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  1. 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. AGI
  2. 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.
    Source: Dictionary of Mining, Mineral, and Related Terms

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