Plastic deformation of metamorphic pyrite: new evidence from electron-backscatter diffraction and forescatter orientation-contrast imaging

Abstract

Application of new scanning electron microscope techniques to the study of deformed metamorphic pyrite reveals evidence for plastic deformation not readily recognised by more traditional methods. Specifically, use of forescatter solid-state detectors in conjunction with tilted polished specimens of pyritic ore produces high quality crystallographic orientation contrast images, which map the distribution of deformation domains within grains. Use of electron-backscatter diffraction allows quantification of the crystallographic misorientations shown by the orientation contrast images. Combination of these techniques shows that the pyrite studied deforms by slip on {100} and more rarely {110} systems. Slip is often associated with distributed rotation of up to 20° about <100> and more rarely <110> axes. Pyrites may have simple histories involving rotation about a single <100> axis, or more complex histories involving rotation about different <100> axes, and more rarely <110>, in different domains of the same pyrite grain, or sequential rotations about quite different systems, typically distributed rotation about <100> followed by discrete rotation about a non-crystallographic axis.