Starting materials

The nanomaterials are prepared from crushed Verzasca gneiss powder (d ≤ 200 µm) of the same origin as the microcrystalline fault rocks described in previous studies^12,30,31. Image analysis of BSE micrographs shows that the initial powder material consists of 37% quartz (Qtz), 33% plagioclase (Plg), 28% potassium feldspar (Kfs) and minor micas (~2%). Chemical analysis by X-ray fluorescence (XRF) shows that the starting powder consists of 77 wt% SiO₂, 13.3 wt% Al₂O₃, 4.63 wt% K₂O, 3.16 wt% Na₂O, and ≤1 wt% of other elements³⁰.

To grind this material down to nanometric size, the powder is milled for a total of 12 min in deionized water with 0.1 mm diameter zirconia balls using a planetary ball mill (Fritsch Pulverisette 7). After each ~2 min of milling, the holder is left to cool for ~20 min. As the milling is performed in water the temperature increase during milling is buffered at ≤100 °C. The particle size distribution of the unsheared starting material falls in the range 0.01 ≤ d ≤ 1 μm, with a median of ~0.1 μm as determined by a laser particle meter (Fritsch Analysette 22). Note that this measurement likely represents the “apparent” grain size as nanoparticles tend to cluster together⁴⁸. After the milling, the powders are dried in an oven at 110 °C for >48 h to evaporate any residual H₂O. Supplementary Fig. ¹ shows the SEM-BSE and secondary electron (SE) topographic images of the dried nanomaterial. All minerals are mixed due to ball milling so it is impossible to distinguish individual grains. The material forms a compact shard that can be easily disintegrated using a mortar and pestle. Nanomaterials are known to be very reactive, and their properties could change over short periods of time even at room conditions^29,32. To minimize this inherent uncertainty, the nanomaterial used in this study came from one batch that went through the same history and was used over the period of ≈1 year. The fact that the microstructural as well as mechanical data are consistent between the individual runs indicates that the starting materials at elevated temperatures and pressures of the experiments have similar properties regardless of the “age” of the starting material.