Todorokite is a complex hydrous manganese oxide mineral with generic chemical formula (Na,Ca,K,Ba,Sr)1-x(Mn,Mg,Al)6O12·3-4H2O.

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.

[11][12] The tunnels accommodate water molecules and large cations such as potassium K+, barium Ba2+, silver Ag+, lead Pb2+, calcium Ca2+ and sodium Na+.

[15] Todorokite occurs as spongy banded and reniform (kidney-shaped) aggregates composed of minute lathlike crystals.

[4][7][8] Minerals of the hollandite-cryptomelane and romanèchite groups also have fibrous or acicular habits and two perfect cleavages parallel to the fiber axis.

The Berman balance measures the relative weights of the specimen in air and in water; when todorokite was tested in this way it gave a value of 3.49.

[1][11] The type locality is the Todoroki mine, Akaigawa Village (25 km SW of Ginzan), Shiribeshi Province, Hokkaido, Japan.

[17] Although todorokite is a common product of oxidation and leaching of primary manganese carbonate and silicate minerals, its major occurrence is in deep-sea ferromanganese nodules.

[13] At the type locality in Akaigawa, Hokkaido, Japan, todorokite occurs as an alteration product of inesite (Ca2Mn2+7Si10O28(OH)2·5H2O) and rhodochrosite MnCO3).

[11] It is found as very fine fibrous flakes about 0.05 mm in length, loosely aggregated in sponge-like masses in druses in gold-bearing quartz veins.

[4] At Huttenberg, Carinthia, Austria, todorokite occurs as nodular masses about 5 cm in diameter, with a fibrous structure and coarse concentric layering.

[4] At Charco Redondo in Oriente Province, Cuba, fibrous todorokite has been found with fibers to 10 cm in length.

[14] At Farragudo in the Algarve, Portugal, a stalactitic mass of todorokite was found in a small collection of secondary manganese minerals, chiefly cryptomelane (K(Mn4+,Mn2+)8O16).

[4] Large crystals have been found in the Smartt Mine, Hotazel, and elsewhere in the Kuruman district, Cape Province, South Africa.

[8] At Sterling Hill, New Jersey, todorokite has been identified in a small collection of secondary manganese oxides from the old surface workings.

Associated with chalcophanite (ZnMn4+3O7·3H2O) and secondary calcite (CaCO3) crystals in franklinite (ZnFe3+2O4) – willemite (Zn2SiO4) ore.[4] At Saipan in the Mariana Islands, in the Pacific Ocean, todorokite has been found with the same mode of aggregation and color as that from Charco Redondo in Cuba.