when desiccated. The degree of hydration of the cations and their corresponding hydrated radii explain the swelling or the shrinking behaviour of phyllosilicates. Other cations such as Mg2+ and K+ ions exhibit even a more contrasted effect: highly hydrated magnesium ions are "swellers" as in vermiculite (totally expanded interlayer) while poorly hydrated potassium ions are "collapsers" like in illite (totally collapsed interlayer). As the interlayer space of smectites is more open and so more easily accessible to water and cations, smectites exhibit the highest cation-exchange capacity (CEC) of clay minerals commonly found in the soils. Only more expandable vermiculite and some rarer alumino-silicate minerals (zeolites) with inner channel structure can exhibit a higher CEC than smectite. Formation process Smectites are formed from the weathering of basalt, gabbro, and silica-rich volcanic glass (e.g., pumice, obsidian, rhyolite, dacite). Many smectites are formed in volcanic hydrothermal system (such as geyser system) where hot water percolating through the porous matrix or the cracks of the volcanic ash deposit (pumice, pozzolan) dissolves most of amorphous silica (up to 50 wt.% of SiO2 can be dissolved), leaving smectite in place. This mecanism is responsible of the formation of the bentonite deposit (Serrata de Nijar) of Cabo de Gata in the south-east region of Almeria in Andalusia (Spain). Wyoming MX-80 bentonite was formed in a similar way during the Cretaceous Period when volcanic ashes were falling in an inner sea on the American continent. The highly porous (with a large and easily accessible specific surface) and very reactive volcanic ashes rapidly reacted with seawater. Because of silica hydrolysis, most of silica was dissolved in seawater and removed from the ashes giving rise to the formation of smectites. Smectites found in many marine clay deposits are often formed in this way as it is the case for the Ypresian Clays found in Belgium and very rich in smectites. Industrial applications Smectites are commonly used in very diverse industrial applications. In civil engineering works, it is routinely used as a thick bentonite slurry when excavating deep and narrow trenches in the ground to support the lateral walls and to avoid their collapse. It is also used as mud for drilling fluids. Smectites, more commonly called bentonite, are candidate as buffer and backfill materials to fill the space around high-level radioactive waste in deep geological repositories. Smectites also serve as additive in paints or as thickening agent for various preparations.