Structural models for the islandlike modulated phyllosilicates, stilpnomelane and parsettensite, were examined by distance least-squares (DLS) calculations. Parsettensite occurs naturally only near the Mn end-member, whereas stilpnomelane may be both Mn and Fe2+ rich. Models for compositions between Fe2+ and Mn2+ end-member stilpnomelane demonstrated that the stilpnomelane structure is geometrically stable across the composition range. In principle, therefore, stilpnomelane may occur at Mn end-member compositions. DLS calculations indicated also that Fe2+- and Mn2+-rich stilpnomelane-cell edge lengths are geometrically controlled; c is fixed at approximately 17.835 Å (d001 = 12.13 Å). DlS-derived atomic coordinates are given for stilpnomelane.
Misfit in stilpnomelane between the tetrahedral and octahedral sheets is relieved by warping of the octahedral sheet and by adjustments to the double six-membered ring interisland connectors. These adjustments include an in-plane tetrahedral rotation and out-of-plane tilting. In Mg- or Fe3+-rich stilpnomelane, Si to Si distances within the interisland connectors are reduced and Si to Si repulsions increase. Thus, tetrahedral tilting in the interisland connectors increases, along with the magnitude of the d001 value. In contrast to parsettensite, however, only limited tilting of the tetrahedra in these connectors may occur, which accounts for the different d001 values in the two model structures.
In parsettensite, double four-membered ring interisland connectors do not allow for tetrahedral rotation, and, therefore, parsettensite requires the nearly perfect alignment of islands. Thus, parsettensite probably cannot have a composition with smaller octahedral cations (e.g., Fe2+).