Fig. 6

Photos of shear surfaces and corresponding Raman spectra transects. a Sample 111 (1 cm/s), and b Sample 112 (100 µm/s), showing discontinuous concentric rings of powder on sample surface. Raman spectra from transect scans are shown for each sample (c corresponds to sample 111, d corresponds to sample 112). The appearance and disappearance of the sharp quartz peaks along the transect can be correlated to the amount of visible wear material in the photos. See text for additional discussion