A COMPARISON OF THE STRUCTURES AND GEOMETRIC STABILITIES OF STILPNOMELANE AND PARSETTENSITE - A DISTANCE LEAST-SQUARES (DLS) STUDY

Structural models for the islandlike modulated phyllosilicates, stilpn omelane and parsettensite, were examined by distance least-squares (DL S) calculations. Parsettensite occurs naturally only near the Mn end-m ember, whereas stilpnomelane may be both Mn and Fe2+ rich. Models for compositions between Fe2+ and Mn2+ end-member stilpnomelane demonstrat ed that the stilpnomelane structure is geometrically stable across the composition range. In principle, therefore, stilpnomelane may occur a t Mn end-member compositions. DLS calculations indicated also that Fe2 +- and Mn2+-rich stilpnomelane-cell edge lengths are geometrically con trolled; c is fixed at approximately 17.835 angstrom (d001 12.13 angst rom). DLS-derived atomic coordinates are given for stilpnomelane. Misf it in stilpnomelane between the tetrahedral and octahedral sheets is r elieved by warping of the octahedral sheet and by adjustments to the d ouble six-membered ring interisland connectors. These adjustments incl ude 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, tetrahe dral tilting in the interisland connectors increases, along with the m agnitude of the d001 value. In contrast to parsettensite, however, onl y limited tilting of the tetrahedra in these connectors may occur, whi ch 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 req uires the nearly perfect alignment of islands. Thus, parsettensite pro bably cannot have a composition with smaller octahedral cations (e.g., Fe2+).