Complex stacking sequences in a lepidolite from Tordal, Norway Available to Purchase

Lepidolite from Tørdal, Norway, has been described as the complex stacking polytype 3M₂ = [121] of space group C2. Subsequent optical second-harmonic analysis indicated a center of symmetry possibly present, and Weissenberg photographs showed weak, extra six-layer spots. Of the 402 possible six-layer mica structures, only eight have the observed monoclinic symmetry and unit-cell geometry. Stacking arrangement 6M₁ = [121121] of space group C2/c, which can be generated by adding a center of symmetry in the octahedral sheet of the 3M₂ structure, fits the observed intensity-data best. The 6M₁ structure would refine only to R = 23%, however, and anomalies on DED maps in sections where the 3M₂ and 6M₁ structures differ suggest that the 3M₂ structure is dominant. Refinement of the data as a 3M₂ structure proceeded better to R = 12%, but is incorrect in detail.

hrtem imaging showed that both three-layer and six-layer stacking sequences are present in the same crystal, that twins formed by stacking faults of the three-layer structure exist, and that material with substantial stacking disorder also exists.