Electron-microscope and diffraction studies of polytypism in stilpnomelane

Several hitherto unreported, long-period polytypes of the complex layer silicate stilpnomelane have been identified by a combination of electron diffraction and high-resolution lattice-imaging techniques. The rich diversity of polytypes present in a Californian sample comprises two distinct 2-1ayer structures, one possessing triclinic, the other trigonal symmetry, a 3-layer trigonal variant, and further structures of, as yet, undetermined symmetry with 4, 5, 9 and 14 layers in the unit cell. Coherent intergrowths of the 9 and 14- layer polytypes have been observed in bright-field lattice images, and all polytypes show well-ordered electron-diffraction patterns. Specimens from this geological source, as well as two others (North Wales and New Zealand) display, in addition, disordered examples of the accepted, X-ray based, l-layer triclinic structure. The fine details of the stacking arrangements (vector sequences) in the 2 and 3-layer polytypes have been determined from electron-diffraction data, and, from similarities in the variation of contrast with sample thickness in lattice images of 2 and 14-layer structures, it is possible to formulate a model for the structures of the longer-period polytypes.