Laser Raman spectroscopic studies on graphites from Dharwar craton: A possible metamorphic grade indicator for the host rocks

We present here the results of Laser-Raman Spectroscopic studies on graphit e samples from the Archaean metasedimentary rocks of Dharwar craton to corr elate the structural parameters of graphite with the metamorphic grade of t he host rocks. The present studies show that the first order Raman peak (O) of the graphite samples appears in the wave number range from 1581 to 1585 cm(-1); the second order Raman peak (S) appears in the wave number range f rom 2699 to 2721 cm(-1); and the disorder Raman (D) peak appears in the wav e number range from 1349 to 1356 cm(-1). It is suggested that the intensity ratios of the Raman peaks (D/O)(a) are the inverse function of in-plane cr ystallite size of the graphite. The observed values of intensity ratio (DIO )(h) varies from 0.095 to 0.591, (D/D+O)(a) varies from 0.18 to 0.46 and (S /O)(h) varies from 0.21 to 0.32. This range of values indicates that the gr aphite samples are well crystallized. A correlation of (D/O) area ratios al ong with the in-plane crystallite size La (50 to 193 Angstrom) and the meta morphic grade shows that the metamorphic grade of the host rocks of the stu died samples lies in the range between upper chlorite zone and lower sillim anite zone. The estmated peak metamorphic temperatures of the host rocks ar e in the range of 575 to 625 degrees C and they are in excellent agreement with temperatures obtained by the Powder X-ray diffraction studies and conv entional geothermometry studies.