Cu12+xSb4S13(x = 0.0, 0.5, 1, 1.5) and Cu12+xSb4S12Se (x = 0.5, 1, 1.5) compounds are synthesized by conventional solid state reaction followed by spark plasma sintering and their thermoelectric properties were investigated. The results reveal that the intrinsically low thermal conductivity of polycrystalline Cu12+xSb4S13 materials can be further reduced to 0.25 W.m−1.K−1 with the aid of exsolution process. Furthermore, we realize a substantial power factor enhancement for Cu12+xSb4S13(x = 0.5, 1, 1.5) via Se solid solution. By properly balancing electrical and thermal transport, a maximum zT of 1.1 associated with a power factor of 1.2 mW.m-1.K-2 at 723 K for Cu13.5Sb4S12Se is reported. Finally, we demonstrate that zT of mineral based thermoelectric materials can be greatly improved by synergistic integration of band structure engineering and exsolution process.