ZEOLITIZATION OF VOLCANICLASTIC SEDIMENTS - THE ROLE OF TEMPERATURE AND PH

This paper addresses the question of why some volcanic ashes are zeoli tized and others are not. The pH of interstitial water is the most imp ortant control on zeolitization, Surface water and groundwater in volc anic areas is acidic, but zeolitization requires neutral to alkaline c onditions. Therefore, acidic interstitial water must be removed or neu tralized before zeolitization can take place. Conditions favoring zeol itization are deposition or redeposition under water, or reheating of ash after acidic interstitial water has been removed, or influx of sli ghtly alkaline (bicarbonate-bearing) groundwater from overlying or nei ghboring limestones.