On the Genesis of Indian Beryls

Beryl samples collected from pegmatites of Bihar, Rajasthan, Andhra Pradesh and Kerala. Showed marked correlations between chemical constituents and physical parameters. The following are the major observations of this study: (i) Specific gravity and refractive index vary with alkali elements, (ii) unit cell dimensions vary with divalent metal ions and alkali elements, (iii) Be content correlates with refractive index, unit cell dimension c and also with Li content. Beryl samples are alkali-poor and form two distinct populations with alkali concentration of 0.594-0.863 wt% and 1.468-1.871 wt%. These two chemical types of beryls possibly tally with two generations, one of primary zones and the other of late phase cleavelandite. The data also showed that the analysed beryls from Indian pegmatites contain extremely low concentrations of rare alkalies. Correlation of Al with divalent metal ions; and Be with Li signifies that both octahedral and tetrahedral substitutions can take place in these beryls. In the structural formula the sum (Al³⁺+Me²⁺), in all cases, showed close proximity to 2 justifying octahedral substitution.