Crystal‐plastic deformation is one of the main mechanisms that can accommodate large amounts of strain within the lithosphere. Despite the requirement of understanding dislocation nucleation and arrangement, the only accepted method for direct observation of dislocations in geological materials so far is transmission electron microscopy. Herein, we present a study using a combination of electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) to visualize and analyse crystal defects in pyrite deforming close to the crystal plastic to brittle transition zone. Structures in focus include (a) dislocation nucleation at crack‐tips and (b) the reactivation of mode I cracks accompanied by the nucleation of dislocations and crystal‐plastic behaviour resulting in the development of complex dislocation structures and low‐angle grain boundaries. EBSD maps reveal an increase in misorientation towards micro‐cracks, consistent with a greater dislocation density along cracks observed by ECCI.