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| огдоэдрия (the point group of a crystal is called ogdohedry if it is a subgroup of index 8 of the point group of its lattice; in the three-dimensional space there is only one ogdohedry: it corresponds to the geometric crystal class 3 of crystals belonging to the hexagonal lattice system [in case of rhombohedral crystals, it corresponds instead to a tetartohedry]) | |||