Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

G. Parthasarathy ¹, S. R. Sharma ¹, T. R. Ravindran ², A. K. Arora ², S. M. Hussain ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, IN

Abstract

We report here powder X-ray diffraction, differential thermal analysis and Laser-Raman spectroscopic data on the graphite from Schirmacher Oasis (70°45' S, 11°40' E), East Antarctica. The sample was collected from Precambrian garnet-biotite gneiss. The present X-ray studies show that the structural parameters like inter-planar spacing, d₀₀₂ (3.3500 A), and crystallite size, L_c (552 A) of this sample. These values are found to be very close to that of well-crystallized graphite. All the five observed X-ray diffraction peaks have been indexed to the hexagonal phase of graphite, indicating the metamorphic origin of graphite. The observed full width at half maxima (FWHM) of the (002) peak is 0.148 A and the estimated graphitizing degree (GD=13lA) showed the high-grade metamorphism of the host rock. The estimated peak metamorphic temperature is found to be ∼973 K (700°C). DTA studies on the same graphite sample show a strong exothermic peak at 1023 K, which corresponds, to the combustion of carbonaceous matter under atmospheric condition. The absence of extra weak peak in the range 1090-1110 K, indicates the absence of any noticeable detntal graphite in the sample. The first order Laser - Raman spectra of the sample shows a strong well-ordered Raman peak (O) at 1581cm"¹ and a weak broad peak due to disorder (D) at 1354cm"¹. The intensity ratio disordered-to-ordered Raman peak (D/O) area has been used to estimate the in-plan crystallite size (La) of the graphite. The experimental values of D/O = 0.143 and D/D+O = 0.125 indicate that the sample is well crystallized graphite, thereby indicating the high metamorphic grade of the host rocks. The structural parameters obtained by Raman spectroscopic method and the estimated in-plane crystallite size (La = 298 A) indicate that the metamorphic grade of the host rock of the studied sample falls in the sillimanite zone. The present study suggests that the peak metamorphic temperature of the host rock could be in the temperature range of about 973 K (700°C).

Keywords

East Antarctica, Graphite, X-Ray Diffraction, Schirmacher Oasis, High-Temperature Dta, East Antarctica.

Full Text

     

G. Parthasarathy ¹, S. R. Sharma ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad -500 007, IN

Abstract

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.

Keywords

Graphitization, DTA, Raman Spectroscopy, Karnataka, Dharwar Craton.

Full Text

     

S. R. Sharma ¹, G. Parthasarathy ¹, T. R. Ravindran ², A. K. Arora ², B. Kumar ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam- 603 102, IN

Abstract

We present here the results of Laser-Raman Spectroscopic studies on graphite samples from the Archaean metasedimentary rocks of Dharwar craton to correlate the structural parameters of graphite with the metamorphic grade of the 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 from 2699 to 2721 cm^-1; and the disorder Raman (D) peak appears in the wave number range from 1349 to 1356 cm^-1. It is suggested that the intensity ratios of the Rarnan peaks (D/O)_a are the inverse function of in-plane crystallite size of the graphite. The observed values of intensity ratio (D/O)_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 graphite samples are well crystallized. A correlation of (D/O) area ratios along with the in-plane crystallite size La (50 to 193 Å) and the metamorphic grade shows that the metamorphic grade of the host rocks of the studied samples lies in the range between upper chlorite zone and lower sillimanite zone. The estmated peak metamorphic temperatures of the host rocks are in the range of 575 to 625°C and they are in excellent agreement with temperatures obtained by the Powder X- ray diffraction studies and conventional geothermometry studies.

Keywords

Raman Spectroscopy, Graphite, Carbon Isotopes, Metamorphic Temperature, Dharwar Craton, Karnataka.

Full Text

     

S. R. Sharma ¹, G. Parthasarathy ¹, B. Kumar ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500007, IN

Abstract

We report here., the results of powder x-ray diffraction studies on six graphite samples from the Archaean metasedimentary rocks of Dharwar craton, with the motivation of correlating the graphitization process of carbonaceous matter during metamorphism with metamorphic grade. Our studies show that interplanar spacing d(002) and crystallinity L_c(002) for all the studied samples are very close to that of well crystallized graphite.

The observed decrease in full width at half maximum (FWHM) of the (002) diffraction peak from 0.20 to 0.102 Å. increase in crystallite size, L_c(002) from 408 to 800 Å and graphitizing degree from 86 to 362 indicate different metamorphic grades. Our results indicate that the rocks from Hampapura are of low metamorphic grade and those from Arjun Bettahalli are highly metamorphosed. The peak metamorphic temperatures of metasedimentary rocks are estimated to be around 600±50°C. The temperature estimates based on graphite crystallinity studies and the temperatures estimated by silicate mineral geothermometry studies of the coexisting mineral assemblages were found to be consistent.

Keywords

Mineralogy, Graphites, X-Ray Diffraction, Metamorphic Grade, Peak Metamorphic Temperatures, Dharwar Craton.

Full Text