NA-K ION-EXCHANGE REACTION BETWEEN RHYOLITIC GLASS AND (NA, K)CL AQUEOUS-SOLUTION UNDER HYDROTHERMAL CONDITIONS

Hydrothermal experiments on ion exchange reactions were carried out using synthetic and natural rhyolitic glasses and (Na, K)Cl aqueous solution at 200-300°C and 1000 bars. The rhyolitic glass did not crystallize during the reaction even under hydrothermal conditions, and it behaved as a Na-K ion exchanger. The Na-K ion exchange properties of the glass were strongly influenced by its thermal history. Quenched glass preferentially fixed K rather than Na, and glass obtained by slow cooling showed the opposite trend. The present experimental results show clearly that glass as well as minerals can control the composition of the coexisting aqueous solution in hydrothermal systems. The present results also suggest that the chemical composition of coastal thermal waters in Japan is controlled by the two-stage episode of reaction between acidic volcanic rocks and seawater. © 1990.