Blue glass: A new impactite variety from Zhamanshin crater, U.S.S.R.

Abstract

A new variety of impact glass has been discovered at Zhamanshin impact crater (U.S.S.R.). The crater has been known as the source of different impact glasses such as irghizites and Si-rich zhamanshinites (Si-rich varieties) and Si-poor (andesitic) zhamanshinites. The newly discovered impact glass is of distinct blue color and shows a layered structure with numerous small vesicles. The blue color ranges between the layers from opaque turquoise to very dark blue. The layers of blue glass are usually connected with layers of greyish or brownish color showing normal Si-rich zhamanshimte composition. The major and trace element chemistry of the blue glass differs from the chemistry of other Si-rich impact glasses from the Zhamanshin crater in several ways. One of the most distinct features is the high CaO content (up to about 7 wt.%), and the different CaO/MgO ratios. Volatile trace elements are generally intermediate between irghizites and Si-rich zhamanshinites, or even higher than in Si-rich zhamanshinites, reflecting the inhomogeneity of the blue glass. REE abundances are slightly larger than in Si-rich zhamanshinites and irghizites and show a less pronounced Eu anomaly. Impact mixing of country rocks present at the crater seems capable of explaining the chemistry of the blue glass.