Symmetry and cation displacements in hollandites: structure refinements of hollandite, cryptomelane and priderite.

Crystal structure determinations are reported for a) a hollandite, (Ba_0.75Pb_0.16Na_0.10K_0.04)(Mn,Fe,Al)₈(O,OH)₁₆, from Stuor Njuoskes, Sweden, with a 10.026(3), b 2.8782(7), c 9.729(3) A, beta 91.03(2)o, space group I2/m; b) a cryptomelane, (K_0.94Na_0.25Sr_0.13Ba_0.10)(Mn,Fe,-Al)₈(O,OH)₁₆, from Chindwara, India, with a 9.956(3), b 2.8705(9), c 9.706(4) A, beta 90.95(3) o, space group I2/m; c) a priderite, (K_0.90Ba_0.35)(Ti,Fe,Mg)₈O₁₆, from the West Kimberley area, Western Australia, with a 10.139(2), c 2.9664(9) A, space group I4/m. The symmetry of hollandite compounds depends on the ratio of the average ionic radius of the octahedral cations to that of the tunnel cations. Structures in which this ratio is >0.48 distort, reducing the tunnel volume, and thereby lowering the symmetry from tetragonal to monoclinic. The position occupied by a tunnel cation is determined primarily by the cation size. Relatively small cations, such as Ba²⁺ in priderite and Pb²⁺ in hollandite, displace from the special position, 2(a), to more stable sites that are at the sum of the ionic radii from the nearest O atoms. This study also indicates that the reduced form of Mn in hollandite and cryptomelane is Mn³⁺; the bond lengths suggest that the Mn³⁺ ion is more easily accommodated in the structures than the larger Mn²⁺.-J.E.C.