*3.3. Raman Analysis of Solid Phases on Fracture Surfaces*

Raman analyses were performed on fracture surfaces that appear dark under an optical microscope in polarized light and on areas where fracture streaks with fibrous traces are present. Figure 12 shows some spectra taken respectively in dark isotropic areas. Given the size of the Raman laser spot used (≈2 μm), the analysis cannot select only the filament areas that are smaller than 1/10 of spot, but will also include areas where there is certainly undisturbed quartz. Despite this, the analysis clearly shows the presence of α-cristobalite and tridymite in very variable proportions (Figure 12). In the first spectrum (white) the 465, 404 and 365 cm−<sup>1</sup> bands of alpha quartz are observed. It belongs to an optically non-isotropic zone. The underlying spectra, indicated as Black1, Black2 and Black3 in Figure 12, are three selected areas within optically isotropic areas of approximately 4 μm<sup>2</sup> each. In the spectra we observe the presence of the most intense bands of quartz, but also of tridymite and cristobalite, albeit of reduced intensity. The identified cristobalite is of the alpha type, while the tridymite is similar to the

PO10 polytype, as per Ruff standard of said mineral. On the right, an enlargement of the spectra is observed in the range of 440 to 320 cm<sup>−</sup>1. The Black1, 2, 3 spectra have been recorded at 200 Mpa; the quantity of these isotropic patches seems to increase with increasing pressure and with time of stay under stress.

**Figure 12.** Raman spectra of stressed quartz fragments; the three spectra below derive from the collection of Raman spectra included in the RRUFF collection (RRUFF Project, Department of Geosciences, University of Arizona, 1040 E 4th, Tucson, AZ, USA. 85721-0077): (Cristobalite (low)-R060648, Tridymite PO10-R040143, Quartz (low)-R050125.3); on the right, an enlargement of the area between 440 and 320 cm<sup>−</sup>1.
