Bentonite clay

Back to Rocks and Minerals Articles

Kathy Feick

“The clay with 1000 uses”

Bentonite was first named Taylorite, after William Taylor, who studied clay deposits in the United States. The use of the name Taylorite stopped when this group of minerals was split up into separate groupings.  In 1898, W C Knight used the name bentonite to refer to this specific type of clay because the first site discovered was near Fort Benton in the Wyoming, Montana region of the United States. Bentonite clay is an aluminum phyllosilicate, which consists mostly of the mineral montomorilonite. It has the incredible ability to increase as much as 14 times its original volume.

Formation:

Bentonite is part of the smectite class of clays. Smectites are clay minerals which are approximately less than 2 micrometres in largest dimension. Although they are rare in large quantities, smectites are common in most earth surface sediments. The formation of large deposits of smectite (i.e., bentonite) requires rather special geologic conditions.

Bentonite is a material derived from the alteration, over geological time periods, of glassy material emitted from volcanoes (tuff or ash), or from the alteration of silica bearing rocks such as granite and basalt. Bentonite only forms in the presence of water. Depending on the nature of formation, bentonite can have a variety of accessory minerals in addition to its constituent mineral montmorillonite. These minerals may include attapulgite, kaolin, mica, and illite as well as minerals like quartz, feldspar, calcite and gypsum. The presence of these minerals may affect the value of a deposit.

Most bentonite deposits date from the Tertiary to Mesozoic periods (up to 230 million years ago). This may be due to the tendency for bentonite convert to another non-swelling clay called illite or because the conditions for initial formation of the bentonite were not favourable.

Types of Bentonite:

Two types of bentonite are recognized, and the uses of each depend on specific physical properties.

Type 1 Sodium Bentonite

Sodium Bentonite is the type of swelling clay. It has single water layer particles which contain Na⁺ as the exchangeable ion

Type 2 Calcium Bentonite

Calcium Bentonite is the non-swelling clay. It has a double water layer with Ca²⁺ as the exchangeable ion.