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

U. K. Pandey ¹, Tikam Chabria ¹, P. Krishnamurthy ¹, R. Viswanathan ¹, B. Kumar ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Carbon isotope studies on calcite - graphite system yield narrow Δ Cat - Gr values, varying from 3.9 to 4.3, indicating equilibrium between coexisting calcite and graphite. Calcite - graphite geothermometry has assigned temperatures ranging from 630° to 700°C for the calc - granulites found to the NE of Usilampatti area. The inferred temperatures correspond to the retrograde temperatures obtained by cation exchange thermometry on the granulites from other areas in the Madurai Block. These temperatures may correspond to the late isobaric cooling event. X-ray diffraction studies indicate well ordered nature of these graphites and corroborate the findings of isotope geothermometric inferences.

Keywords

Geothermometry, Calc-Granulite, Calcite-Graphite, Carbon-Oxygen Isotopes, X-Ray Diffraction, Usilampatti, Tamil Nadu.

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Chanchal Sarbajna ¹, R. P. Sinha ¹, P. Krishnamurthy ¹, K. V. G. Krishna ², R. Viswanathan ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN
2 Atomic Minerals Directorate for Exploration and Research, Nagarbhavi, Bangalore - 560072, IN

Abstract

White coloured alkali beryl from the pegmatites of Mariagalla-Allapatna in Mandya district, Karnataka contains distinctly higher alkali elements (Na, Li, Rb, Ca) and relatively low beryllium oxide (c. 11 % BeO) compared to normal beryl. Beryl occurs mostly between graphic intermediate zone and quartz core in the complexly zoned, spodumene-bearing Li-Cs-Ta (LCT) type pegmatites in this area. The average value (n = 7) for alkali beryl from this area are Na₂O 1.26%, Li₂O 0.6%, Cs₂O, 0.74%, K₂O 0.07 and Rb₂O 0.08%. Cs content in some of the beryls are higher than Na and hence such beryls may be termed as a new Na-Li-Cs type X-ray powder diffraction data for the beryl (a_o = 9.2765-9.339 Å, c_o = 9.2079-9.2487 Å) reveal differences in crystal structure with respect to normal beryls from Royalstone, Massachusetts, USA (a_o = 9.215Å, c_o = 9.192Å) and compares well with the reported alkali beryl from Arizona, USA (a_o = 9.30Å, c_o = 9.20Å) Their refractive index (N_ω) ranges from 1.588 to 1.592 and the specific gravity varies from 2.74 to 2.79.

Keywords

Economic Geology, Geochemistry, Mineralogy, Rare Metal Pegmatites, Beryl, Marlagalla, Karnataka.

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E. V. S. S. K. Babu ¹, O. P. Somani ¹, R. Viswanathan ¹, R. P. Sinha ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN

Abstract

The wodginite group of minerals occur in the Challanpara pegmatite, Bastar District, Chattisgarh. Nearly pure wodginite fractions were obtained by heavy liquid and electromagnetic separation. The mineral fractions havc been studied by X-ray powder diffractometiy and electron-microprobe analysis. Challanpara wodginite deviates from ideal formula MnSnTa₂0₈, or ABC₂O₈ by extensive substitutions at the B site. The Challanpara wodginites also host small hut significant amounts of Zr, W and traces of U. X-ray diffraction study indicates that these wodginites show partial cation disorder.. Upon heating the samples to ∼950°C, cell parameters such as interaxial angle P and cell volume showed notable changes suggesting partial ordering of the crystal structure. 'The calculated index of order (Δ) varies in a narrow range from 0.40 to 0.41 suggesting that the examined phases are "partially disordered wodginites".

Keywords

Wodginite, Pegmatite, X-Ray diffraction, Electron microprobe, Challanpara, Bastar, Chattisgarh.

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