Zincian spinel [(Zn,Fe,Mg)Al 2 O 4 ] is found in a variety of geologic settings including metamorphosed massive sulfide deposits, aluminous metasediments, pegmatites, and marbles. Many spinels in and near metamorphosed massive sulfides have formed by desulfidation and oxidation reactions that involve members of the system Zn-Fe-Al-Si-S-O. Experimental reversals of the simplified reaction: ZnS + FeS + 2Al 2 O 3 + O 2 = ZnAl 2 O (sub 4/) + FeAl 2 O 4 + S 2 , using hydrothermal techniques, have been obtained at 2 and 5 kb and various temperatures using Fe (sub 1-x) S + FeS 2 + Fe 3 O 4 as a buffer. Calculated positions of the spinel-sphalerite equilibria in f (sub O 2 ) -f (sub S 2 ) -X space, for ideal behavior along the ZnAl 2 O 4 -FeAl 2 O 4 join as is suggested by octahedral site preference energies, agree with experimental results and theoretical calculations depending on the choice of the free energy of formation of hercynite. The compositions of spinels in equilibrium with sphalerite and either pyrite or pyrrhotite become more Zn-enriched with increasing f (sub S 2 ) and f (sub O 2 ) values and can be used as a measure of f (sub O 2 ) during metamorphism.Spinel associated with metamorphosed massive sulfide deposits, marbles, and aluminous metasediments is enriched in the gahnite (ZnAl 2 O 4 ), spinel (MgAl 2 O 4 ), and hercynite (FeAl 2 O 4 ) molecules, respectively, whereas spinel associated with pegmatites is depleted in the spinel molecule. Such variability in composition constitutes a potential exploration guide for metamorphosed massive sulfide deposits.