MAPPING THE COEFFICIENT OF FRICTION OF THE SERPENTINE MINERAL CHRYSOTILE RELATIVE TO EFFECTIVE NORMAL STRESS AND TEMPERATURE

We report new frictional strength data for the serpentine mineral chrysotile at effective normal stresses between 40 and 200 MPa in the temperature range 25-280°C. Triaxial friction experiments were performed using a 1-mm-thick layer of chrysotile gouge placed along a sawcut surface in antigorite driving blocks. Overall, the coefficient of friction of water-saturated chrysotile gouge increases both with increasing temperature and effective normal stress, but the temperature-related increases begin at about 100°C. For heated chrysotile, the rate of increase in coefficient of friction with increasing effective normal stress is 3-4 times higher below 100 MPa than above. As a result, a minimum in the coefficient of friction (<0.1) occurs at normal stresses below 50 MPa at about 100°C. Maximum coefficient of friction (>0.55) results from a combination of effective normal stresses >150 MPa and temperatures >250°C. The low-strength region is characterized by velocity-strengthening and the high-strength region by velocity-weakening behavior. Thoroughly dried chrysotile has a coefficient of friction >0.7 and is velocity weakening. The wide range in chrysotile strength can be explained by its tendency to adsorb large amounts of water, that acts as a lubricant during shear. The excess water is progressively driven out with increasing temperature and pressure, causing chrysotile to approach its dry strength.

We combined these new data with our previously published results for chrysotile to construct what may be the first contour map of the coefficient of friction relative to temperature and effective normal stress ever prepared for a geologic material. Depth profiles for a chrysotile-filled fault plotted on this map pass through the strength minimum at about 3-km depth for fluid pressures between hydrostatic and lithostatic, then strength increases rapidly at greater depths. Such a trend would not be predicted from the room-temperature data, which show a continuous, gradual increase in coefficient of friction with increasing effective normal stress. These results illustrate the potential hazards of extrapolating room-temperature strength data to predict fault-zone behavior at depth.