Natural hydration and ion exchange of obsidian: an electron microprobe study

The electron microprobe can be successfully used in the analysis of hydrated glass (perlite), and its ability to analyze very small volumes makes it an ideal tool for studying processes of hydration and ion exchange. Water content, although not measured directly, may be esti-mated quite accurately by dif ference-of-sum of oxide components between associated non-hydrated glass (obsidian) and its hydrated equivalent (perlite). Analyses of four obsidian-perlite pairs with a wide electron beam (lOOprm in diameter) are very similar to wet-chemical analyses of the samples. Analyses of massive perl i te using a narrow (5prm) beam detected about 3 weight percent HrO, but revealed only a very small amount of ion exchange, mainly a sl ight loss in NarO. Analyses made along f ine fractures in the perlite, however, show slightly to significantly higher water contents accompanied by appreciably higher KrO and appreciably lower NazO contents. The amount of water of hydration in a glass is probably governed by the avai labi l i ty of openings in the polymerized glass structure. A higher degree of hydration in a thin layer along fractures is accompanied by more intense ion exchange. This higher degree of hydration and ion exchange, associated with further weakening and disruption of the glass structure, is an early stage in the eventual formation of secondary argillic or zeolitic assemblages. During the time when the material still retains the optical properties of glass the ion-exchange processes appear to be largely governed by the composition of the glass