1. n. 
Clay minerals whose surfaces have been coated with a chemical to make them oil-dispersible. Bentonite and hectorite (plate-like clays) and attapulgite and sepiolite (rod-shaped clays) are treated with oil-wetting agents during manufacturing and are used as oil-mud additives. Quaternary fatty-acid amine is applied to the clay. Amine may be applied to dry clay during grinding or it can be applied to clay dispersed in water. The latter process is much more expensive, requiring filtering, drying and other manufacturing steps. Organophilic bentonite and hectorite, "bentones," are used in oil muds to build rheology for cuttings lifting and solids suspension. They also contribute to low-permeability filter cake. Organophilic attapulgite and sepiolite are used in oil muds strictly to build gel structure, which may not be long lasting due to shear degradation as the mud is pumped through the bit.