Over the past 40 years, two universally accepted maxims of geology dealing with zeolite minerals have been shown to be incorrect: the first is that zeolites occur primarily as cavity fillings in basaltic igneous rocks, and the second is that these zeolites crystallized from so-called hydrothermal solutions, i.e. thermal waters from depth. We recognize today that enormous quantities of zeolites occur as low-temperature, low-pressure alteration products of pyroclastic material in sedimentary rocks, and that many of the zeolites filling vugs and cavities in basalts and traprocks are actually diagenetic in origin, having precipitated directly from ground water percolating through the rock mass (see Walker 1951, 1959, 1960a,see Walker b; Nashar and Davies 1960). This is not to say that zeolite deposits of hydrothermal origin do not exist—far from it. The extensive zones of zeolites surrounding hot-spring and geothermal activity in Yellowstone National Park, Wyoming; Wairakei, New Zealand; Reykjavik, Iceland; Pauzhetsk, Kamchatka, Russia; Lardello, Italy; and Takenoyu, Japan; and in the vicinity of magmatic ore deposits, such as the Kuroko deposits of northern Japan, suggest that waters arising and/or heated from below have indeed played an important role in their formation. Hydrothermal alteration can be defined as the crystallization of mineral material or the alteration of preexisting mineral material that has taken place in the presence of hot solutions, the composition and temperature of which were wholly or partially the result of a deep-seated magmatic body. These reactions usually take place under comparatively high solution/solid ratios over a wide temperature range and under low-pressure conditions. Hydrothermal alteration is controlled by many factors, including solution chemistry, temperature, rock permeability and the composition and stability of the starting material. Although hydrothermal solutions are very diverse in composition, alkaline solutions are generally most favorable for formation of zeolites. The temperature …