A dioctahedral member of the smectite group. An important characteristic of this member is that isomorphous substitution occurs primarily in the tetrahedral sites to produce the net negative charge on the 2:1 layer (Güven, 1988). An idealized formula for beidellite is R⁺_0.33Al₂(Si_3.67Al_0.33)O₁₀(OH)₂.nH₂O, where R is an exchangeable cation, in this case univalent, but other valences are possible. The iron (Fe³⁺) analogue is known as nontronite and intermediate compositions are known. Beidellite is found abundantly as weathering products of volcanic ash, in association with hydrothermal environments, and as diagenetic products. An especially pure end-member beidellite comes from the Black Jack Mine, Idaho, USA.
Cf., smectite.