Single crystals of Rb_1.5Mn₈O₁₆ were grown under the hydrothermal conditions of 450°C and 0.2GPa. The structure was tetragonal (I4/m); a=9.915(9)Å, c=2.866(3)Å and Z=1. The X-ray structure analysis was performed using the 656 independent reflections to R=0.088 and R_w=0.054. The determined structure adopts hollandite type characterized by the 2×2 tunnel. It is noted the electron density distribution of Rb⁺ in the tunnel was elongated along the c-axis, suggesting the distribution of Rb⁺ over the special position (2b).

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Tetragonal cryptomelane from the Tirodi mine, India transforms to monoclinic structure at 710°C in air. The composition of the tunnel is same before and after transformation (Na:K = 1:2), but the site occupancy increase from 68% to 86%. The increase of the occupancy is due to the movement of tunnel cation which were in the part converted to bixbyite. The structure refinements of the both tetragonal and monoclinic cryptomelanes clearly show the change of the MnO₆ framework. In the monoclinic cell, two symmetry-independent octahedra exist. Mn(2) octahedra has greater distortion than the octahedra in tetragonal cell, but Mn(1) octahedra has less distortion. It is possible to consider that some of the Mn cations are reduced to Mn³⁺ to maintain the charge balance with the tunnel cations and Mn³⁺ ions exist mainly in Mn(2) octahedra for greater distortion. The transformation was caused by the increase of the tunnel site cation occupancy.

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The short-range structures of (Na_0.5Ca_0.5)Ga_1.5Ge_2.5O₈ and CaGa₂Ge₂O₈ glasses have been analysed by X-ray diffraction and EXAFS spectroscopy techniques. Structural differences have been found between these two glasses. The structure of (Na_0.5Ca_0.5)Ga_1.5Ge_2.5O₈ glass is similar to that of NaGaGe₃O₈ glass, and seems to be made of GaO₄ and GeO₄ tetrahedra. On the other hand, the coordination number of T (Ga and Ge atoms) atoms of CaGa₂Ge₂O₈ glass is larger than 4 (and about 5) and the T–O distance (=1.83 Å) is longer than that of NaGaGe₃O₈ glass.
Therefore, on CaGa₂Ge₂O₈ glass, it is supposed that the coordination number of Ga atom is larger than 4 because of its larger Ga–O distance obtained by EXAFS analysis.

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Chemical and physical data for five manganoan zinnwaldites from Japan were analysed and were found to be high in fluorine, reflecting the crystallization under relatively high f (HF or F₂) conditions. All specimens belong to 1M polytype. The role of fluorine on the formation of 1M type of mica structure was considered. Tetrahedral rotation angles α of five zinnwaldites were calculated from powder data.

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