ELEMENTS, VOL . 5, PP. 93–98 APRIL 2009 93 1811-5209/08/0005-0093$2.50 DOI: 10.2113/gselements.5.2.93 Geological Aspects and Genesis of Bentonites INTRODUCTION Bentonites are clay rocks that form from volcanic glass and that consist predominantly of clay minerals of the smectite group. Bentonites are used industrially because they have important physical and chemical properties, including crystal structure, chemical composition, small crystal size (hence large specific surface area), high capacity for ion exhange, variety in the type of exchangeable cations, hydration and swelling behaviour, colloidal properties, dehydration, and reactions with organic and inorganic reagents (Odom 1984). Because of these properties, benton- ites are used in a variety of applications, such as drilling, foundry processes, iron ore pelletization, civil engineering, adsorbents, filtering, decolourizing and clarifying. The most recently developed application is the production of nanocomposites. The purpose of this article is to describe the formation of bentonite. Some of the questions we aim to answer are: What controls smectite formation during alteration of volcanic glass? Are there variations in bentonite that are invisible at the macroscopic scale (i.e. cryptic variations)? How can smectite-rich bentonites form from rocks of unfa- vourable composition? Does parent rock composition control smectite composition? Did bentonites form in previous geological epochs and, if so, how can they be used for stratigraphic correlation? PARAMETERS CONTROLLING BENTONITE FORMATION Mechanisms The alteration of volcanic pyro- clastic rocks to smectite is a common process, often producing bentonite deposits of commercial value. Evidence for the volcanic nature of bentonite precursors is the pres- ence of primary igneous minerals (β- quartz, biotite, sanidine, zircon, apatite, ilmenite, magnetite); the presence of glass shards that may be fresh, partially altered or pseudomorphically replaced by smectite; and the distribution pattern of characteristic trace components such as the rare earth elements (REE). Alteration of glass may occur through vapour-phase crystal- lization often associated with welding of ignimbrites, burial diagenesis, contact metamorphism, hydrothermal activity, and by hydrolysis either in alkaline lakes and in marine sediments or by percolating groundwater. Vapour- phase crystallization is not an important process in bentonite formation. Bentonites form as a result of the alteration of glass by a fluid phase, so they form in aqueous environ- ments, usually in shallow seas or lakes. However, alteration of volcanic glass to smectites is frequently incomplete and fresh glass shards may be preserved even in old volcano- sedimentary strata (Weaver 1989). In fact, most commercial bentonite deposits contain fresh glass shards. Economic deposits of bentonite may be formed by any of three mechanisms: (1) diagenetic alteration of volcanic glass, (2) hydrothermal alteration of volcanic glass, and (3) formation of smectite-rich sediments in salt lakes and sabkha environments usually from dissolution of detrital smectites, which is often associated with sepiolite and/or palygorskite. This last process does not necessarily require pyroclastic or volcaniclastic precursors and is observed in arid climates. Sedimentary deposits contain trioctahedral smectites (saponite and stevensite) in small amounts, so their quality is inferior. Regardless of the formation process, leaching of alkali elements and high (Mg 2+ )/(H + ) are required to form smec- tites instead of zeolites during the alteration of volcanic glass (Senkayi et al. 1984; Christidis 1998). Magnesium is often supplied by the fluid phase, especially when the parent rock is acidic (Si-rich). Mass balance calculations show that the large water to rock ratios of an open system are necessary to form smectites, regardless of the parent rock (Christidis 1998). High water to rock ratios are facili- tated by high permeability. Low permeability, thus low water to rock ratio, favours zeolites. Smectites have been B entonites are clay rocks consisting predominantly of smectite. They form mainly from alteration of pyroclastic and/or volcaniclastic rocks. Extensive deposits, linked to large eruptions, have formed repeatedly in the past. Bentonite layers are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration, favoured by fluids that leach alkali elements and by high Mg content. Smectite composition is partly controlled by parent rock chemistry. Recent studies have shown that bentonite deposits may display cryptic variations in layer charge – i.e. the variations are not visible at the macroscopic scale – and these correlate with physical properties. KEYWORDS: bentonite, smectite, glass alteration, stratigraphic correlation, cryptic variation, clay George E. Christidis 1 and Warren D. Huff 2 1 Technical University of Crete Department of Mineral Resources Engineering 73100 Chania, Greece E-mail: christid@mred.tuc.gr 2 Department of Geology, University of Cincinnati Cincinnati, OH 45221, USA E-mail: warren.huff@uc.edu