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

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