FRICTION OF 2:1 SHEET SILICATES - HOW CRYSTAL STRUCTURE INFLUENCES SHEAR STRENGTH

Phyllosilicates are of importance to earthquake research, because they tend to have lower frictional strengths than other common rock-forming minerals and thus provide a possible weakening mechanism for faults. As part of a larger investigation of the frictional properties of layer-structured minerals, we sieved separates of twelve 2:1 sheet silicates to produce synthetic gouges. We ran room-temperature frictional sliding experiments on sawcut rock samples containing a 1-mm layer of each gouge. The samples were dried at 120 degrees C overnight and then immediately tested in a triaxial apparatus at 100 MPa normal stress and 0.0005 mm/sec sliding velocity. The samples were sheared dry to 4 mm displacement, then water was introduced to a pressure of 10 MPa and sliding was resumed at 100 MPa effective normal stress. Overall, the coefficient of friction (=shear stress/effective normal stress) of a given mineral correlates directly with its calculated (001) bond strength; the water-saturated values range from about 0.2 for talc to 0.7 for the brittle micas margarite and clintonite (typically, the coefficient of friction is about 0.8 for major rock-forming minerals at these conditions). All dry values of coefficient of friction are 0.1-0.2 higher than the water-saturated values, indicating that water acts as a lubricant during shear of phyllosilicate gouges. Shear of the gouge layers is highly localized to a few major shear planes in which the platy grains are oriented parallel to the shear surfaces. We propose that the water in the saturated gouges forms thin structured films between the plate surfaces, and that shear is concentrated in the water films. The wide range in the water-saturated coefficient of friction reflects the strength of the bonding of the water films to the plate surfaces, which in turn is a function of the strengths of the important contributors to the (001) bond. For the 2:1 sheet silicates, the charge on the tetrahedral sheet is the dominant control on shear strength. In addition, the coefficient of friction is 0.10-0.15 higher for a dioctahedral mineral than for its trioctahedral equivalent. The frictional strength of any 2:1 sheet silicate should therefore be predictable from its composition.