Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10⁻⁵s⁻¹ using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H₂O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and ...