Monthly Archives: March 2012

Colors of Bentonite

Bentonite is a common clay material with an uncommon number of uses. Bentonite is widely distributed – it is particularly abundant in the Big Bend region of the Trans-Pecos because past volcanic activity produced such abundant supplies of the materials from which it is made. Bentonites were formed by mechanical and chemical weathering of volcanic materials such as glasses and tuffs. (Tuffs are made of volcanic ash.)

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Being a clay, bentonite weathers quickly, constantly exposing fresh material at the surface. Two properties of bentonite make its deposits visually distinct – it expands dramatically when wet, and it readily adsorbs a wide range of metals and other minerals. Bentonite’s constant expanding and contracting destroys the root systems of seedlings and/or plants such as cacti that have widespread, shallow roots. The result is that very little grows on exposed bentonite deposits – bentonite hills are completely bare on top.

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On the other hand, bentonite readily adsorbs a variety of minerals which give it color. So most deposits have wide bands of varying colors. In West Texas, reds, yellows, blues and greens are common colors.

Because it weathers so fast, bentonite badlands are usually characterized by numerous canyons, ravines, gullies, hoodoos and other such geological forms. The combinations of weird shapes and colors give bentonite badlands a beauty all of their own.

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Although its exact chemical composition is variable, the properties that make bentonite useful, and beautiful, are the result of its physical structure. It is composed of a number of layers, each approximately 10 angstroms thick, and each consisting of two tetrahedral and one octahedral unit. This structure is responsible for three important characteristics of bentonite: the ability to absorb large amounts of moisture, the ability to exchange ions of one element for another, and the property of thixotropy.

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Water absorption – Bentonite usually occurs in the form of tiny flakes ranging in length from 0.01 micrometers to 10 micrometers. The flakes aggregate to form thin particles. These particles have truly extraordinary surface areas. Twelve grams of bentonite have the surface area of a football field. So bentonite can absorb huge amounts of water and other polar fluids. The water accumulates in the spaces between layers. These spaces can grow to 10 times the width of the layers surrounding them, and the mineral as a whole can swell up to 20 times its regular volume.

Cation exchange – Bentonite is always electrically unbalanced by atoms that have replaced aluminum – elements such as magnesium, iron, or calcium. The result is a positively charged space between the layers. This space becomes filled with cations (negatively charged particles) which are positioned, but weakly held, near the tetrahedral layers and which serve to balance the charge. The cations are extremely mobile and can be replaced or exchanged with almost any other cation. This ability to exchange cations is an important characteristic of bentonite.

Thixotropy – When saturated with water, bentonite becomes thixotropic. Thixotropy is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. This characteristic makes bentonite ideal for use in drilling muds and a wide variety of other construction applications where it is applied as a fluid and expected to remain in place indefinitely. Here is a picture of an industrial lubricant containing molybdenum disulfide; the molybdenum provides lubrication in high-temperature situations where parts of the lubricant components may fail.

 

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Uses

Bentonite is used as a sealing agent to seal landfills, tunnels, dams, and other water agents. It is added to concrete and mortars as an admixture. When used in drilling muds, bentonite lubricates cutting heads and seals cracks and other leaks around pockets of water and/or other fluids that are not the primary object of the drilling. It is widely used in West Texas to line stock ponds, settling tanks, and other open-air fluid containers where leakage to the ground below is undesirable. Probably the cattle and oil industries are the largest consumers of bentonite in this area.

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Bentonite is sticky when damp and very resistant to high-temperature degradation which makes it ideal a use as a binder in moulding mixes. The metallurgy industries are major buyers of bentonite.

Bentonite is used as a plasticizer in ceramic materials and in the production of ointments, tablets, medications and cosmetic creams. It is used as a thickener in paints, as a binder in granulated feed-stuffs, as a strengthener in plastics, rubber, explosives, glass and mineral fibers and fertilizers.

The food industry uses bentonite for cleaning, decolorizing and stabilizing vegetable and animal fats and oils. Wine makers, juice makers and brewers use it to stabilize and clarify their products.

During the past 20 years, bentonite has been used by the nuclear industry where it is compacted to high densities and applied as blocks between the canisters and surrounding rock where it takes up water and swells to completely fill spaces. Even if new spaces open, bentonite will swell to seal the gaps. This prevents radiotoxic nuclides from entering the biosphere.

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Perhaps the application most familiar to everyday householders is in kitty-litter. Bentonite absorbs urine and helps deodorize solid waste. More than two million tons of bentonite pass through American kitty litter boxes every year. And you’re likely to find kitty-litter, or something like it in garages where it is very useful in cleaning up oil spills.

There is a marvelous exposure of bentonite on the western side of Big Bend National Park. The colors and shapes of these hills make them endlessly fascinating to explore. When I’m hiking out there though, somehow I just can’t seem to forget that these exotic canyons are actually made of kitty-litter! Who would have thought?

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