28 Handbook of Natural Zeolites, 2012, 28-51 Vassilis J. Inglezakis and Antonis A. Zorpas (Eds) All rights reserved-© 2012 Bentham Science Publishers CHAPTER 3 Zeolite Formation and Deposits Ioannis Marantos 1* , George E. Christidis 2 and Mihaela Ulmanu 3 1 Institute of Geology and Mineral Exploration, Olympic Village, 136 77 Athens, Greece; 2 Department of Mineral Resources Engineering, Technical University of Crete (TUC), 73100 Chania, Greece and 3 National R&D Institute for Non- Ferrous and Rare Metals, IMNR, Bd. Biruintei 102, 77145 Pantelimon, Romania Abstract: Zeolites are the main mineral components in altered volcaniclastic rocks ranging in age and composition. They form by alteration mainly of volcanic glass in various geological environments, under variable geochemical and temperature conditions. Proposed genetic models of zeolite deposits include weathering, diagenesis in open or closed hydrologic systems, low temperature hydrothermal systems, primary magmatic environments and impact craters. The most common zeolite species, which may occur in mineable deposits, are clinoptilolite-heulandite, mordenite, chabazite, analcime, and phillipsite. Mineable zeolite deposits are widespread in many countries worldwide. The world annual production of natural zeolites remains essentially constant over the last 10 years at ca 3 million tons. Although several large high grade zeolite deposits are currently under operation and numerous studies on the suitability of the zeolite materials in various applications have been carried out, most of the annual zeolite production is consumed in massive low value applications like additives in pozzolanic cement and lightweight aggregates. Keywords: Formation, deposits, geology, volcaniclastic rocks, geothermal gradients, metamorphic processes, hydrothermal zeolites, geoautoclaves, metamorphism, chemical gradients, zonation. INTRODUCTION The term zeolite was first used by Cronsted [1] in 1756 to describe crystals of a mineral from the Svappari copper mine in Sweden, which were “boiling” during heating. Later Murray and Renard [2], recognized zeolite minerals in deep sea sediments and more recently mineralogists and geologists described zeolite minerals in volcanic, volcaniclastic and sedimentary rocks. Today it is generally * Address correspondence to Ioannis Marantos: Institute of Geology and Mineral Exploration, Olympic Village, 136 77 Athens, Greece; E-mail: marantos@igme.gr, yanmarantos@yahoo.gr