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 reorganization. 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.