A monoclinic mineral, (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂.nH₂O ; smectite group; expansive and cation exchangeable; perfect basal cleavage; in bentonite clays formed by 2054 alteration of volcanic ash or mafic igneous rocks. The chief constituent of bentonite and fuller's earth. Compare: beidellite; hectorite.