A morphological study of the sulfurisation of digenite to covellite using reflected polarised light microscopy

Abstract A series of copper rods were reacted with sulfur vapour in evacuated glass ampoules at ∼445 °C. Product materials were characterised by powder X-ray diffraction and reflected polarised light microscopy. Copper sulfurised rapidly to digenite, γ-Cu 2-x S, under these conditions, whereas the subsequent sulfurisation to covellite, CuS, was notably slower, yielding texturally distinguishable inner (secondary) and outer (primary) CuS regions. A two-stage partial sulfurisation of γ-Cu 1.8 S resulted in the external growth of two successive layers of primary CuS, which demonstrates decisively that covellite − besides being a p-type metal − is ionically conducting at 445 °C, although considerably less so than digenite. We infer that the growth of platy covellite crystals and their radial alignment in the primary CuS layer are a consequence of copper ion mobility being restricted to the basal plane of the covellite structure. Sulfurising a coil of copper wire at ∼445 °C is an effective method for synthesising covellite.