The concentration of Cu in bulk silicate Earth is ~30 ppm, slightly less than its average concentration (~72 ppm) in fresh mid-oceanic ridge basalt (MORB) glass.  and  form a variety of sulfides (often in association with Fe), as well as carbonates and hydroxides (e.g., chalcopyrite, chalcocite, cuprite and malachite). In mafic and ultramafic rocks, Cu tends to be concentrated in sulfidic materials. In freshwater, the predominant form of Cu is free Cu; in seawater, Cu complexes with carbonate ligands to form  and .