THE USE OF ELECTRON-OPTICAL METHODS TO DETERMINE THE CRYSTAL-STRUCTURE OF A MODULATED PHYLLOSILICATE - PARSETTENSITE

The crystal structure of parsettensite, approximately 7.5+(Mn,Mg)49(Si 64.5,Al7.5)SIGMA=72O68-(OH)50.nH2O where M+ is an exchangeable cation such as K, Na, or Ca, was solved by trial-and-error methods based on h igh-resolution transmission electron microscopy images and electron di ffraction data. A simple tilting experiment was utilized to determine qualitatively the magnitude of dynamical diffraction effects for refle ctions in a diffraction plane. Thus, it was determined that a kinemati cal approximation could be used, since dynamical diffraction effects w ere minimized by using very thin grains and superstructure reflections streaked parallel to c were not affected significantly by orientatio n changes near the (001) plane. Derived structural models of a single layer projected down c were compared with hk Fourier transforms. The t echnique was capable of discriminating among models. A distance least- squares (DLS) refinement confirmed that the tetrahedral linkages are d imensionally reasonable, and DLS-derived atomic coordinates are given. The ideal three-dimensional model conforms to C2/m symmetry with cell parameters, derived from X-ray powder methods, of a = 39.1(1), b = 22 .84(5), c = 17.95(6), d001 = 12.56 angstrom, and beta 135.6(2)-degrees . Parsettensite is a modulated 2:1 layer silicate. It consists of a co ntinuous Mn-rich octahedral sheet coordinated by silicate tetrahedral rings forming islands three tetrahedral rings wide. Pairs of islands a re linked by inverted and partially tilted tetrahedra that form four-m embered ring interisland connectors, with junctions of three islands f orming also 12-membered ring connectors. Layers are cross-linked throu gh two sets of four-membered ring connectors (double four-membered rin gs). Although the tetrahedral island connectors are different from tho se in stilpnomelane, the tetrahedral islands are similar, which explai ns the similarities in the diffraction patterns of the two minerals, a s noted by earlier workers.