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).

An obsolete term for lepidolite.

See pyrosmalite.

An obsolete term for muscovite.

See point defect.

An obsolete term for what is believed to be a mixture of opal and allophane.

An obsolete term, probably a nickel-bearing chlorite.

An obsolete term for lepidolite.

Symmetry involving a rotation about an axis followed by a translation parallel to the axis. The rotations correspond to a 2-, 3-, 4- or 6-fold axis and requires a translation following each rotation. The symbol for a screw axis is n_m, where n is the rotation component and m/n is the translation component. For example, in a 4₁ screw axis, the translation is 1/4 of a unit cell and the rotation corresponds to a 4-fold axis. Thus, symmetry consistent with a 4₁ screw axis is generated by a 90^o rotation followed by a 1/4 cell translation, with the process continuing until closure.
Cf., rotation symmetry, rotoinversion axis, symmetry, unit cell

See line defect.