What is Bentonite?

Content:

• Introduction



• Bentonite origin



• Physico-chemical properties
  & composition

See Fact Sheet on Bentonite

Introduction

Bentonite is an industrial name for an ore which contains mainly smectite, the most common form in geological terms being montmorillonite-like, with particular properties of swelling and water absorption.

Indeed bentonite presents strong colloidal properties and its volume increases several times when coming into contact with water, creating a gelatinous and viscous substance.

The clay was usually formed from volcanic rock or ash that has been transformed over millions of years and now consists of very small clay particles (<2 µm) each of which is extremely thin. The clay particles associate with each other by becoming stacked on top of one another in a smectic fashion that is they lie so that their orientations are broadly parallel to each other.

Bentonite origin

Bentonite is a clay mineral derived from the alteration, over geological time periods, of glassy material emitted from volcanoes - tuff and ash or from alteration of silica bearing rocks such as granite and basalt.

The environmental requirements for the formation of the clay that is the main component found in Bentonite are only approximately known and different climatic and hydrological environments together with the different ages and depths of occurrence produce minor variations in this clay.

Most Bentonite deposits date from the Tertiary and Mesozoic periods (i.e. up to 230 million years ago). Bentonite swells in water but this clay can, over long periods, convert to another non-swelling clay type called Illite. The lack of older Bentonite deposits may be due to conversion of the clay to Illite or because the conditions for initial formation of the water swelling clay were not favourable.

The water swelling clay nowadays referred to as Bentonite was first named Taylorite after William Taylor who studied deposits in the USA.


In 1898, W C Knight used the name Bentonite because the first site discovered was near Fort Benton in the Wyoming/Montana region of the USA, in upper cretaceous tuff. The name Bentonite has become widely accepted and has taken on broader usage now being applied to clays of the same basic type as present near Fort Benton but whose conditions of formation cause them to exhibit lesser natural swelling capacity in water.




Bentonite's main constituent is the clay mineral montmorillonite. This in turn derives its name from a deposit at Montmorillon, in Southern France.

Physico-chemical properties & composition

Bentonite is a naturally occurring material consisting predominantly of the clay mineral montmorillonite. Montmorillonite is a material species in the family of sheet silicates called smectites. Other smecitite group minerals include hectorite, stevensite and saponite (Figure).

Indeed, the definition of bentonite can be expanded to include these other smectites, but deposits in which they predominate are more commonly referred to by other names (hectorite, urasite, etc...)

Smectites are three-layer clay minerals. They consist of two tetrahedral layers of interconnected SiO4-tetrahedrons which enclose a central M(O,OH)6 -octahedron layer (M=Al, Fe, Mg and others). The silicate layers have a slightly negative charge that is compensated by exchangeable ions in the intermediate layers. The charge is so weak that the cations (in natural form, predominantly Ca2+-, Mg2+- or Na+-ions) can be adsorbed with an associated hydrate shell (innercrystalline swelling).

Montmorillonite-like Structure:

Courtesy Oil-Dri Corporation of America

An essential characteristic of all smectite minerals is their ability to absorb tremendous amounts of water and other liquids into their sheet structures. This gives bentonite extraordinary swelling and adhesive properties that are exploited commercially by many industries. The ability of smectite to absorb water is due in part to by the inherently small grain size of individual smectite crystals (typically much less that 2µ) and to the fact that individual sheets possess a negative surface charge which tends to attract polar molecules (Figure). This negative charge is also responsible for another essential attribute of smectite - its ability to absorb positively charged ions from solutions, an attribute which, like adhesion, is also exploited commercially.

Montmorillonites and related smectite minerals are capable of forming in many types of geologic environments and through a variety of mechanisms. However, an essential ingredient in their formation is water. During the reaction of water with rock, the surfaces of unstable minerals dissolve and contribute ions to the surrounding solution. As these solutions become saturated, new minerals precipitate that are more stable under the prevailing geological conditions. If the chemistry of the rock being dissolved is suitable, these new minerals will include smectites. In actuality, smectites are common constituents of most earth surface sediments and can form through the reaction of a wide variety of rock types so long as these rocks are rich in silicon and other essential elements. However, the formation of large deposits of smectite (i.e., bentonite) require rather special geological conditions.

Because smectites are water-born minerals, volcanic environments provide among the most favorable conditions for the formation of large deposits of smectite. This is because the hot circulating waters often associated with such environments tend to increase the intensity and rate of water-rock reactions. The geological probability of producing a large bentonite deposit is further increased if the reacting rock consists of volcanic glass because volcanic glasses are chemically unstable under earth surface conditions and are often fractured, leading to highly reactive surfaces. Deposits rich enough in smectite to be considered bentonite can also form during low temperature weathering of non-volcanic rocks, but these deposits tend to be less extensive.

Finally, depending on the nature of their genesis, bentonites will contain a variety of accessory minerals in addition to montmorillonite. These may include lesser amounts of other clay minerals such as attapulgite, kaolin, mica and illite as well as non-clay minerals like quartz, feldspar, calcite and gypsum. The presence of these minerals will affect the industrial value of a mined deposit, reducing but sometimes increasing its value depending on the application.