocks corresponding to sillimanite zone. The values of interplanar spacing d002 (0.3346 nm), crystallite size (or stacking height; 54.83 nm) and degree of graphitization (135.5) indicated the peak metamorphic temperature of the host rock to be about 750 +/- 30 degrees C, and hence the high temperature that prevailed along the EGB shear zone.