Todorokite

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Todorokite
Specific gravity
3.5 to 3.8
Optical propertiesBiaxial
Refractive indexGreater than 1.74
Birefringencenearly 0.02[3]
PleochroismX = dark brown, Z = yellowish brown[4]
FusibilityDoes not fuse[3]
SolubilitySoluble in acids[3]
References[5][6][7][8]
Polyhedral representation of the todorokite structure.[9]

Todorokite is a complex hydrous

manganese nodules
.

Structure

Manganese occurs in different oxidation states, including Mn2+, Mn3+ and Mn4+.[10] 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.[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+.[11] 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.[9] This structure is similar to that of hollandite (Ba,Mn2+)Mn4+7O16 and romanèchite (Ba,H2O)2(Mn4+,Mn3+)5O10, but with larger tunnels.[12] 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.[11][13]

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).[5][6][8] 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°[5][6]
  • a = 9.7570(15) Å, b = 2.8419(5) Å, c = 9.5684(14) Å, β = 94.074(14)°[8]
  • a = 9.75 Å, b = 2.849 Å, c = 9.59 Å β = 90°[7][14]

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

Sterling Hill and the Takhte-Karacha deposit.[15]

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.[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.[11] Todorokite is dark brown to brownish black in color and brown in transmitted light.[7] It has a black to dark brown streak[6][8] 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

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.[3][4]

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,[3] and a later investigation put it even higher, greater than 2.00.[4] 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.[3] 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.[4] The orientation of optical directions with respect to the lattice parameters is Y parallel to b and Z near or parallel to c.[4]

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.

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.[14]

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).[3]

Other characteristics

When todorokite was first discovered, in 1934, modern methods of mineral analysis were not available, and one of the standard techniques was to use a blowpipe to heat a small sample of the mineral, and observe its behavior. Todorokite was tested in this way, and it was noted that under the blowpipe it turned brown and lost its metallic luster, but did not fuse.[3]

X-ray diffraction lines at 9.5 to 9.8 Å and 4.8 to 4.9 Å are characteristic. This is also true of buserite Na4Mn14O27·21H2O.[1][11]

Type locality

The

Akaigawa Village (25 km SW of Ginzan), Shiribeshi Province, Hokkaido, Japan.[17] Type material is conserved at the Harvard Mineralogical Museum, Cambridge, Massachusetts, US, reference 106214.[17]

Occurrence and associations

Although todorokite is a common product of

silica (SiO2).[4]

Other localities

Austria

At

Carinthia, Austria, todorokite occurs as nodular masses about 5 cm in diameter, with a fibrous structure and coarse concentric layering. It is fragile, soft and porous, and so light that it can float on water. It is brown, but lighter in color than samples from Charco Redondo in Cuba. The luster of fracture surfaces is dull, but the smooth outer surface of the nodules has a faintly bronze-like appearance. It has a relatively high barium content, and also Mn4+, perhaps due to a small admixture of pyrolusite (Mn4+O2).[4]

Brazil

Todorokite is a rare constituent of the

metamorphic country rock that contains spessartine (Mn2+3Al2(SiO4)3) and other manganese minerals.[4]

Cuba

At

hot springs.[14]

Portugal

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). It is soft and porous, with a fibrous mode of aggregation and dark brown to brownish black color.[4]

South Africa

Large crystals have been found in the Smartt Mine, Hotazel, and elsewhere in the Kuruman district, Cape Province, South Africa.[8]

United States

At Sterling Hill, New Jersey, todorokite has been identified in a small collection of secondary manganese oxides from the old surface workings. It forms soft, dark brownish-black masses with a confused fibrous structure. 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. It is relatively brittle and hard because it is intimately admixed with finely divided silica, amounting in some cases to as much as 50% of the sample. The content of magnesium is greater than that of barium or calcium.[4]

See also

References

  1. ^ a b c "IMA Mineral List with Database of Mineral Properties".
  2. S2CID 235729616
    .
  3. ^ a b c d e f g h i j k Foshag W F (1935) New mineral names, American Mineralogist 20, 678–678. Summary of article by Yoshimura in the Journal of the Faculty of Science, Hokkaido Imperial University, Ser. IV, Geol and Min 2, no 4:289–297 (1934)
  4. ^ a b c d e f g h i j k l m n Frondel, Marvin and Ito (1960) American Mineralogist 45: 1167–1173
  5. ^ a b c d e Gaines et al (1997) Dana’s New Mineralogy Eighth Edition. Wiley
  6. ^ a b c d e f Webmineral data
  7. ^ a b c d e f Mindat.org
  8. ^ a b c d e f g h Handbook of Mineralogy
  9. ^ a b Post, Heaney and Hanson (2003) American Mineralogist 88: 142–150
  10. ^ Manceau, Marcus and Grangeon (2012) American Mineralogist 97: 816–827
  11. ^ a b c d e f Burns, Burns and Stockman (1983) American Mineralogist 68:972–980
  12. ^ a b Post and Bish (1988) American Mineralogist 73: 861–869
  13. ^ a b Bodeï et al. (2007) Geochimica et Cosmochimica Acta 71: 5698–5716
  14. ^ a b c Straczek, Horen, Ross, and Warshaw (1960) American Mineralogist 45: 1174–1184
  15. ^ American Mineralogist (1979) 64:1333 Extracted from Chukhrov, Gorschkov, Sivtsov and Berezovskaya (1978) Izvest. Akad. Nauk SSSR, Ser. geol., no 12:86–95 (In Russian)
  16. ^ Polariscope, Gemstone Buzz
  17. ^ a b "Catalogue of Type Mineral Specimens". Archived from the original on 2013-12-06. Retrieved 2012-01-15.
  18. ^ Manceau, Lanson and Takahashi (2014) American Mineralogist 99: 2068–2083
  19. ^ Golden, Chen and Dixon (1986) Science 231: 717–719
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