It’s important to understand it so you can provide the best service to your customer.
By Ronald B. Peterson
I was on a project recently discussing pond sealing. The project manager looked at me and said, “What is bentonite? That stuff is magic.”
That is not the first time I have been asked that or something similar, so I thought this month I would try to simplify it.
What Is It
Bentonite is a member of the smectite clay group. Its official name is sodium montmorillonite, as it is named after a deposit found in Montmorillon, France. The name bentonite comes from a deposit near Fort Benton, Montana.
Montmorillonite is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm [micrometer] and a thickness of 0.96 nm [nanometer]. Using an electron microscope, magnification of about 25,000 times is required to see the individual clay particles. Members of this group include saponite, nontronite, beidellite, and hectorite.
I have been assured that if you took one cubic inch of sodium montmorillonite and separated it into its smallest particles, then laid them out flat, you would cover 66 football fields. And if anyone has the time and dedication to do that, please let me know if it really is true!
Montmorillonite is a subclass of smectite, a 2:1 phyllosilicate mineral characterized as having greater than 50% octahedral charge. Its cation exchange capacity is due to isomorphous substitution of magnesium for aluminum in the central alumina plane. The substitution of lower valence cations in such instances leaves the nearby oxygen atoms with a net negative charge that can attract cations.
The individual crystals of montmorillonite clay are not tightly bound, hence water can intervene, causing the clay to swell. This means montmorillonite is a characteristic component of swelling soil.
The water content of montmorillonite is variable and it increases greatly in volume when it absorbs water. Chemically, it is hydrated sodium calcium aluminum magnesium silicate hydroxide (Na,Ca)(Al,Mg)(Si4O10)(OH)2·nH2O.
Potassium, iron, and other cations are common substitutes, and the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and kaolinite.
The quality of bentonite is in proportion to the concentration of sodium. The higher the sodium level, the better the qualities of the bentonite and the more reactive it is. This is the reason you will see people referring to drilling fluid grade bentonite as sodium montmorillinite.
There are other cations commonly associated with bentonite such as calcium, magnesium, and manganese plus others.
Montmorillonite is used in the oil drilling industry as a component of drilling mud, making the mud slurry viscous, which helps in keeping the drill bit cool and removing drilled solids.
It is also used as a soil additive to hold soil water in drought-prone soils, in the construction of earthen dams and levees, and to control the leakage of fluids. It is even used as a component of foundry sand and as a desiccant to remove moisture from air and gases.
Montmorillonite clays have been extensively used in catalytic processes. Cracking catalysts have used montmorillonite clays for more than 60 years. Other acid-based catalysts use acid-treated montmorillonite clays.
Like many other clays, montmorillonite swells with the addition of water. Montmorillonites expand considerably more than other clays due to water penetrating the interlayer molecular spaces and concomitant adsorption.
The amount of expansion is due largely to the type of exchangeable cation contained in the sample. The presence of sodium as the predominant exchangeable cation can result in the clay swelling to several times its original volume.
Hence, sodium montmorillonite has come to be used as the major constituent in nonexplosive agents for splitting rock in natural stone quarries in an effort to limit the amount of waste, or for the demolition of concrete structures where the use of explosive charges is unacceptable.
Several Grades and Qualities
This swelling property makes montmorillonite-containing bentonite useful also as an annular seal or plug for water wells and as a protective liner for landfills. Other uses include as an anti-caking agent in animal feed, in papermaking to minimize deposit formation, and as a retention and drainage aid component. Montmorillonite has also been used in cosmetics.
Sodium montmorillonite is also used as the base of some cat litter products due to its adsorbent and clumping properties. Bentonite is also used in the pharmaceutical and food industries.
Bentonite truly has thousands of uses.
It is the ability to provide viscosity or carrying capacity and sealing characteristics that enhance its value in the drilling and construction industry.
Bentonite was formed because of volcanic ash being deposited in a body of water and then altered because of geological activity. There are bentonites of various qualities around the globe, but the bentonites found in northern Wyoming and South Dakota are the highest quality due to their sodium content.
Since there are so many different grades and qualities of bentonite, you need to make sure that you know what properties you need and that you convey that information to your vendor to ensure that you get the correct bentonite for your water well project.
Make sure you ask enough questions to be sure you acquire the product that you need to complete your task. Also make sure you understand how to use it in order that you achieve the best results for your customer.
Remember the only dumb question is the one that you have but don’t ask.
Ronald B. Peterson has been involved with the drilling industry for more than 40 years. He previously worked for Baroid Industrial Drilling Products and is now with Mountainland Supply Co., a supply company in Orem, Utah. He served as The Groundwater Foundation’s McEllhiney Lecturer in 2015 and was given NGWA’s most prestigious award, the 2013 Ross L. Oliver Award. He can be reached at firstname.lastname@example.org.