A comparative study on structural properties of graphites from Dharwar craton, India: A high-temperature DTA study

We report here new experimental results on the high temperature differentia l thermal analyses (DTA) on six graphite samples from the Dharwar craton, a long with the structural data obtained by powder X-ray diffraction, carbon isotopic and Laser Raman spectroscopic studies. Degree of metamorphism and crystallite size obtained by X-ray studies indic ates systematic variation in graphite with peak metamorphic temperatures. X -ray and Laser Raman spectroscopic studies do not show the presence of rhom bohedral graphite phase which is the most characteristic of the fluid-depos ited (abiogenic) graphite. The most reliable information on the origin of t he graphite is provided by the carbon isotopic ratio. However, the interpre tation of the isotopic data is usually not straightforward because other pr ocesses may influence the original signature. But isotopic fractionation do es not alter once the graphite is well crystallized, because of very sluggi sh kinetics of carbon diffusion. Present studies show that the main exother mic DTA peak temperature of graphite samples decreases linearly with increa se of the interplanar d-spacing d (002) of the hexagonal graphite structure and follows the relation d(boz) (Angstrom) = 3.5197 - 1.67 X 10(-4) T (K). These studies also reveal that DTA/ thermogravimetric analysis is an effec tive tool for distinction of detrital graphite material from the pearly cry stallized organic matter. Raman spectroscopic results on the same samples i ndicate systematic change in the metamorphic grade of the host rocks with t he intensity ratio of disordered to ordered Raman peaks. These findings con firm that the carbon in the graphite of the metasedimentary host rocks from Dharwar craton is of metamorphic and biogenic origin, thereby confirming t he findings of the isotopic studies on the same samples.