Effects of the stacking faults on the calculated electron density of mica polytypes - The Durovic effect

The occurrence of residues in the Fourier map of OD structures (polytypes i n which pairs of building modules are geometrically equivalent) in the posi tions 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 refin ement of the structure with a single scale factor for both the family and t he nonfamily 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 refle ctions. The,virtual atoms' occur in the same positions of the atoms corresp onding to the stacking faults, but the spurious peaks are in a quantitative relation with them only if die stacking faults diffract coherently. The ca se of mica polytypes is illustrated also with the aid of examples taken fro m the literature.