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A  Clay
Bentonite  is a member of the Smectite clay family which is one of seven major clay groupings, and Smectite is further subdivided into seven sub-groupings of its own.   Clays are classified amongst known colloids.  By definition clays are fine-grained sediments less than 0.0039 mm in size.   Smectite’s general chemical formula is:

     [Ca, Na, H) (Al, Mg, Fe, Zn) 2(Si, Al) 4O10(OH)2 - xH2O]

     Usually, Bentonite is comprised of about 80% Montmorillonite, another member of the Smectite Group, along with minor amounts of Smectite’s additional five other prominent members.  In alphabetical order they are: Nontronite, Pyrophyllite, Saponite, Sauconite, and Talc.   All members of the Smectite Group are 2:1 clays, meaning they are composed  principally of three-layers consisting of two tetrahedral sheets for every one octahedral sheet.   They are hydrophilous; Aluminum phyllo-silicates with a crystalline form.

 
     Oil Well Drilling   Bentonite is most commonly known for its use as a major component of drilling mud, particularly for oil wells, but is also widely used as an additive for viscosity and filtration control.  When exposed to water, Bentonite, characteristic of all clays, swells considerably, making it ideal for protecting subterranean formations from invasion by the other drilling fluids making up the mud by providing a thixotrophic effect.  However, it is mainly used as circulation mud in rotary drilling systems.  Bentonite’s chief purpose as a drilling mud is to lubricate and to cool the cutting bits, to carry away cut rock fragments, and to act as a seal against the escape of gas from the bore hole.  Other related functions of such Bentonite based fluids is to prevent the hole from blowing out, as well as, to condition the wall of the drill hole to prevent caving-in.  

     Its major component, Montmorillonite forms when basic rocks, such as tuff or volcanic ash in fresh water calderas and shallow marine environments, are altered.  This process is known as devitrification and comprehends a chemical change of such glassy igneous material. Although Bentonite contains prodigious amounts of its sister clay, Montmorillonite, Bentonite is much superior for drilling because it is often enriched with an usually high sodium content.  Hence the name “Sodium Montmorillonite”, or (Na) Montmorillonite.  

     Another form of Bentonite, “Calcium Montmorillonite” or (Ca) Montmorillonite is deemed to be inferior for this purpose.  Calcium Montmorillonite requires additives such as Sodium Carbonate, long-chain synthetic polymers, carboxymethylcellulose (CMC), polyphosphates, or starch to be injected in order to help make the final product meet quality specifications as a drilling mud.  Unfortunately, these ingredients may not remain effective in the long-term when in use on a drilling rig, due to bacterial attack, high temperature, mechanical shear-degradation, hardness ions in the water, and other factors that can render such additives ineffective.
Originally, the Smectite Group was better known as the Montmorillonite Group, hence Bentonite, Saponite, Talc, etc., may be called “Montmorillonoids”.   For additional background concerning Bentonite and its sister clays, consult the website hosted by Window Peak Trace Minerals, an important supplier of Montmorillonite for application in agriculture, and animal and human nutrition.

     Historical sketch of Certain Smectite Clays’ Nomenclature http://www.specialplacestostay.com/maps/browsemap/france/09d/
Despite what you may read on other websites, Bentonite and Montmorillonite, are not exactly the same thing.  Montmorillonite was a name coined in 1847 for a discovery near the Prefecture of Montmorillon (départment of Vienne in the region of Poitou-Charentes), France.  [Mauduyt: Un mot sur un morceau de quartz d´une variété particulière, ainsi que sur une substance minérale trouvée dans le department de la Vienne. Bull Soc Géol France 4 (1847) 168-170].  Mauduyt’s chosen name for the mineral, “Montmorillonniste” was short-lived. Monsieurs Damour and Salvetat suggestion of the label “Montmorillonite” has stuck ever since.
 
 
     Bentonite proper (aka “Pascalite” http://en.wikipedia.org/wiki/Pascalite), was first discovered in 1830 by the French-Canadian fur trapper Emile Pascal atop the 8600 feet http://www.bighornmountains.com/ high Big Horn Mountains in Wyoming, but was not distinguished from the former until much later.  Pascal soon developed a mining partnership with Ray Pendergraft calling the clay after himself.  By the late 1880s William Taylor became the biggest producer of this variety of Smectite in the Rock Creek area of Wyoming, and the common name “Taylorite” was born in his honor.  However, it was just before the turn of the 20th Century that American geologist W. C. Knight devised the name Bentonite [Eng Min J (1898) 66: 491] for a deposit he found near Fort Benton, Montana --a portion of the Fort Benton Formation geological stratum that extends into the Rock Creek area of eastern Wyoming.  Pascalite and Taylorite, along with many other names, long since have fallen into synonymy with Bentonite, the preferred name by serious geologists.

     Interestingly enough, Fort Benton was originally called Fort Lewis (founded in 1846 as a well-known American Fur Company outpost by Major Alexander Culbertson, but was renamed in 1850 for Thomas Hart Benton.  After military service in the War of 1812, Benton settled in St. Louis, Missouri, and in 1815 and became editor of the St. Louis