HETEROGENITE
Heterogenite – CoO(OH) – is a relatively rare hydroxide that occurs as a secondary mineral in some Co deposits. Due to the volumes in which it is sometimes found, it can be an important cobalt ore. Macroscopically, it closely resembles psilomelane.
An older term for heterogenite is “stainerite” (or “stainierite”).
According to more recent literature, the correct formula for the mineral is HCoO2.
There is a nickel-rich variety known as “heubachite.”
Its frequent association with radioactive minerals requires caution when handling it!
1. Characteristics
Crystal system: Hexagonal dihexagonal dipyramidal.
Color: Black, brownish-black, reddish.
Habit: Massive, reniform, globular, stalactitic, in crusts, earthy. Prismatic crystals are very rare, up to 1 mm.
Cleavage: On {0001}.
Tenacity: No information available.
Twinning: Common.
Fracture: Irregular to conchoidal.
Mohs Hardness: 3 – 5
Parting: No.
Streak: Very dark brown, black.
Lustre: Metallic to dull.
Diaphaneity: Opaque.
Density (g/cm³): 4.13 – 4.47
2. Geology and Deposits
Heterogenite is typically a secondary mineral, resulting from the alteration of primary cobalt-bearing minerals. It can form from skutterudite, linnaeite, cobalt-rich pyrites, or olivines.
It also occurs widely in residual soils (eluvium or bauxites) overlying ultramafic rocks.
In asbolane occurrences, heterogenite is a major component, formed through the leaching of olivine or cobalt-rich pyrites.
3. Mineral Associations
Heterogenite occurs associated with quartz, carbonates (calcite), and:
– primary Co minerals (skutterudite, linnaeite),
– secondary Co minerals (roselite, erythrite, cobaltiferous calcite, smolyaninovite, spherocobaltite, etc.),
– secondary Cu minerals (malachite, chrysocolla, cuprite, cornetite, torbernite),
– Fe oxides and hydroxides, and Mn hydroxides (psilomelane, etc.),
as well as others, such as curite (a Pb+U hydroxide) and pharmacosiderite (a K-Fe arsenate).
4. Transmitted Light Microscopy
Not applicable, as heterogenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: When heterogenite occurs in compact masses, it takes a good polish with relative ease. Earthy aggregates, by their nature, require impregnation and are more difficult to polish.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white with a brownish tint; it strongly resembles magnetite.
Compared to the color of hematite, the color of heterogenite is more yellowish.
Compared to the color of magnetite, the color of heterogenite is very similar.
Compared to the color of goethite, the color of heterogenite is significantly lighter.
Pleochroism: No.
Reflectivity: 16.7 – 25.47%
Bireflectance: Strong, ranging from nearly pure white with a cream undertone to a much darker color – a grayish, pinkish brown with a chocolate hue.
Basal sections show no bireflectance.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong anisotropy.
Sources report different colors: (i) between brown and gray or (ii) bluish-white to yellowish-white. Extinction is complete, and basal sections are isotropic.
Internal reflections: No.
May be confused with: Magnetite, but magnetite is isotropic.
General Characteristics:
Grain shape: it always occurs as very small crystals, forming spherulites and aggregates with a radial structure. A fine concentric texture may be visible within these aggregates. Typically, the central portions of these spherulites are very fine-grained; the grain size increases toward the edges of the spherulite or aggregate. It may form dendritic or skeletal aggregates with subparallel branches. Basal sections can exhibit perfect hexagonal shapes.
Cleavage or parting is frequently visible in larger crystals.
Twinning is common.
Zoning may be observed in some crystals.
Modifications, cryptocrystalline or colloform, may be present, appearing isotropic.
