The ultrastructure of pyroxenoid chain silicates. II. Direct structure imaging of the minerals rhodonite and wollastonite

Abstract

The feasibility of direct structure imaging of pyroxenoid chain silicates by high-resolution electron microscopy at 500 kV is demonstrated both experimentally and theoretically for the minerals wollastonite, CaSiO 3, and rhodonite, MnSiO 3. Image simulations with a simplified multi-slice approach indicated that the crystal projection most suitable for direct observation of the tetrahedral chain arrangement corresponded to the overlap of the metal cations with the tetrahedral sites, a situation which occurs in all pyroxenoids. Electron micrographs were also obtained, with brightfield axial illumination at 500 kV, which could be closely matched with the simulated images, at least for very thin crystal regions. The implications of these results, in particular for studies at the atomic level of solid-state transformations involving the pyroxenoids, are briefly discussed.

The ultrastructure of pyroxenoid chain silicates. III. Intersecting defects in a synthetic iron–manganese pyroxenoid

“…The presence of disordered intergrowths in various types of pyroxenoid chain silicates has been firmly established by high-resolution electron microscopy (Alario-Franco, Czank & Liebau, 1980;Ried & Korekawa, 1980) The original assignment of defects observed in (GOD lattice images to variations in the chain periodicity has been confirmed by combined computer simulation/electronmicroscopic studies (Jefferson, Pugh, Alario-Franco, Mallinson, Millward & Thomas, 1980), and it is now possible to observe the tetrahedral arrangement in such structures directly (Smith, Jefferson & Mallinson, 1981), hence assigning chain repeats unambiguously. The existence of defects of the types reported confirms the original suggestion (Liebau, 1972) that the pyroxenoids form a continuous structural series, adapting to variations in cation size/ pressure/temperature in a continuous manner.…”

Conditions for direct structure imaging in silicon carbide polytypes

“…However, in thicker specimens there is increasing divergence between experimental micrographs and theoretical simulations (e.g. Smith, Jefferson & Mallinson, 1981), and some adequate means for incorporating inelastic scattering into the image simulations needs to be developed in order initially to close this gap and, eventually, to enable some sort of inversion whereby experimental micrographs might be correctly interpreted. This is obviously much more serious in the presence of crystal defects of unknown structure -it then becomes necessary to rely on simulations of any neighbouring perfect material so that at least the imaging conditions (i.e.…”

Electron microscopy

“…It is often difficult to obtain three-dimensional information rather than merely projections of structures. The usable thickness of sample is usually limited to about 50 A for 100-keV electrons with only a small increase at higher voltages if the best possible resolution is achieved (15) .…”