Figure 6
Face forms tetragonal pyramid {12l} and {31l}, projected along the common tetragonal axis, and the coset of the four alternate [Sigma]5 twin reflection planes m'. These planes belong to the oriented eigensymmetries of both forms, which are mapped upon themselves by this twinning. Thus, the corresponding reflection sets {12l} and {31l} are superimposed with their twin-related sets and have equal F moduli (diffraction case A). For the tetragonal bipyramids, four additional twofold axes (parallel to the traces of the mirror planes m') belong to their eigensymmetry and the twin law. These coincidence and intensity characteristics hold, more generally, for the sets {h, 2h,l}and {3h,h,l} with h, l any positive or negative integer, including the limiting cases of tetragonal prisms (l = 0) and pedion or pinacoid (h = 0).  [article HTML]

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