Comblainite

Chemical formula: Ni²⁺₄Co³⁺₂(OH)₁₂(CO₃)·3H₂O

A rare nickel-cobalt hydroxycarbonate with a characteristic blue-green color, most often forming rosette aggregates and crusts.

## Characteristics Comblainite is a hydrated nickel-cobalt hydroxycarbonate belonging to the pyroaurite group. It occurs as very small, hexagonal platelets, which typically form rosette aggregates, spherical clusters, or thin crusts on the host rock. Due to the small size of its crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Comblainite crystals exhibit a vitreous to pearly luster on cleavage surfaces. The mineral is translucent. Its Mohs hardness is approximately 2, and its calculated density is 2.96 g/cm³. It is a very soft and brittle mineral. ## Colors and Varieties Comblainite has a characteristic, vivid blue-green color. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1980 by P. Piret and M. Deliens. Its name honors the Belgian geologist, Dr. Guillaume Comblain (1921-1976), who was the director of the Geological Survey of Belgium. It was discovered in material originating from the Shinkolobwe mine in the Democratic Republic of Congo. ## Uses Comblainite has no industrial application. It is solely a mineral of scientific and collector's interest, sought after for its rarity and attractive color.

Properties

Mohs hardness
2
Color
Turquoise-blue, yellow-green under a microscope
Luster
Vitreous, Pearly
Streak
Pale green
Density
3.05
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Comblainite is identified primarily by its characteristic blue-green color, its occurrence in the form of small rosettes or crusts, and its association with other nickel and cobalt minerals. Reaction with hydrochloric acid (vigorous effervescence) is a diagnostic feature, but it destroys the sample. ## Distinguishing from Similar Minerals It can be confused with other secondary nickel and cobalt minerals of similar color, such as reevesite or honessite. Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Crystal Forms It forms very small, thin, hexagonal platelets that combine into characteristic rosette or spherical aggregates. It also occurs as thin coatings and crusts.

Geological environment

## Genesis Comblainite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits containing nickel and cobalt, most often in dolomitic or carbonate rocks. ## Mineral Associations It often co-occurs with other secondary minerals, such as reevesite, honessite, as well as with uraninite, quartz, talc, and minerals from the serpentine group. ## Localities The most important and classic locality, from which the type material originates, is the Shinkolobwe mine in Katanga Province, Democratic Republic of Congo. This mineral has also been identified in several other locations worldwide, including the Kmety mine in Slovakia and Carr Boyd Rocks in Western Australia.

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens are those with well-formed, rosette aggregates visible under magnification, displaying an intense, blue-green color. Contrast with the rock matrix and absence of mechanical damage are also important. Due to the microscopic size of the crystals, specimens are usually small. ## Popular Localities The most known and sought-after specimens by collectors come from the type locality - the Shinkolobwe mine in the Democratic Republic of Congo.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its softness and reactivity, contact with water and any chemical agents should be avoided. ## What to Avoid Contact with acids must be absolutely avoided, as they cause its rapid dissolution with the release of carbon dioxide. The mineral is very soft, so it must be protected from scratches and impacts. It should not be heated or exposed to prolonged moisture. ## Storage Comblainite specimens are best stored in closed, dry containers or display cases, away from other minerals that could scratch them. It should be protected from dust and moisture.

External references

Sources

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