Copper Thioarsenates

Cupric Orthothioarsenate, Cu₃(AsS₄)₂, is formed as a dark brown precipitate when sodium orthothioarsenate is added to a solution of a cupric salt. The reaction, however, is complex, sulphides of copper and arsenic also being formed. A similar precipitate is formed when ammonium hydrosulphide or hydrogen sulphide is added to a solution of arsenic acid and a copper salt, and the proportion of sulphide and thio-salt in the precipitate varies with the concentration of the reactants. Copper hydroxide reacts with alkali thioarsenates to form copper sulphide and alkali arsenate, but some copper orthothioarsenate is formed and remains in solution in excess of alkali thioarsenate.

Cuprous Orthothioarsenate, Cu₃AsS₄, occurs widely distributed as the minerals enargite and elarite. These occur in massive or granular form or in greyish-black monoclinic or rhombic crystals.