TODOROKITE
Todorokite – (Na,Ca,K,Ba,Sr)1-x(Mn,Mg,Al)6O12·3-4H2O – is a relatively common oxide found in Mn ores.
The crystals appear as very small fibers, up to 0.05 mm, forming bands with the fibers arranged perpendicularly to the band surface. The fibers can be compact or isolated; they can group together in very porous aggregates, form earthy aggregates, and sometimes have a reniform, botryoidal, or stalagtitic habit.
Macroscopic recognition of todorokite is very difficult because the mineral is similar to several other Mn oxides. When micaceous in habit, it can be confused with ranciéite, and when fibrous, it is similar to pyrolusite. Microscopic identification is also problematic due to the extremely small size of the crystals; the fibrous habit also does not help.
Several classic literatures on ore minerals do not even mention it. Other analytical methods are necessary for the conclusive identification of todorokite, as is the case with several other Mn ore minerals.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Dark brown, black-brown, gray-purple.
Habit: Fibers, compact, reniform, etc.
Cleavage: {100} perfect, {010} perfect.
Tenacity: Brittle.
Twinning: Common, contact twins or as “trillings”.
Fracture: No information available.
Mohs Hardness: 1.5
Parting: No.
Streak: Black, gray, or dark brown.
Lustre: Sub-metallic to dull, resinous, sub-vitreous
Diaphaneity: Opaque.
Density (g/cm³): 3.5 – 3.8
2. Geology and Deposits
Todorokite is formed as a product of oxidation/leaching or hydrothermal alteration of primary manganese minerals such as carbonates and silicates. In the type-locality (Todoroki Mine, Japan), todorokite formed from inesite and rhodochrosite.
It is the main Mn oxide of deep-sea polymetallic nodules. It also occurs in Mn oxide deposits and in marine sedimentary deposits derived from volcanic fumaroles. It forms late as a coating on granitic pegmatites. It can occur as friable spongy masses in druses of auriferous quartz veins.
It is possible that it forms as a mineralization from fungi, as it is found in cave deposits and in water. It is also possible that most natural todorokite is derived from birnessite.
3. Mineral Associations
It is associated with other Mn oxides and hydroxides such as pyrolusite, cryptomelane, romanechite, manganite, birnessite, and hollandite. Also with Mn minerals such as rhodochrosite and inesite.
Due to its occurrence in pegmatites, it is associated with typical minerals of this paragenesis, such as albite, microcline, muscovite, fluoroapatite, quartz (including amethyst variety), and others.
In other geological situations, it is associated with carbonates (calcite, aragonite, magnesite), barite, gypsum, opal, zeolites (levyne, chabazite, stilbite), colemanite, franklinite, tainiolite, halloysite, and nontronite.
4. Transmitted Light Microscopy
Literature indicates that todorokite is transparent in very thin lamellae.
Refraction indices: nα : 2,310; nβ: 2,350
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Brown, with pleochroism visible in brown.
Relief: Very high.
Cleavage: No information available.
Habits: Fibers, compact, renoform, etc.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: 2.310, corresponding to black colors.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: Common.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: No information available.
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: Polishing todorokite aggregates is always very difficult because during the grinding and polishing process the fibers come loose, creating holes and scratching the already polished surface.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray to dark gray.
Pleochroism: In various shades of gray.
Reflectivity: 9.41 – 16.62%
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, ranging from gray-brown to dark blue.
Internal reflections: No, but the finer fibers exhibit internal reflections.
May be confused with: other Mn oxides/hydroxides.
General Characteristics:
Grain shape: always occurs in fibers, which can be parallel forming bands or arranged radially forming botryoidal, reniform aggregates, etc.
Twins are parallel to the length of the fibers, as can be seen by the anisotropy.