We report a combined experimental and computational study of the polar nature of twin walls (TWs) in calcite. Second-harmonic generation (SHG) imaging reveals that $\left\{01\overline{1}8\right\}$ TWs exhibit clear second-harmonic  activity, whereas $\left\{10\overline{1}4\right\}$ TWs show no detectable response under any polarization configuration. Polarization-resolved SHG measurements demonstrate that the SHG signals at $\left\{01\overline{1}8\right\}$ TWs are consistent with a symmetry reduction from the bulk centrosymmetric structure ($R\overline{3}m$) to point group $m$. Molecular dynamics simulations clarify the microscopic origin of this polarity: At $\left\{01\overline{1}8\right\}$ TWs, charge-center displacements are confined to two atomic layers, giving rise to localized electric polarization, while at $\left\{10\overline{1}4\right\}$ TWs, alternating displacements in adjacent layers effectively cancel the net electric polarization. The simulations further reveal strong coupling between interfacial electric polarization and shear stress, as well as suppression of electric polarization with increasing temperature. These findings establish the atomic-scale mechanism of polarity at calcite TWs and provide a framework for understanding polar domain boundaries in centrosymmetric crystals.

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• Domain walls
• Ferroelectricity
• Molecular dynamics
• Optical second-harmonic generation