• Nanoindentation experiments on a high-angle grain boundary (60 • misorientation) in a pure forsterite bicrystal reveal that the interface acts as a source of dislocations. • Nanoindentation experiments on a high-angle grain boundary (60 • misorientation) in a pure forsterite bicrystal reveal that the interface acts as an obstacle to incoming dislocations, leading to pileups of dislocations. • Nanoindentation experiments on a subgrain boundary (13 • misorientation) in a pure forsterite bicrystal do not detect the impact of the interface on dislocations.