EXPERIMENTAL-STUDY OF INTERACTION OF ACID VOLCANIC GLASSES WITH WATERAND PROBLEM OF GENESIS OF POTASSIUM RHYOLITES

This paper reports original experimental data on the interaction of na tural obsidians and artificial quartz glass with water. The experiment s were carried out at 400 degrees C. Both hydratability of samples and dynamics of the change in concentrations of Na and K in water above t he samples were under study. Despite the similarity of chemical compos itions, the obsidian samples behaved in differential ways in the proce ss of experiments. One obsidian suffered devitrification, another rema ined amorphous. The level of alkali contents in a fluid was also diffe rent, and their concentrations varied with time, Na2O/K2O changing in a different way. The experimental results indicate a different thermod ynamic state of obsidians. Crystallization involved with volatile bodi es of glassy acid volcanites provides conditions for autometasomatic p henomena inside these bodies. The inhomogeneous nature of obsidians le ads to their irregular devitrification which in turn can cause a consi derable redistribution of alkalies despite the uniform chemical compos itions of rocks. Cation-exchange interactions between devitrified and amorphous parts of the rocks may lead to the appearance of composition s close to potassium rhyolites.