See pyrosmalite.

An obsolete term for muscovite.

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

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)

Crystal growth from nanoparticles to larger particles that occurs by an assembly of aggregates, sometimes known as “oriented attachment”.

A mineral formed by an alteration process; the chemical constituents of a secondary mineral may be derived from the decomposition of a primary mineral and/or derived from outside the system during the alteration process.
Cf., primary mineral