Todorokite is a rare complex hydrous manganese oxide mineral with the chemical formula . It was named in 1934 for the type locality, the Todoroki mine, Hokkaido, Japan. It belongs to the prismatic class 2/m of the monoclinic crystal system, but the angle β between the a and c axes is close to 90°, making it seem orthorhombic. It is a brown to black mineral which occurs in massive or tuberose forms. It is quite soft with a Mohs hardness of 1.5, and a specific gravity of 3.49 - 3.82. It is a component of deep ocean basin manganese nodules. Structure Manganese occurs in different oxidation states, including , and . Todorokite is made up of octahedra that share edges to form triple chains. These chains share corners to form roughly square tunnels parallel to the b crystal axis. The tunnels accommodate water molecules and large cations such as potassium , barium , silver , lead , calcium and sodium . The octahedra at the edges of the triple chains are larger than those in the middle and therefore are likely to accommodate the larger cations (magnesium , manganese , copper , cobalt , nickel etc.), whilst the middle octahedra are occupied by the smaller cations. This structure is similar to that of hollandite and romanèchite , but with larger tunnels. Although tunnels formed from triple chains of octahedra are most common in todorokite, occasional tunnels have been observed in crystals from both terrestrial and manganese nodule deposits that have one pair of sides formed by triple chains, but with the other pair of sides formed from chains 4, 5, 8 or more octahedra wide. Unit cell The unit cell has six manganese sites and twelve oxygen sites constituting the octahedral framework. Mg and Al may substitute for Mn, and the tunnels contain large cations and water molecules. There is one formula unit per unit cell (Z = 1). The side lengths are a = 9.8 Å, b = 2.8 Å and c = 9.6 Å, with angle β = 94.1°. More detailed values given in the references are: a = 9.764 Å, b = 2.842 Å, c = 9.551 Å, β = 94.14° a = 9.7570(15) Å, b = 2.8419(5) Å, c = 9.5684(14) Å, β = 94.074(14)° a = 9.75 Å, b = 2.849 Å, c = 9.59 Å β = 90° Varieties also occur with a =