AbstractA new exsolved copper‐iron‐sulfur phase in sphalerite has been found in the Dajing cassiterite‐sulfide copper‐silver‐tin deposit, Inner Mongolia, China. After investigating the optical properties, chemical composition, crystal structure and exsolved characters of the exsolved phase, the authors present important evidence for its being a new exsolved copper‐iron‐sulfur phase mineral. Its formation mechenism is discussed in considering heating experiments, phase equilibrium data and the geology o