- any of the six main classifications of crystals and of crystal lattices according to their symmetry: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic.
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- crystallog any of six, or sometimes seven, classifications of crystals depending on their symmetry. The classes are cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. Sometimes an additional system, trigonal, is distinguished, although this is usually included in the hexagonal systemSee also crystal class
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