The term glauconite as currently used has a twofold meaning. It is used as both a mineralogic and morphologic term. The mineral glauconite is defined as an illite type of clay mineral. It is dioctahedral with considerable replacement of aluminum by iron and magnesium. Structural substitutions result in a charge deficiency in both the tetrahedral and octahedral sheets, and interlayer cations seem to balance both of these charges. Calcium and sodium as well as potassium are the interlayer cations. A fundamental characteristic of glauconite is that the unit cell is composed of a single silicate layer rather than the double layer of most other dioctahedral micas.