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Bentonite is a clay mineral composed of the smectite group (montmorillonite) of clay minerals. It is formed through the alteration of volcanic ash, with hydration and loss of alkalies and bases. The hydration process results in the formation of octahedral dioctahedral smectite cations and illite cations. Its cation exchange capacity depends on its sodium or calcium content. It has a high water absorption capacity and swelling capability, which make it useful for a wide variety of applications. It is often used as a sealing liner in earth construction, waste deposits and in water engineering applications such as lakes and reservoirs. It is also used as an ingredient in cat litter due to its water absorption, swelling and odour-controlling properties.

The bentonite chemical formula is a tetrahedral, aluminium silicate with a structure that consists of two silicate layers that are bent or folded over each other. The lower layer consists of montmorillonite and the upper layer consists of silica.

Sodium bentonite, or Wyoming bentonite, has the greatest swelling capacity in water. When wet, it swells up to a volume of approximately 20 times its original size, and separates into small sheets of alumina-silicate. These sheets provide an immense surface area over which cation exchange, adsorption and hydrogen bonding can occur. Unlike calcium bentonite, which tends to clump on swelling and provides less surface area for fining wine, a small amount of sodium bentonite can greatly improve the efficacy of fining wine (see Marchal et al, 2002).

Activated calcium bentonite is used in winemaking because it is inexpensive, readily available and has a higher proportion of exchangeable calcium than sodium bentonite. The presence of exchangeable calcium increases its pH range and makes it more suitable for use in acidic wines. Moreover, it is much more stable than sodium bentonite when exposed to extremes of temperature, thereby providing an advantage in the production of wines with higher temperatures.