Electron-microscope and diffraction studies of polytypism in stilpnomelane

Abstract

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.

The crystal structure of ganophyllite, a complex manganese aluminosilicate. I. Polytypism and strutural variation

“…Accepted cell constants for the minerals ganophyllite and bannisterite (Smith & Frondel, 1968) A2/a lographic forms. In contrast to the polytypes of stilpnomelane (Crawford, Jefferson & Thomas, 1977) these were not the result of varying the stacking sequence of layers parallel to (001). Consequently, it was necessary to elucidate the exact nature of the nonspace-group absences and intergrowths of the various crystallographic variants of ganophyUite before proceeding with a complete structure determination, in order to reduce the large number of parameters which had to be evaluated.…”

High resolution electron microscopic and X-ray studies of non-random disorder in an unusual layered silicate (chloritoid)

“…is th a t it involves no averaging, since the local structure is revealed directly. And provided due caution is taken in interpreting the results, it is possible, as we have dem onstrated recently for the complex layered silicate stilpnomelane (Crawford, Jefferson & Thomas 1977) to attack directly the problem of the nature and distribution of poly types. Accordingly we have used both h.r.e.m.…”

“…In a similar way, information regarding the true structural periodicity is also evident in the image, and in effect multiple scattering is actually an aid to polytype iden tification, particularly with thicker crystals where the effect is more pronounced. Advantage was taken of this phenomenon in the identification of new polytypes in stilpnomelane by Crawford, Jefferson & Thomas (1977).…”

“…One of the principal reasons why high resolution electron microscopy (h.r.e.m.) has made such an impact in inorganic chemistry is that, under favourable circum stances, it affords a direct picture of the projected charge density of the structure of a solid (Iijima 1971;Goodman & Moodie 1974;Cowley 1975;Cowley & Iijim a 1976). Details of the conditions necessary for the attainm ent of such im agesoptimal structure, specimen thickness and imaging conditions -have been given previously (Cowley 1975;Anderson 1977), but the general nature of the results achieved requires emphasis.…”

57Fe Mossbauer study of the oxidation state of iron in stilpnomelane from granite pegmatites in Poland

“…The 24 tetrahedral groups are linked together by 6-membered rings of tetrahedra, with their apices pointing in the opposite direction from those of the islands. These rings have a more trigonal configuration, and their apices are also the apices of similar but inverted 6 member rings linking the islands coordinating to the next octahedral sheet (Eggleton 1972;Crawford et al 1977).…”