• Mineral interfaces in granite exhibit transitional mechanical properties, with elastic modulus and hardness intermediate between adjacent mineral phases.
• Higher plasticity indices at interfaces indicate enhanced plastic deformation capacity of hard-phase minerals in these regions.
• Fracture toughness of mineral interfaces ranges from 0.90 to 1.63 MPa m0.5, providing critical data for micromechanical modeling.
• A significant positive linear correlation exists between elastic modulus and fracture toughness at the rock scale, but this correlation weakens at the mineral scale, revealing a scale effect.