Structure
There are two forms of Cu2S 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 Cu1.96S which is non-stoichiometric (range Cu1.934S-Cu1.965S) and has a monoclinic structure with 248 copper and 128 sulfur atoms in the unit cell. Cu2S and Cu1.96S are similar in appearance and hard to distinguish one from another.
Read more about this topic: Copper(I) Sulfide
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