The chemistry of opaque spinel, delafossite (Cu₂O·Fe₂O₃), cuprite (Cu₂O), cassiterite, nickel-olivine (Ni₂SiO₄), and bunsenite (NiO) from some copper converter slags has been investigated by electron microprobe analysis. The spinel has a complex composition containing up to 33.89% NiO and 47.69% SnO₂, ranging from essentially Ni₂SnO₄ to (Ni,Fe2+)Fe23+O4. The associated delafossite contains up to 19.18% NiO and 38.15% SnO₂. The chemical variation of the mineral phases is evaluated, and it appears that Ni²⁺ enters the spinel and delafossite to charge-balance the octahedrally coordinated Sn⁴⁺.