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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.
== Structure ==
Manganese occurs in different states, including Mn2+, Mn3+ and Mn4+. Todorokite is made up of (Mn4+O6) octahedra that share edges to form triple chains. These chains share corners to form roughly square tunnels parallel to the b crystal axis.〔Burns, Burns and Stockman (1983) American Mineralogist 68:972-980〕〔Post and Bish (1988) American Mineralogist 73: 861–869〕 The tunnels accommodate water molecules and large cations such as potassium K+, barium Ba2+, silver Ag+, lead Pb2+, calcium Ca2+ 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 Mn3+, copper Cu2+, cobalt Co, nickel Ni etc.), whilst the middle octahedra are occupied by the smaller Mn4+ cations. This structure is similar to that of hollandite (BaMn2+Mn4+7O16) and romanèchite ((Ba,H2O)2(Mn4+,Mn3+)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.〔
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