ON THE STRUCTURE OF FAULTS IN PYRITES

Images produced by transmission electron microscopy (TEM) under two-be am dynamic conditions have been used to determine the direction of the displacement vector of planar faults in specimens of FeS2 pyrite (Pa (3) over bar). The nature of the fringe contrast arising from the faul ts in TEM images confirms their identity as antiphase domain boundarie s (APBs). Experimental and computer-simulated selected area diffractio n patterns and experimental convergent beam electron diffraction patte rns have been used to account for the relatively high intensity of the kinematically forbidden reflections which are observed experimentally . The high intensity of the forbidden reflections in the experimental selected area diffraction patterns from faulted pyrites have been succ essfully modelled using a diffraction simulation of a pyrite lattice c ontaining an APE. The experimental and simulated data indicate that th e intensity of the kinematically forbidden ref-lections from pyrite ar ises from three separate mechanisms: (a) double diffraction from the t ranslational symmetry elements in the pyrite lattice (a dynamic scatte ring effect which increases with specimen thickness); (b) from small m isorientations of the specimen away from the zone axis; and (c) from t he structure and configuration of the APBs. The latter may be a more d ominant effect when boundaries in the plane of the thin-foil specimen are considered.