Cavitation damage and ductile fracturing is a common phenomenon observed in high-temperature, ambient pressure deformation of superplastic metals and ceramics, but hardly described for geological materials. We performed high-pressure, high-temperature (400 MPa, 950°C–1200°C) torsion experiments on fine-grained (size ≈4 μm, aspect ratio ≈2.5) synthetic feldspar aggregates containing <3 vol% residual glass. Samples deformed at constant strain rates (≈2 × 10⁻⁵ – 2 × 10⁻⁴ s⁻¹) to high strain (≈2.8–5.6) reveal strain hardening at the lower strain rates. Microstructures show pronounced cavitation and formation of porosity bands containing redistributed glass, presumably associated with grain boundary sliding and shape-preferred orientation of high-aspect ratio feldspar grains. Sudden failure by strain-induced nucleation, growth and coalescence of the cavities occurred in one-third of the samples before deformation was terminated. In natural mylonites cavitation damage may produce increased porosity enhancing fluid flow in high-temperature shear zones.