XRD patterns were calculated for 2M1 and 3T dioctahedral mica structural models in which trans-vacant (tv) and cis-vacant (cv) layers were interstratified and arranged, independent of octahedral cation distribution, as in the periodic 2M1 and 3T mica polytypes. The XRD patterns of the interstratified cv/tv2M1 and cv/tv3T models were shown to be similar to those of the tv2M1 and tv3T mica structures, respectively. Because both the positions and shapes of hkl reflections in the XRD patterns of the tv3T and cv/tv3T as well as of the tv2M1 and cv/tv2M1, respectively, are the same qualitative X-ray identification of the tv2M1, cv/tv2M1, tv3T and cv/tv3T mica varieties should be a complex problem, especially when the amount of cv layers is either rather low (10–15 %) or high (30–40 %) with significant tendency to segregation of the layer types.

The diffraction features of cv/tv2M1 and cv/tv3T mica structures make it possible to use the Rietveld technique for the structural refinement. Correct application of this technique to the cv/tv2M1 and cv/tv3T structures should reveal partial occupancy of trans-octahedra because of the ability of diffraction to average structural parameters. Pavese et al. (2001) found partial occupancies of trans-octahedra in 3T and 2M1 powder phengite samples. To explain these results the authors postulated the ± b/3 stacking disorder. However, the calculations have shown that such disorder strongly modifies the XRD patterns. Therefore, in the paper an alternative, crystal-chemically more justified hypothesis is suggested according to which the occupancy of trans-sites in 3T and 2M1 phengites results from coexistence of tv and cv layers in their structures. Diffraction features a dioctahedral mica sample given as a tv3T standard in textbooks (Brindley, 1980; Bailey, 1984) are quite similar to those characteristic for a 3T structure in which tv and cv 2:1 layers are interstratified.

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