1. n. 
A material composed of clay minerals, predominantly montmorillonite with minor amounts of other smectite group minerals, commonly used in drilling mud. Bentonite swells considerably when exposed to water, making it ideal for protecting formations from invasion by drilling fluids. Montmorillonite forms when basic rocks such as volcanic ash in marine basins are altered.
2. n. 
A clay mineral that is composed principally of three-layer clays, such as montmorillonite, and widely used as a mud additive for viscosity and filtration control. Commercial bentonite ores vary widely in amount and quality of the swelling clay, sodium montmorillonite. Ores of lower quality, those with more calcium-type montmorillonite, are treated during grinding by adding one or more of the following: sodium carbonate, long-chain synthetic polymers, carboxymethylcellulose (CMC), starch, or polyphosphates. These help make the final product meet quality specifications. Unfortunately, the additives may not remain effective in "the real mud world" when in use at the rig due to hardness ions in the water, high temperature, bacterial attack, mechanical shear-degradation, and other factors that can render these additives ineffective.
See: cation exchange capacity, clay extender, conventional mud, hectorite, low-solids mud, low-yield clay, native clay, organophilic clay, peptized clay, phosphate salt, prehydrated bentonite, seawater mud, smectite, smectite clay, sulfonated polystyrene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, water, oil, and solids test