Bentonite and Montmorillonite clays often interchange names and have similar properties yet vary from deposit to deposit. This commonality can be confusing when first learning about clays.
Actually it is not a significant difference. They are both Smectites. Most Smectite clays have Montmorillonite properties. The Montmorillonite name was actually given to a Smectite clay that was found in the Montmorillon area of France. Clays are often named after their location. Bentonite was originally named for Smectite clay found near Fort Benton Wyoming. Most Smectites are now referred to as Bentonites or Montmorillonites. Like Kleenex was the only tissue for so long that most everyone says, ‘Please hand me a Kleenex,’ instead of a tissue or by another brand name.
There are very few true 100% Montmorillonite clays. Some Montmorillonite properties are found in different percentages in most clays. It has become a buzz word by association. It is often found in your Bentonites but also in Illites, Kaolins and Chlorites as well, all to different degrees.
Below are several references where Bentonite and Montmorillonite are used interchangeably.
This is a definition from Wikipedia:
Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the Smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet. The particles are plate-shaped with an average diameter of approximately 1 micrometre. It is the main constituent of the volcanic ash weathering product, Bentonite.
Montmorillonite and Bentonite's water content is variable and it increases greatly in volume when the absorbs water. Chemically Montmorillonite is hydrated sodium calcium aluminum magnesium silicate hydroxide (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron, and other cations are common substitutes; the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite and kaolinite.
“Bentonite, also referred to as Montmorillonite, is one of the most effective and powerful healing clays used to treat both internal and external maladies.”
Bentonite consists of nanometer scale of Smectite clay minerals (mainly Montmorillonite) and micrometer scale of macro-grains (mainly quartz). Properties of saturated Bentonite are characterized by hydrated clay minerals.
"Bentonite is a dark-grey to dark-green clay-rich rock composed of mostly Montmorillonite, with minor concentrations of cristobalite, zeolite, and quartz, among others.”
Some other interlacing names are mentioned in MineralsZone.com.
“BENTONITE and FULLER'S EARTH are the two important naturally occurring clays of great commercial importance possessing inherent bleaching properties. They are, therefore commonly called bleaching clays. They fall mainly under Montmorillonite group containing a varying amount of attapulgite.”
Bentonite and Montmorillonite have similar mineral properties. The most important of which are attributed to the Smectite Family of clays. The Cation Exchange Capacity CEC and the Specific Surface area of Smectites are considerable larger than other families of clays being as they are predominately 2:1 clays. There absorption capacity is as much as 8 times greater than other clays.
All clays are natural unless they have been processed or tampered with by man. Most clays are evolved from volcanic ash that has weathered for millions of years therefore they qualify as natural. Pure refers to the cleanliness of the deposit. The surface clays most likely have been exposed to pollutants and fecal contamination. That is why you want a subsurface deep deposit of the clay. Look for clay that has a Microbial Lab Test Report.
Green clay is also a popular term and can be a Montmorillonite or a Bentonite. Historically green clays have been known to be the most popular healing clays and are predominately of the Bentonite or Montmorillonite clays, though they can be from other families as well. Green clays are generally from marine life from lakes or inland seas or rivers that the volcanic ash was deposited into.
According to Surface Mining, 2nd Edition by B. A. Kennedy, Bentonite is clay consisting essentially of Smectite minerals. The term Smectite is applied as a group or family name and Montmorillonite is a mineral species name.
So, in answer to the question, are Montmorillonite and Bentonite the same? No not specifically. Are they similar in properties and abilities? Yes. Are they used interchangeably and synonymously? Yes.
And that’s all I have to say about that subject.