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.

A British term for underclay.
Cf., underclay

A rock or sedimentary layer underlying a coal bed with physical characteristics suggesting that it served as a soil which supported the vegetation that produced the coal. (Modified from Huddle and Patterson, 1961)