Final answer:
Magnetite is the mineral among the listed options that is classified as an oxide, with the chemical formula Fe3O4.
Step-by-step explanation:
The question asks which mineral is an oxide. Among the minerals listed, magnetite is the correct answer, as it is an iron oxide with the chemical formula Fe3O4. In contrast, calcite is a carbonate mineral with the formula CaCO3; galena is a lead sulfide mineral with the formula PbS; and chalcopyrite is a copper iron sulfide mineral with the formula CuFeS2. Minerals like hematite (Fe2O3) would also be classified as oxides, but it is not listed in the given options. Magnetite aligns with the reference to iron(III) oxide where iron is 3+ and oxide is 2-; Fe3O4.