Permeability and frictional strength of cation‐exchanged montmorillonite

Abstract

[1] Smectites, such as montmorillonite, are abundant throughout the upper crust and are commonly found in fault gouge. They are known for their weak frictional strength and low permeability. Smectites are swellable clays in which the interlayer cation can easily be exchanged. In this study, we measure permeability and frictional strength of montmorillonite exchanged with Na + , K + , Ca 2+ , and Mg 2+ using a triaxial shear apparatus. We find that the interlayer cation influences both permeability and the friction coefficient. K-montmorillonite is significantly stronger ( = 0.26) than Na-montmorillonite ( = 0.15) and Ca-or Mg-montmorillonite ( = 0.11). Permeability for all four clays is between 10 -21 and 10 -23 m 2 in the pressure range between 10 MPa and 100 MPa, with K-montmorillonite the most permeable clay gouge. Mg-, Ca-, and Na-montmorillonite show relative similar permeabilities of which Na-montmorillonite is least permeable. We relate the higher frictional strength and permeability of K-montmorillonite to reduce interlayer water content caused by differences in cation hydration and adsorption. The results show that the rapid cation exchange process in montmorillonite can influence macroscale parameters, such as permeability and strength, which can contribute to changes in local pressure conditions and frictional strength of shallow fault zones.Citation: Behnsen, J., and D. R. Faulkner (2013), Permeability and frictional strength of cation-exchanged montmorillonite,

Frictional strength of wet and dry montmorillonite

“…[], and Behnsen and Faulkner [, ]. In principle, these data should represent a material property of the clay and fall in a narrow band, taking into account the differences in frictional strength between, for example, Na‐ and Ca‐montmorillonite as described by Behnsen and Faulkner [], and the data should be reproducible between different laboratories. Behnsen and Faulkner [] report all friction values just after the early elastic portion of the loading curve transitions into shear sliding in order to avoid strain‐hardening effects altogether.…”

A “slice-and-view” (FIB–SEM) study of clay gouge from the SAFOD creeping section of the San Andreas Fault at ∼2.7 km depth

“…As current molecular models indicate that divalent ions concentrate around a medium line, with the water molecules protruding outwards toward the negatively charged interlayer surfaces, slip can be easily accommodated by the breakage and rebuilding of weak hydrogen bonds. In fact, only hydration of divalent interlayer cations have so far yielded friction coefficients equivalent to those measured for the creeping gouge, whereby the type of interlayer cation and its hydration structure explain the weak behaviour of smectite in laboratory shear experiments (Behnsen and Faulkner, 2013;Ikari et al, 2007). We posit that in addition to clay fabric, the state of cation hydration is a key contributor to creep along the SAFOD segment at~2.7 km depth and the origin of fault weakness as measured under laboratory conditions.…”

Exchangeable cation composition of the smectite‐rich plate boundary fault at the Japan Trench

“…Another significant implication of our result is that the anomalous composition of exchangeable cations can potentially perturb the mechanical state of the fault. Several experiments have examined the frictional properties of cation-exchanged smectite [e.g., Shimamoto and Logan, 1981;Moore and Lockner, 2007;Ikari et al, 2007;Behnsen and Faulkner, 2013], though most studies have focused on Ca-smectite and Na-smectite, and Mg-smectite has been only investigated in Behnsen and Faulkner [2013]. The frictional coefficient of Na-smectite is higher than Mg-smectite over a wide normal stress ranges [Behnsen and Faulkner, 2013].…”