a. Symbol in structural petrology for the direction of tectonic transport,
similar to the direction in which cards might slide over one another. Striations
in a slickensided surface are parallel to direction a.
b. A crystallographic axis: In the isometric system each axis is designated a;
in hexagonal, tetragonal, and trigonal systems the nonunique axes are a; in the
orthorhombic system a is always shorter than b with c either the longest or the
shortest axis; in the monoclinic and triclinic systems a may be determined by
one of several conventions.
Dictionary of Mining, Mineral, and Related Terms