Mineralogical and Chemical Features of Radioactive Biotites from Chhotanagpur Gneiss, Kudri Hills, Sonbhadra District, Uttar Pradesh

Textural, mineralogical and chemical features of radioactive biotites from the melanosomes and leucosomes of migmatites in Chhotanagpur Gneiss at Kudri Hills are described. Biotite from melanosome is usually large and contains profuse inclusions of zircon, allanite, uranothorite and uraninite, as compared to thc biotite from leucosome. The melanosome biotite also shows contrasting microtextures, viz., presence of Fe-Ti oxide granules along cleavages and as rims around the flakes, as well as sieve texture as compared to leucosome biotite. The FeO(t)/MgO value in melanosome biotite varies from 5.0 to 5.3, whereas it is 5.9 to 6.5 in leucosome biotite. Structural formulae of biotite, calculated on the basis of 24 (0, OH, CI, F), show that the 12-coordinated cations of X-group do not reach the ideal figure of 2 in either case. The octahedral sites occupied by Y-group cations have a wide range in leucosome biotite in contrast to the very narrow range in melanosome biotite.

Anomalous concentration of rare and radioactive elements, viz., U (up to 1200 ppm), Th (up to 656 ppm), Zr (up to 31076 ppm), Ce (up to 1576 ppm), Nb (up to 193 ppm) and Y (up to 2053 ppm) in melanosome biotite seems to result from its favourable crystal structure, conducive physico-chemical attributes and stability at high temperature of amphibolite facies. The most important feature of melanosome biotite is the presence of inclusions of radioactive minerals like uraninite, uranothorite, allanite and zircon.