Mineralogy and geochemistry of alkali beryl from the rare metal bearing pegmatites of Marlagalla-Allapatna, Mandya district, Karnataka

White coloured alkali beryl from the pegmatites of Marlagalla-Allapatna in Mandya district, Karnataka contains distinctly higher alkali elements (Na, Li, Rb, Ca) and relatively low beryllium oxide (c. 11% BeO) compared to nor mal beryl. Beryl occurs mostly between graphic intermediate zone and quartz core in the complexly zoned, spodumene-bearing Li-Cs-Ta (LCT) type pegmati tes in this area. The average value (n=7) for alkali beryl from this area a re Na2O 1.26%, Li2O 0.6%, Cs2O 0.74%, K2O 0.07 and Rb2O 0.08%. Cs content i n 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(0)= 9.2765-9.339 Angstrom, c(0)=9.2079-9.2487 Angstrom) reveal differences in c rystal structure with respect to normal beryls from Royalstone, Massachuset ts, USA (a(0)=9.215 Angstrom, c(0)=9.192 Angstrom) and compares well with t he reported alkali beryl from Arizona, USA (a(0)=9.30 Angstrom, c(0)=9.20 A ngstrom). Their refractive index (N-omega) ranges from 1.588 to 1.592 and t he specific gravity varies from 2.74 to 2.79.