Electron channelling contrast imaging of individual dislocations in geological materials using a field-emission scanning electron microscope equipped with an EBSD system

Abstract

Dislocation-related defects in minerals govern globally important rheological processes such as mantle convection in the deep Earth where dislocation creep is a common deformation mechanism of the constituent minerals. Understanding such processes requires the direct observation of individual dislocations in minerals and across their interfaces. Using electron channelling contrast imaging (ECCI) in a conventional field-emission scanning electron microscope (FE-SEM), we successfully observed individual dislocations in experimentally deformed polycrystalline ferropericlase as well as in natural olivine without special surface treatments to highlight dislocations such as oxidation decoration or chemical etching. Combining backscattered electron (BSE) imaging with electron backscatter diffraction (EBSD) techniques, we refined the two-beam condition of a Kikuchi band at the Bragg orientation to maximize the visibility of individual dislocations with a particular Burgers vector. The dislocation microstructures of progressive subgrain rotation as dominant recrystallization mechanism and the co-activation of two non-coplanar slip systems are demonstrated in ferropericlase and olivine, respectively. Inclined dislocations in olivine are also visualized in end-on view in ECCI. Orientation optimized ECCI in a FE-SEM may serve as an alternative to diffraction contrast imaging in transmission electron microscopy. ECCI shows promise as a non-destructive imaging of individual dislocations of rock-forming minerals.