The Zn²⁺-rich trioctahedral smectite (Ross, 1946) with the ideal composition of (M⁺_x ^. nH₂O)Zn₃(Si_4-xAl_x)O₁₀(OH)₂ where M is the exchangeable cation in the interlayer (univalent example given here), x is the number of substitutions for Si by Al in the tetrahedral sites, and n is variable. Zn content may vary commonly from 1.48 to 2.89 atoms per O₁₀(OH)₂, and the octahedral site may contain vacancies. Mg, Fe³⁺, and Al are known to replace Zn (Faust, 1951).