Over the last 40 years, a number of occurrences of zeolite minerals, especially analcime and clinoptilolite, have been reported as alteration products of volcanic ash and other pyroclastic materials and as ground mass material in bentonite clay (19) The alteration of a volcanic glass to mordenite was reported (20). The zeolite, comprising about 1% of the parent rock, was identified as a mordenite by x-ray powder data. Zeolites may occur commonly as intermediate products in a breakdown of volcanic material. The chemical composition of mordenite, based upon the unit cell, was (Mg0.2 Ca0.8 Na3.7 K1.8) Al7.7 Si40.3O96.22.9 H2O. The chemical composition of the original glass is very nearly the same.