Cu5FeS4 bornite nanoparticles with sizes in the range from 10 nm to 40 nm were synthesized by high energy mechanical alloying (MA) and annealing. The phase evolution during MA and the related optical properties were studied. X-ray diffraction analysis showed the crystal structure of synthesized powders to adopt the high bornite phase after 2 h of MA. Following an annealing treatment at 300 degrees C for 20 min, the nanoparticle crystal structure changed to low bornite. The band gap of bornite nanoparticles was estimated to be about 0.87 eV for low bornite and 1.23 eV for high bomite. The photoresponse of low bomite nanoparticles indicated a transient photocurrent with fast recombination of photo-induced electron-hole pairs. The thermal conductivity of consolidated nano powder was 0.46 W m(-1) K-1 at room temperature and remained very low over the measured temperature range of 30-320 degrees C. A peak ZT value of 0.28 was obtained at 320 degrees C for consolidated nano-powder. As low bornite is stable at room temperature, the results suggest the possible use of this material, made of inexpensive and earth abundant elements, for energy conversion through thermoelectrics or photocatalysis. (C) 2017 Elsevier B.V. All rights reserved.