Todorokite
A variety of Minerals
What is Todorokite?
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.
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Key Characteristics
Characteristics of Todorokite
Todorokite has perfect cleavage parallel to the plane containing the b and c axes, and parallel to the plane containing the a and c axes. Contact twins occur frequently. The mineral is very soft, with hardness only 1 ⁄2. It is generally fibrous, making it difficult to measure the specific gravity accurately. 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. The pycnometer measures the mass and the volume of the specimen directly; this method gave a value of 3.66 to 3.82 for todorokite. The pycnometer is more likely to give an accurate reading for a fibrous material.
Appearance of Todorokite
Todorokite occurs as spongy banded and reniform (kidney-shaped) aggregates composed of minute lathlike crystals. The crystals are flattened parallel to the plane containing the a and c crystal axes, and elongated parallel to the c axis. Minerals of the hollandite-cryptomelane and romanèchite groups also have fibrous or acicular habits and two perfect cleavages parallel to the fiber axis. Todorokite is dark brown to brownish black in color and brown in transmitted light. It has a black to dark brown streak and the luster is metallic to dull, but silky in aggregates. It is opaque in all but the thinnest slivers, which are transparent.
Composition of Todorokite
Manganese occurs in different states, including Mn, Mn and Mn. Todorokite is made up of (MnO6) 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 K, barium Ba, silver Ag, lead Pb, calcium Ca and sodium Na. 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 Mg, manganese Mn, copper Cu, cobalt Co, nickel Ni etc.), whilst the middle octahedra are occupied by the smaller Mn cations. This structure is similar to that of hollandite (BaMnMn7O16) and romanèchite ((Ba,H2O)2(Mn,Mn)5O10), 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.
Quick Facts
Physical Properties
- Color
- Purplish gray, brown to black; brown in transmitted light
- Hardness (Mohs)
- 1.5
- Density
- 3.65 g/cm³
- Streak
- Brown, black, lead-gray
Chemical Properties
- Chemical Formula
- (Na,Ca,K,Ba,Sr)1-x(Mn,Mg,Al)6O12 · 3-4H2O Also given as (Ca,K,Na,Mg,Ba,Mn)(Mn,Mg,Al)6O12.3H2O.
- Elements
- Al, Ba, Ca, H, K, Mg, Mn, Na, O, Sr
Also Known As
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