<p>Epidote is ubiquitous and abundant within granitoid rocks in active geothermal reservoirs typically as alteration patches on natural fractures/faults. This highlights the necessity of understanding the impacts of rock epidotization and its heterogeneity in defining frictional strength and stability of earthquake-capable faults. We complete fault reactivation experiments on laboratory faults containing both homogeneous and lateral heterogeneous epidote-quartz mixed gouges to explore evolution of rheology at room temperature and low confining stress. Fault lateral heterogeneity is obtained via varying the size of epidote or quartz gouge patch on the fault surface, rather than mixing the two gouges homogeneously. Epidote contents span 0 to 100 <i>vol</i>.% in homogeneously mixed gouges and with epidote patch size increased from 0 to 100 <i>area</i>% in lateral heterogeneous faults. Coefficients of friction in pure quartz and epidote gouges approach 0.65 and 0.63, respectively and are less sensitive to structure as either homogenous or lateral heterogeneous mixtures. Frictional stability for pure quartz and epidote gouges is markedly different with quartz gouge exhibiting only stable frictional sliding but epidote primarily unstable sliding. Elevating the epidote proportion or patch size enhances instability with a transition from stable to unstable frictional sliding occurs at &gt; 60 <i>vol</i>.% epidote content for homogeneous faults, but with no apparent transition to instability for lateral heterogeneous faults. These observed transitions in frictional stability with epidote proportion and structure are partially explained by the relative dominance of localized shear evolving in epidote gouges and granular cataclastic flow within quartz gouges. These results aid in understanding the enhanced potential for instability in candidate EGS reservoirs relating to progressive epidote alteration.</p>