The occurrence of residues in the Fourier map of OD structures (polytypes in which pairs of building modules are geometrically equivalent) in the positions of the ‘virtual atoms' of the corresponding family structure derives from the presence of stacking faults inside an otherwise ordered (periodic) matrix. These residues are commonly spurious peaks deriving from the refinement of the structure with a single scale factor for both the family and the non-family reflections, which may be instead on a different scale because of the different peak shape and background characterizing the two types of reflections, resulting in broadening and streaking of the non-family reflections. The ‘virtual atoms' occur in the same positions of the atoms corresponding to the stacking faults, but the spurious peaks are in a quantitative relation with them only if the stacking faults diffract coherently. The case of mica polytypes is illustrated also with the aid of examples taken from the literature.

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