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Crystallography emphasizes the long‐range order or crystal structure of crystalline substances. It focuses on the symmetry of crystalline materials and on the ways in which their long‐range order is related to the three‐dimensional repetition of fundamental units of pattern during crystal growth. In minerals, the fundamental units of pattern are molecular clusters of coordination polyhedra or stacking sequences (ssss1). The ways in which these basic units can be repeated to produce crystal structures with long‐range order are called symmetry operations. Crystallography is also focused on the description and significance of planar features in crystals including planes of atoms, cleavage planes, crystal faces, and the forms of crystals. In addition, it is concerned with crystal defects which are local imperfections in the long‐range order of crystals.