Friction in clay-bearing faults increases with the ionic radius of interlayer cations

Smectite can dramatically reduce the strength of crustal faults and may cause creep on natural faults without great earthquakes; however, the frictional mechanism remains unexplained. Here, our shear experiments reveal systematic increase in shear strength with the increase of the ionic radius of interlayer cations among lithium-, sodium-, potassium-, rubidium-, and cesium-montmorillonites, a smectite commonly found in faults. Using density-functional-theory calculations, we find that relatively small sodium ions fit in the ditrigonal cavities on the montmorillonite surfaces, resulting in weakening of interlayer repulsion during sliding. On the other hand, relatively large potassium ions do not fit in the ditrigonal cavities, resulting in a larger resistance to sliding due to electrostatic repulsion between potassium ions. Calculated shear strength is consistent with our shear experiments by considering the partial dehydration of the frictional contact area. These results provide the basis for developing a quantitative model of smectite-bearing fault rheology.