Montmorillonite and bentonite are both types of clay minerals, but they have some differences in their composition and properties. Here are the main differences between montmorillonite and bentonite:

Composition: Montmorillonite is a specific type of clay mineral that belongs to the smectite group, which also includes other clay minerals like bentonite. Bentonite, on the other hand, is a broader term that refers to a group of clay minerals, of which montmorillonite is one of the key components.

In simpler terms, montmorillonite is a type of clay mineral found within bentonite.

Water Absorption: Montmorillonite has a high water absorption capacity due to its unique layered structure. It can expand greatly when exposed to water and has a high swelling ability.

Bentonite, as a whole, exhibits similar water absorption and swelling properties, but its overall performance may vary depending on the specific composition and ratio of minerals present.

Cation Exchange Capacity (CEC): Montmorillonite has a high cation exchange capacity, which refers to its ability to attract and hold positively charged ions (cations) within its structure. This property allows montmorillonite to bind and exchange various ions, which can be beneficial in applications such as soil conditioning. Bentonite, as a group, also exhibits significant cation exchange capacity.

Applications: Montmorillonite and bentonite are both widely used in various industries and applications. Montmorillonite is commonly used in drilling muds, geotechnical engineering, cat litter, and as a binder in ceramics. Bentonite, which includes montmorillonite as one of its components, has a broader range of applications. It is utilized in areas such as drilling fluid, foundry, paper production, cosmetics, and pharmaceuticals.

Overall, the main difference lies in their specific definitions and classifications. Montmorillonite is a specific type of clay mineral, whereas bentonite is a broader term referring to a group of clay minerals, including montmorillonite. Bentonite can contain other clay minerals in addition to montmorillonite, and its properties and applications may vary based on the specific composition.

Montmorillonite clays and Bentonite clays 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.

various brands have used different names for the same thing. This has caused confusion.

Bentonite is an absorbent aluminium phyllosilicate clay consisting mostly of montmorillonite. It was named by Wilbur C. Knight in 1898 after the Cretaceous Benton Shale near Rock River, Wyoming 

 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.

Bentonite is an absorbent aluminium phyllosilicate clay consisting mostly of montmorillonite. It was named by Wilbur C. Knight in 1898 after the Cretaceous Benton Shale near Rock River, Wyoming 

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.]

References

http://durablehealth.net/