**5. Conclusions**

In this work we have definitively shown that anisotropic stress on quartz crystals causes the formation of amorphous silica and, from it, at least two silica polymorphs, cristobalite and tridymite, in nanocrystalline form. These phases accumulate with the advancement of the fractures and can profoundly modify the behavior of the solid in terms of friction coefficient. Amorphous silica is, therefore, not a simple friction melting product of the surface portion of the rock subject to friction, but it is a precursor of far more complex phases that represent the memory of the event. This observation contrasts sharply with what had been hypothesized by several authors, who aim to explain the lowering of the friction coefficient of the quartz subjected to shear stress due to the simple presence of silica gel.

Given the extreme speed with which cristobalites and tridymites are formed, we can argue that these mineral phases are generated while the fracture phase is still active and that their accumulation is such as to heavily modify the rock's mechanical behavior.

Cristobalite can thus be the main cause of the collapse of the friction coefficient occurring in the friction tests on quartz and on rocks rich in quartz.

**Author Contributions:** Conceptualization: G.M. and P.P.; Methodology: G.M. and P.P.; Analyses and investigation: P.P., E.T., E.P.; Writing—original draft preparation: P.P.; Writing-review and editing: G.M. and P.P.; Resources and project administration: F.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Thanks are due to two anonymous reviewers who contributed, with their suggestions, to improve the quality of the present paper. The Chinese Academy of Sciences Visiting Professorship partially supported Giovanni Martinelli for Senior International Scientists (2018VMA0007).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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