Metals occur as: (1) Native/free — Au, Ag, Pt, Cu (sometimes) — low reactivity metals. (2) Combined — as oxides (haematite Fe2O3, bauxite Al2O3.2H2O, cuprite Cu2O), sulphides (galena PbS, zinc blende ZnS, copper pyrites CuFeS2, cinnabar HgS, argentite Ag2S), carbonates (limestone CaCO3, siderite FeCO3, calamine ZnCO3), halides (rock salt NaCl, fluorspar CaF2, carnallite KCl.MgCl2.6H2O). Mineral = any naturally occurring compound of metal. Ore = mineral from which metal is extractable profitably. All ores are minerals; not all minerals are ores. Gangue = earthy impurities (silica, clay) in the ore. Flux = substance added to react with gangue to form fusible slag: CaO (basic flux for acidic gangue SiO2 → CaSiO3), SiO2 (acidic flux for basic gangue FeO → FeSiO3).