Typical evolution of the coefficient of friction with shear displacement. Results are shown for experiments on wet illite/quartz gouge at 200°C and 500°C (RSRS07 and RSRS43 respectively; previously reported by Den Hartog et al. 2012a), on pure illite gouge at 500°C (RSI3), and on dry illite/quartz gouge at 500°C (RSRS66). The applied velocity steps are indicated in micrometre per second (μm/s), with the location of a change in velocity indicated using dashed lines connecting the same velocity step in different experiments. Shear strain γ is not indicated on the horizontal axis as it was different for all samples.