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 differential thermal analyses (DTA) on six graphite samples from the Dharwar craton, along 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 indicates systematic variation in graphite with peak metamorphic temperatures. X-ray and Laser Raman spectroscopic studies do not show the presence of rhombohedral graphite phase which is the most characteristic of the fluid-deposited (abiogenic) graphite. The most reliable information on the origin of the graphite is provided by the carbon isotopic ratio. However, the interpretation of the isotopic data is usually not straightforward because other processes may influence the original signature. But isotopic fractionation does not alter once the graphite is well crystallized, because of very sluggish kinetics of carbon diffusion. Present studies show that the main exothermic DTA peak temperature of graphite samples decreases linearly with increase of the interplanar d-spacing d (002) of the hexagonal graphite structure and follows the relation d₀₀₂ (Å) = 3.5197 - 1.67 × 10^-4 T(K). These studies also reveal that DTA/thermogravimetric analysis is an effective tool for distinction of detrital graphite material from the poorly crystallized organic matter. Raman spectroscopic results on the same samples indicate systematic change in the metamorphic grade of the host rocks with the intensity ratio of disordered to ordered Raman peaks. These findings confirm that the carbon in the graphite of the metasedimentary host rocks from Dharwar craton is of metamorphic and biogenic origin, thereby confirming the findings of the isotopic studies on the same samples.