Copper(I) sulfide
Copper(I) sulfide is a copper sulfide, a chemical compound of copper and sulfur. It has the chemical compound CuS. It is found in nature as the mineral chalcocite. It has a narrow range of stoichiometry ranging from CuS to CuS. Preparation and reactions. CuS can be prepared by heating copper strongly in sulfur vapour or HS. The reaction of copper powder in molten sulfur rapidly produces CuS, whereas pellets of copper require much higher temperature. CuS reacts with oxygen to form SO: In the production of copper two thirds of the molten copper sulfide is oxidised as above , and the CuO reacts with unoxidised CuS to give Cu metal: Structure. There are two forms of CuS: a low temperature monoclinic form ("low-chalcocite") which has a complex structure with 96 copper atoms in the unit cell and a hexagonal form stable above 104 °C. In this structure there are 24 crystallographically distinct Cu atoms and the structure has been described as approximating to a hexagonal close packed array of sulfur atoms with Cu atoms in planar 3 coordination. This structure was initially assigned an orthorhombic cell due to the twinning of the sample crystal. There is also  a crystallographically-distinct phase (the mineral djurleite) with stoichiometry CuS which is non-stoichiometric (range CuS-CuS) and has a monoclinic structure with 248 copper and 128 sulfur atoms in the unit cell. CuS and CuS are similar in appearance and hard to distinguish one from another.