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Potential problems in the interpretation of powder X-ray diffraction patterns from fine-dispersed 2M₁ and 3T dioctahedral micas

Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky Street, 119017 Moscow, Russia

^* E-mail: dritsva{at}geo.tv-sign.ru

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.

Key-words: cis-vacant layers, trans-vacant layers, ± b/3 disorder, interstratification, 2M₁, 3T mica polytypes, simulation of XRD patterns.

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