Hydration of volcanic glass and genesis of perlite deposits based on oxygen and hydrogen isotope data

Perlite is an important industrial mineral, resulting from secondary hydration of silicic volcanic glass. In the Western Carpathians, there occur three economic deposits and several occurrences, hosted by Neogene volcanics. Chemical and isotopic compositions of perlites were studied for the purpose of understanding the mechanism of hydration of glass in perlites. Strong correlations between water content (expressed as loss on ignition - LOI), Na2O, delta O-18 and delta D were determined. Modelled isotopic composition of perlites in equilibrium with present day meteoric liquid and vapour indicates hydration with a significant proportion of vapour. Advanced hydration occurs in porous glass, where the vapour is able to penetrate tiny pores, which speeds-up the process of hydration. Hydration starts at elevated temperatures, but a higher degree of hydration happens by removal of alkali elements accompanied by exchange for diffusing ions of hydrogen at significantly lower temperatures. This probably occurs in geological settings that enable slow cooling so that the decreasing speed of diffusion with decreasing temperature is compensated by a longer time of opening of the system. The proposed model of perlite hydration was tested by modelling of perlite data from very different climatic conditions on New Zealand and in Yellowstone, USA.