The concentration of Zn in bulk silicate Earth is ~55 ppm, while its average concentration in fresh mid-oceanic ridge basalt (MORB) glass is ~87 ppm. Like Cu, Zn commonly associates with Fe to form a variety of zinc sulfide minerals such as sphalerite. Additionally, Zn associates with carbonates and hydroxides to form numerous diverse minerals (e.g., smithsonite, sweetite, etc.). In mafic and ultramafic rocks, Zn tends to concentrate in oxides such as spinel and magnetite. In freshwater, Zn predominantly complexes with water to form an octahedrally coordinated aqua ion . In seawater, Cl ions replace up to four water molecules in the Zn aqua ion, forming ,  and .