The effect of mineralogy and effective normal stress on frictional strength of sheet silicates

Phyllosilicates are common constituents of upper crustal faults and subduction forearcs. We studied the effect of mineralogy and controlled effective normal stress (between 5 MPa and 100 MPa) on frictional strength. Although the effect of mineralogy on frictional strength of single-phase phyllosilicate gouges has been previously studied, the influence of effective normal stress has not. We conducted water-saturated and vacuum-dry frictional tests on single-phase phyllosilicate gouges using a triaxial apparatus. Minerals included talc, pyrophyllite, kaolinite, lizardite, illite, montmorillonite, chlorite, muscovite, phlogopite, and biotite (particle size <30 μm). Results show friction coefficients between 0.22 – 0.44 (dry) and 0.12–0.38 (wet). Wet strength is always lower than dry strength for the same phyllosilicate, and those with hydrophilic surfaces are especially weakened by water. Tri-octahedral minerals are weaker than di-octahedral minerals with otherwise similar structures. The dependence of friction on interlayer bond strength is less clear than previously suggested. At effective normal stresses > 20 MPa dry friction coefficients are constant, and wet friction coefficients show a small increase. This is attributed to loss of water and increased contact area. The results indicate that frictional strength of clay-rich faults increases at depths less than ≈1 km under hydrostatic pore fluid pressures.