A metallic element, Mn, atomic number 25, and atomic weight 54.9380 g/mole. Manganese is one of the transition elements of the first long period of the periodic table, falling between chromium and iron. The principal properties of manganese are given in the table. It is the twelfth most abundant element in the Earth's crust (approximately 0.1%) and occurs naturally in several forms, primarily as the silicate (MnSiO₃) but also as the carbonate (MnCO₃) and a variety of oxides, including pyrolusite (MnO₂) and hausmannite (Mn₃O₄). Weathering of land deposits has led to large amounts of the oxide being washed out to sea, where they have aggregated into the so-called manganese nodules containing 15–30% Mn. Vast deposits, estimated at over 10¹² metric tons, have been detected on the seabed, and a further 10⁷ metric tonnes is deposited every year. The nodules also contain smaller amounts of the oxides of other metals such as iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). The economic importance of the nodules as a source of these important metals is enormous.