Alteration Of Tektite To Form Weathering Products

RECENT use of tektites as evidence for a bolide impact at the Cretaceous/Tertiary (K/T) boundary has focused attention on their long-term stability 1-5. It was proposed in these studies that residual clay features with the spherical tektite morphology result from in situ alteration of the original glassy material. By contrast, examination of tektite alteration as an analogue for the long-term degradation of nuclear waste glass has revealed no evidence of alteration, hydration or devitrification either for samples found in nature or for those reacted in the laboratory 6-9: no residual clay minerals were observed, and therefore the glass was interpreted as having reacted by a complete dissolution or etching process 10-12. Here we show that these apparently incongruent observations can be reconciled through understanding the relationship between the environment in which the glass reacts and the chemical processes that control the reaction rate. We have examined both natural and experimental alteration of tektites and have found that, under conditions of restricted water contact, tektite reaction is dominated by water diffusion and in situ hydrolysis of the glass structure, followed by restructuring of the silicate network to form clays. Over time, the effective rate for these processes is lower than that for etching. Thus alteration of tektites to clays, as observed at the K/T boundary, can proceed only under conditions of limited water contact.