Montmorillonite clays are Bentonite clays and Bentonite clays are Montmorillonite clays. They are not two separate minerals as people think. They are one and the same thing. All types of Bentonite clays are grouped together under the Montmorillonite or Smectite group of clays. To speak of one is to speak of the other.
Marketing of various brand names using the different terminology for the same thing, in the same text, is often responsible for the confusion that arises.
Montmorillonite was named after its discovery locality, Montmorillon, France in the 1800's. Bentonite was named after Fort Benton, Montana, near which it was discovered.
Bentonite is typically regarded as an impure ore of clays that contain both the crystalline structure Montmorillonite and additional crystalline structures. The degree of Bentonite's purity is equal to its degree of Montmorillonite.
In today’s world, finding a sizable source of minerals of 100% purity is rare. It is also rare that the providers of Bentonite would separate the Montmorillonite from the source’s composition, which is why it is distinctly considered Bentonite, as opposed to pure Montmorillonite. Essentially, Bentonite and Montmorillonite are the same thing, with those specified as 100% Montmorillonite are the purest on the earth.
Montmorillonite, a phyllosilicate, is a soft type of mineral that exists in small crystals which accumulate to form clay. Phyllosilicates or sheet silicates, are a group of minerals that include the mica, chlorite, serpentine, talc, and the clay minerals. The clay is named after Montmorillon (France) where it was first discovered.
There exist two types of Montmorillonite, sodium and calcium. Sodium Montmorillonite is commonly known as sodium Bentonite. The presence of sodium as the predominant exchangeable cation can result in the clay swelling to many times its original volume. Almost all natural clays have value in promoting human health. Some may be consumed, others are best used only externally, and some are best reserved for industrial purposes.
[Ubick, Suzanne. "Mud, Mud, Glorious Mud". Magazine of the California Academy of Sciences. California Academy of Sciences. Retrieved 2012.]