Atomistic origins of interface-controlled failure in polycrystalline aragonite

Polycrystalline aragonite exhibits a striking mechanical scale transition: single crystals reach tensile and shear strengths of 4-6 GPa, yet micropillar compression experiments on polycrystalline specimens fail near 462 MPa, indicating that interfaces rather than lattice defects govern macroscopic behaviour. Using molecular dynamics simulations with a force field validated to within 5% of experimental elastic constants, we determine atomistic deformation and failure mechanisms in three limiting interface types: crystalline twins, hydrated protein-mediated interfaces, and water-separated mineral interfaces. Three-dimensional tensile-shear failure surfaces reveal a strength hierarchy spanning 0.3-6.5 GPa controlled entirely by interface composition. Crystalline twins fail through shear-coupled boundary migration, whereas hydrated interfaces show lower strength but high damage tolerance via progressive hydrogen-bond network reorganisation. Water layers exhibit thickness-dependent cohesion, from strong electrostatic coupling in thin films to compliant sliding in thicker layers. Reinterpreting micropillar compression through a Schmid-type relation yields a resolved shear strength of ∼ 230 MPa that lies within the range spanned by the simulated interfaces, corroborating that bulk failure is governed by interfacial slip rather than crystalline fracture. These quantitative structure–property relationships provide transferable parameters for multiscale modelling and clarify the molecular origins of strength reduction in polycrystalline inorganic materials.