Felspars from the pegmatites of Kodarma, Bihar

Conclusion

The felspars of the pegmatites of Kodarma have been classified with special reference to their perthitic textures. It has been suggested that there have been at least two distinct periods in the evolution of the felspars of the pegmatites, the first is represented by orthoclase and microcline perthites of the composite ‘vein’ and ‘film’ type and the second by albiteoligoclase of ‘antiperthitic’ and ‘pure’ type. The former phase preceded the latter phase. The felspar of the second phase shows a replacing relation to that of the first phase when the two occur together. These two phases of the generation of felspars may be explained as due to the differentiation of the granitic residue into potash-rich and soda-rich fractions. The origin of the various types of perthitic intergrowths—‘films’, ‘stringers’, ‘veins’, ‘inclusions’ and ‘antiperthitic’ patches—have been discussed on the basis of their textural features, and the rôle of deuteric replacement has been emphasised in the formation of vein-perthites.