Dependence of Simulated Fault Gouge Frictional Behavior on Mineral Surface Chemistry Quantified by Cation Exchange Capacity

The slip behavior of crustal faults is known to be controlled by the composition of the fault gouge, but the exact mechanisms, especially considering the role of water-rock interactions, are still under investigation. Here, we use a geochemical approach measuring the cation exchange capacity (CEC) of several phyllosilicate minerals and non-clays, using CEC as a proxy for the ability to bind water to the mineral surfaces and/or develop electrostatic forces between particles. Laboratory shearing experiments show that low CEC materials (<3 mEq/100 g) tend to exhibit high friction, low cohesion, and velocity-weakening frictional behavior. Phyllosilicate minerals exhibit CEC values up to 78 mEq/100 g and correspondingly lower friction coefficients, higher cohesion, and a tendency for velocity-strengthening friction. Zeolite behavior is atypical, exhibiting a high CEC value typical of phyllosilicates but the strength and frictional characteristics of a non-clay with low CEC. This suggests that the structure of the mineral is important for non-phyllosilicates. For phyllosilicates, our results can be explained by water bound to mineral surfaces, creating bridges of hydrogen or van der Waals bonds between particles. The enhanced particle bonding for high CEC materials is consistent with high cohesion under zero effective stress conditions, and lowered friction by trapping bound water between the mineral surfaces under normal load. Bound water may explain the tendency for velocity-strengthening friction in high CEC materials by hindering a Dieterich-type time-dependent frictional strengthening mechanism.