Asbolane

Chemical formula: Mn<sup>4+</sup>(O,OH)<sub>2</sub>·(Co<sup>3+</sup>,Ni<sup>2+</sup>,Mg,Ca)<sub>x</sub>(OH)<sub>2x</sub>·nH<sub>2</sub>O

Asbolane is a hydrated manganese and cobalt oxide, forming black, earthy or crusty coatings, valued for its cobalt content.

## Characteristics Asbolane is a mineral from the oxide group, chemically a complex hydrated manganese oxide with significant admixtures of cobalt, as well as nickel, magnesium, and calcium. It does not form macroscopic crystals, most often occurring as earthy, powdery, or sooty aggregates. It is also found in reniform, botryoidal, and crusty forms, often as coatings and films on other minerals and rocks. Its appearance is usually dull and inconspicuous, resembling black paint or sediment. ## Physical Properties Asbolane is a relatively soft mineral, with a hardness ranging from 1 to 2 on the Mohs scale, although contaminated, compact aggregates can be harder. It is brittle and light, with variable density depending on composition and porosity. The luster is most often dull or earthy, less commonly pitchy in more compact forms. It is opaque. ## Colors and Varieties The color of asbolane is uniform - from steel-gray, through bluish-black, to black. The mineral's name refers to its soot-like appearance. No named varieties are distinguished, however, its chemical composition, especially the content of cobalt and nickel, is highly variable, which affects its physical properties. ## History and Name The name asbolane comes from the Greek word *asbolē* (ἀσβόλη), meaning "soot," in reference to its typical appearance. This mineral has been known since antiquity, but its formal scientific description dates from later centuries. Due to its cryptocrystalline nature, for a long time it was classified as "wad" or "cobalt earth" before being recognized as a distinct mineral species. ## Uses Historically, asbolane was a minor source of cobalt, used to produce blue pigment (cobalt blue). Currently, its industrial significance is small, but it remains an important object of interest for collectors specializing in oxidation zone minerals and cobalt and manganese minerals.

Properties

Mohs hardness
1-2
Luster
Earthy
Streak
Black, brownish-black
Density
2.8-3.8
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Asbolane is recognized by its characteristic black color, low hardness, and earthy or sooty form. It is soft, soils fingers, and leaves a black or blackish-brown streak. It often forms thin coatings on other minerals, especially in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals Asbolane can be confused with other black manganese oxides, such as pyrolusite, cryptomelane, or hollandite. Pyrolusite is usually harder and often forms radial or fibrous aggregates. Cryptomelane and hollandite are also harder and occur in botryoidal or compact forms. It is distinguished from black iron oxides (e.g., hematite) by its lower hardness and black streak (hematite's streak is reddish-brown). ## Crystal Forms Asbolane is an amorphous or cryptocrystalline mineral and does not form crystals visible to the naked eye. It occurs exclusively as earthy, powdery, sooty aggregates, as well as in crusty, reniform, and botryoidal forms.

Geological environment

## Genesis Asbolane is a typical secondary mineral, forming in the oxidation (weathering) zones of ore deposits containing manganese, cobalt, and nickel. It forms as a result of chemical weathering of primary minerals of these metals, such as cobaltite, skutterudite, or smaltite, in the presence of manganese minerals (e.g., rhodochrosite). It is a product of processes occurring near the Earth's surface. ## Mineral Associations It most often co-occurs with other secondary minerals of the oxidation zone. Typical associated minerals include erythrite (cobalt bloom), annabergite (nickel bloom), heterogenite, as well as iron oxides (goethite, limonite) and other manganese oxides (pyrolusite, cryptomelane, hollandite). It can also occur in association with quartz and calcite. ## Localities Significant occurrences of asbolane have been reported in many places around the world. Classic localities include Schneeberg and Annaberg in Saxony (Germany) and Jáchymov in the Czech Republic. It also occurs in Bou Azzer in Morocco, in the Shaba (Katanga) region of the Democratic Republic of Congo, in New Caledonia (as an important component of lateritic cobalt ores), and in numerous places in the USA (e.g., Arizona and California).

Rarity

Not very common

For collectors

## Quality Criteria The collector's appeal of asbolane depends on several factors. Specimens with well-developed reniform or botryoidal forms, with a distinct, pitchy luster, are most valued. Association with other colorful secondary minerals, such as pink erythrite or green annabergite, which create an attractive contrast with black asbolane, is also very important. Specimens from classic, historical localities (e.g., from Saxony) are also more highly valued. ## Popular Localities For collectors, specimens from historical German and Czech mining districts are most desirable. High-quality asbolanes in association with erythrite come from Bou Azzer in Morocco. Specimens from the Democratic Republic of Congo are also valued due to the richness of forms and mineral associations.

Care and storage

## Cleaning Asbolane specimens are very soft, brittle, and often dusty. They should be handled gently. Cleaning should be kept to a minimum. If necessary, a soft brush can be used to remove loose impurities. Water should be avoided, as it can dissolve or damage earthy aggregates. ## What to Avoid Ultrasonic cleaners, chemicals, acids, and detergents should be absolutely avoided. The mineral is sensitive to shocks and abrasion - even rubbing with a finger can leave a mark. Store away from moisture. ## Storage It is recommended to store asbolane specimens in closed, padded boxes or containers to prevent crumbling, abrasion, and dusting. It should be isolated from harder minerals that could scratch it. Due to dusting, it should not be stored in open display cases with other minerals.

External references

Sources

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