Nanoscale Tensile, Shear, and Failure Properties of Layered Silicates as a Function of Cation Density and Stress

Mechanical properties of layered silicates on the nanometer scale have been associated with large uncertainty. We attempt to clarify the linear elastic properties including tensile moduli, shear moduli, and potential failure mechanisms for the minerals pyrophyllite, montmorillonite, and mica in the order of increasing cation exchange capacity (CEC) under a broad range of stress using electronic structure calculations, semiempirical classical molecular dynamics simulation, and the comparison to available macroscopic experimental data. In-plane tensile moduli (xx and yy) are similar to 160 GPa independent of CEC and stress, whereas perpendicular tensile moduli (zz) range from 5 to 60 GPa as a function of CEC at low stress (0.01 to 1 GPa) and approach in-plane values at high stress. In-plane shear moduli (xy) are similar to 70 GPa independent of CEC and the shear strength increases from similar to 1 to similar to 3 GPa with increasing cation density. Shear moduli parallel to the layers (xz and yz) are between 2 and 20 GPa as a function of CEC, with a shear strength of 0.2 to 1 GPa beyond which the layers exhibit lateral shear flow. Tensile zz moduli, shear moduli, and shear strength in the xz and yz direction reach a local minimum at a cation density of 0.3 relative to mica. The simulation suggests sliding of the layers, in-plane kinks, and cation intrusion into the layers as potential failure mechanisms equal to amorphization on the macroscale. The anisotropy and stress-dependence of the mechanical properties is determined by the presence of rigid layers and flexible interlayer spaces of variable cation density. Current classical simulation models tend to overestimate in-plane moduli (xx, yy, xy) in a systematic way relative to electronic structure (DFT) and experimental results.