Todorokite

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 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 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. 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 Mg2+, manganese Mn3+, copper Cu2+, cobalt Co2+, nickel Ni2+ etc.), whilst the middle octahedra are occupied by the smaller Mn4+ cations. This structure is similar to that of hollandite (Ba,Mn2+)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, 6, 8 or more octahedra wide.

Unit cell The unit cell has six manganese Mn4+ sites and twelve oxygen O2− 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 = 14.6 Å and a = 24.38 Å, having the same b and c values as above. Epitaxial intergrowths of elongated crystallites resembling the twinned variety of acicular rutile have been observed by electron microscopy in todorokites from an iron-manganese concretion from the Pacific Ocean (a = 14.6 Å) and from the Bakal deposit (a = 14.6 Å and 24.4 Å). Todorokites with around 25 Å have been found in samples from Sterling Hill and the Takhte-Karacha deposit.

Appearance 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.

Optical properties Todorokite is biaxial, as are all monoclinic (and orthorhombic) minerals. In the polariscope, and in the polarizing microscope, specimens may be illuminated from below by light that is polarized by the polarizer, and viewed from above through an analyzer that transmits light of only one direction of polarization. When the directions of polarization of the polarizer and analyzer are at right angles, the specimen is said to be viewed between crossed polars. When todorokite is rotated between crossed polars it appears dark and light in turn, being dark when the crystal face or cleavage face is parallel to one direction of polarization. This is called parallel extinction. All uniaxial minerals display parallel extinction, but so do orthorhombic biaxial minerals such as olivine and orthopyroxenes. The refractive index of todorokite has not been determined, except insofar as it is very high; the original report gave it as greater than 1.74, and a later investigation put it even higher, greater than 2.00. For comparison, diamond has a refractive index of 2.42 and quartz 1.54. A biaxial crystal has three mutually perpendicular optical directions, X, Y and Z, with different refractive indices α, β and γ for light vibrating in planes perpendicular to these directions. The birefringence is the numerical difference between the greatest and the least of these indices; for todorokite it is nearly 0.02. Todorokite is distinctly pleochroic, appearing dark brown when viewed along the X direction, and yellowish brown when viewed along the Z direction [3], but the strength of the effect varies from faint to strong in material from different localities. The orientation of optical directions with respect to the lattice parameters is Y parallel to b and Z near or parallel to c.

Physical properties 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+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.

Solubility Todorokite is soluble in hydrochloric acid (HCl) with the evolution of chlorine (Cl2), and in concentrated sulfuric acid (H2SO4) forming a purple-red solution. It is also soluble in nitric acid (HNO3) forming a residue of manganese dioxide (MnO2).

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