Room-temperature experiments on antigorite serpentinite indicate that at least two mechanisms accommodate frictional sliding of the serpentinite: a rate-weakening, history-dependent mechanism which dominates at fast slip velocities and a rate-strengthening mechanism with no apparent history dependence which dominates at slow velocities. We present a two-mechanism model that successfully describes the two types of behavior observed in the friction experimental data without changing the constitutive parameters over the range of experimental conditions. Multiple mechanisms have been identified in higher-temperature experiments by previous workers and may be required for proper representation of all types at low slip rates and elevated temperatures.