Experimental study of pH effect on hydrothermal alteration of volcanic ashes of Damavand



Volcanic ashes are pyroclastic materials that may convert to secoundary mineral associations through hydrothermal alteration. The most important factors for changes are chemistry and temperature of hydrothermal fluids which can normally form low temperature minerals such as zeolites. In the present study, phillipsite, hydroxy-sodalite and analcime are synthesized hydrothermally from volcanic ashes at alkaline conditions in the laboratory. In these experiments NaOH, KOH, NaCl and KCl solutions in 1.25-7.5 mol/lit concentrations were used as source of Na+, K+ and OH- ions and all of them performed at 150?C for 96h at hydrothermal conditions.
The results show that philipsite forms by phase transition of volcanic ashes in presence of alkaline solutions from low to high concentrations of KOH solutions with or without extra salts such as NaCl whereas analcime is only synthesized in presence of NaOH solutions alone.