Alkaline alteration of rhyolitic tuffs and formation of hydroxy sodalite under hydrothermal conditions

In order to investigate the alteration products of rhyolitic tuffs under alkaline hydrothermal condition, samples of acidic pyroclastic rocks from Darabad area (N-E Tehran) were selected. Rock samples were examined under petrographic microscope, X-Ray powder diffraction, X-Ray Fluorescence and were eventually powdered and mixed with Na+ and K+ bearing solutions in hydrothermal autoclaves. The effect of changes in temperature, time and concentration of cations in solution, alteration of aforementioned tuffs have been studied. The solutions with 1.25- 7.5 mol/lit cation concentrations from KOH and NaCl salt have been prepared and used in hydrothermal experiments in 100 °C and 150 °C. The results revealed that in alteration of rhyolitic tuffs, in presence of KOH and NaCl solutions, with increasing temperature, concentration of Na+ and pH, the quality of quartz decreases and the quality of albite increases (sodic alteration). At 150 °C and high concentration of Na+ and K+ (7.5 mol/lit), quartz dissolves completly, albite changes to orthoclase with cationic exchange of Na+ by K+ (potassic alteration) and forms hydroxy sodalite (zeolitic alteration). XRD studies proposed cubic system with unit cell dimensions of a0= b0= c0= 8.8760 Å for hydroxy sodalite.