Dukeite

Chemical formula: Bi³⁺₂₄Cr⁶⁺₈O₅₇(OH)₆·3H₂O

Dukeite is a very rare bismuth chromate-hydroxide, forming microscopic, transparent crystals of an intense yellow color.

## Characteristics Dukeite is an extremely rare mineral from the chromate group, chemically classified as a hydrated bismuth chromate-hydroxide. It occurs as very small, tabular or hexagonal crystals, rarely exceeding fractions of a millimeter. It typically forms small aggregates or coatings on other minerals. Its most characteristic feature is its intense, bright yellow color. ## Physical Properties Dukeite crystals are transparent and exhibit a resinous luster. The Mohs hardness is relatively low, ranging from 3 to 4. The mineral is brittle, and its fracture is uneven. Due to the microscopic size of the crystals, precise determination of density is difficult. ## Colors and Varieties Dukeite occurs in shades from bright yellow to dull yellowish-brown. The intensity of the color depends on impurities and crystallization conditions. No varieties are distinguished. ## History and Name The mineral was first described in 1993 by P. J. Dunn, D. R. Peacor, W. B. Simmons Jr., and R. A. Gain. Its name honors Joseph A. Duke (1920–1990), an American mineral collector who specialized in rare species and was the first to draw attention to this material. ## Applications Due to its extreme rarity and microscopic size, dukeite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and scientists.

Properties

Mohs hardness
3-4
Luster
Resinous
Streak
Bright yellow
Density
0
Fracture
Irregular/Uneven
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of dukeite is based on its characteristic, intensely yellow color, resinous luster, and co-occurrence with other bismuth minerals in oxidation zones. Due to the microscopic size of the crystals, final confirmation requires advanced analytical methods, such as Raman spectroscopy or EDS analysis. ## Distinguishing from Similar Minerals Dukeite can be confused with other secondary yellow bismuth minerals, such as bismutite or preisingerite. Distinguishing it "by eye" is practically impossible and requires chemical analysis. The presence of chromium in dukeite's composition is key. ## Crystal Forms Dukeite forms very small, hexagonal or pseudohexagonal, tabular crystals. It occurs as individual crystals, rosette-like aggregates, or fine coatings on the surface of the host rock.

Geological environment

## Genesis Dukeite is a secondary mineral, forming in the oxidation zones (gossans) of granite pegmatites rich in bismuth. It forms as a result of the weathering of primary bismuth minerals in the presence of chromium-bearing solutions. ## Mineral Associations This mineral co-occurs with other secondary bismuth minerals, such as bismutite, bismutotantalite, preisingerite, as well as with quartz, muscovite, and hematite. ## Localities The only confirmed locality of dukeite in the world is its type locality – the Posse mine, located in São José de Brejaúba, Minas Gerais state, Brazil. This is a historic mine where gemstones and tantalum were extracted.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of dukeite specimens is primarily assessed based on the abundance and visibility of crystals on the rock matrix. The most desirable specimens are those with numerous, well-formed, intensely yellow crystals that form distinct aggregates. Color contrast with the substrate (e.g., white quartz) also enhances the specimen's attractiveness. ## Popular Localities The only source of dukeite is its type locality in the Posse mine in Brazil. All specimens available on the collector's market come from this single location.

Care and storage

## Cleaning Due to the extreme brittleness and small size of the crystals, mechanical cleaning is not recommended. If necessary, compressed air can be used from a distance to remove dust. Avoid contact with water and chemicals. ## What to Avoid Avoid contact with any liquids, acids, and detergents. The mineral is very soft and brittle, so it should be protected from scratches, impacts, and vibrations. Store away from heat sources. ## Storage Dukeite specimens should be stored in stable, sealed "micromount" boxes, which protect them from dust, mechanical damage, and sudden changes in humidity. They should not be exposed to direct sunlight.

External references

Sources

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