Todorokite is a rare complex hydrous manganese oxide mineral with the formula (Na,Ca,K,Ba,Sr)_1-x(Mn,Mg,Al)₆O₁₂·3-4H₂O. 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.