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The average crystal structures of two natural porous titanosilicates of the labuntsovite group, lemmleinite-Ba and lemmleinite-K, ideally Na 4 K 4 Ba 2 Ti 8 (Si 4 O 12 ) 4 O 4 (OH) 4 ⋅8H 2 O and Na 4 K 4 K 2 Ti 8 (Si 4 O 12 ) 4 O 2 (OH) 6 ⋅8H 2 O, respectively, have been refined from single crystal X-ray diffraction data. Both samples represent an extensive solid solution with labuntsovite sensu strictu Na 4 K 4 D 2 Ti 8 (Si 4 O 12 ) 4 O 4 (OH) 4 ⋅10H 2 O where D = Mn, Fe, and Mg. In addition to the sharp Bragg reflections both crystals, space group C2/m, a = 14.3, b =13.8, c = 7.75 Å, β = 117°, exhibit diffuse layers at c*/2 intervals indicating faulty superstructures with c = 15.7 Å. The diffuse layers consist of two types of reflections. The dominant type is strongly diffuse and smeared along a* indicating an I-centered Bravais lattice. The other type is very sharp but also weak and is in agreement with a C-centered lattice. Models for both superstructures have been developed on the basis of crystal-chemical principles and their theoretical diffraction patterns have been calculated and compared with the observed diffuse layers yielding excellent qualitative agreement.

Origin of diffuse superstructure reflections in labuntsovite-group minerals