The crystal structure of parsettensite, approximately where M+ is an exchangeable cation such as K, Na, or Ca, was solved by trial-and-error methods based on high-resolution transmission electron microscopy images and electron diffraction data. A simple tilting experiment was utilized to determine qualitatively the magnitude of dynamical diffraction effects for reflections in a diffraction plane. Thus, it was determined that a kinematical approximation could be used, since dynamical diffraction effects were minimized by using very thin grains and superstructure reflections streaked parallel to c* were not affected significantly by orientation changes near the (001) plane. Derived structural models of a single layer projected down c were compared with hk Fourier transforms. The technique was capable of discriminating among models. A distance least-squares (DLS) refinement confirmed that the tetrahedral linkages are dimensionally reasonable, and DlS-derived atomic coordinates are given. The ideal three-dimensional model conformsto 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 Å, and β = 135.6(2)°.
Parsettensite is a modulated 2:1 layer silicate. It consists of a continuous Mn-rich octahedral sheet coordinated by silicate tetrahedral rings forming islands three tetrahedral rings wide. Pairs of islands are linked by inverted and partially tilted tetrahedra that form four-membered ring interisland connectors, with junctions of three islands forming also 12-membered ring connectors. Layers are cross-linked through two sets of four-membered ring connectors (double four-membered rings). Although the tetrahedral island connectors are different from those in stilpnomelane, the tetrahedral islands are similar, which explains the similarities in the diffraction patterns of the two minerals, as noted by earlier workers.