A high-resolution electron micrograph of the twin boundary in pyrite

Abstract

Minute fragments of an iron-cross twin of pyrite containing the twin boundary were studied by high-resolution electron microscopy with electron-diffraction control. Bright-field images show, looking down c, two mutually perpendicular sets of dark equidistant fringes (one set for each crystal in the twin) arising from interference of the 000 beam with the dynamically scattered 100 reflexions. The fringes of each set are 5.4 Å (one cell edge) apart. In each crystal the fringes are perpendicular to the [010] direction and hence to the macroscopic striations on (001), which are parallel to the edge (102):({\bar 1}02). Where the two crystals overlap and the fringes intersect to give a square net, the boundary surface can be seen. It is three-dimensional, irregular, consisting of planar rectangular regions parallel to the cube faces, ma × na Å2 in area, where integers m and n range from 1 to ~ 25. No evidence was found for defects or impurity atoms at the twin boundary. On the atomic scale the twin operation is a glide reflexion, with glide component equal to (a1 +a2)/2, in a (1{\bar 1}0) plane passing through point 0, ½, 0; but the arrangement of the S2 doublets along the twin glide plane remains indeterminate. The iron lattice complex F is coherent throughout the twin.