As a particularly common minerals in granites, the presence of feldspar and altered feldspar-chlorite gouges at hydrothermal conditions have important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K-feldspar and albite) and altered feldspar-chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐ stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35-90 MPa and temperatures of 120-400°C representative of in situ conditions for such reservoirs. Our experiment results show that the feldspar gouge is frictionally strong (μ~0.71) at all experimental temperatures (~120-400℃) but transits from velocity-strengthening to velocity-weakening at T>120°C. Increasing the pore pressure increases the friction coefficient (~0.70-0.87) and the gouge remains velocity weakening, but this weakening decreases as pore pressures increase. The presence of alteration-sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection-induced seismicity on pre-existing faults if devoid of chlorite alteration.