A category of clay minerals that have a three-layer crystalline structure (one alumina and two silica layers) and that exhibit a common characteristic of hydrational swelling when exposed to with water. Montmorillonite is a well-known smectite clay mineral to those working in drilling and drilling fluids. Its sodium form, bentonite, is a widely-used water mud additive. It is also used as an oil-mud additive when made oil-dispersible by surface treatment. Smectite clays that occur naturally in shales cause wellbore and mud-control problems due to their hydrational swelling and colloidal dispersion characteristics.