Straczekite

Chemical formula: (Ca,K,Ba)V⁴⁺₂V⁵⁺₆O₂₀·3H₂O

Straczekite is a rare vanadium mineral with a characteristic greenish-black color and low hardness, occurring as fibrous aggregates.

## Characteristics Straczekite is a hydrated calcium, potassium, and barium oxyvanadate, belonging to the straczekite group. It forms aggregates of small, bladed or fibrous crystals, which often arrange into rosette-like or radial aggregates. Its typical appearance is mats of felted fibers or coatings on other minerals. It is opaque and has a characteristic dark, greenish-black color. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1-2, meaning it can be scratched with a fingernail. It has a density of approximately 3.09 g/cm³. It exhibits one-directional cleavage. The luster is described as dull or earthy in the case of aggregates. ## Colors and Varieties Straczekite occurs in a uniform, greenish-black color. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1983. Its name honors John Straczek (1916-1980), an American geologist who for many years was the chief of the mineral exploration branch at the United States Geological Survey (USGS). The type locality, from which the first described specimens originated, is the North Wilson Pit mine in Arkansas, USA. ## Uses Due to its rarity and small crystal size, straczekite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals, especially vanadium minerals.

Properties

Mohs hardness
1-2
Luster
Dull
Streak
Greenish black
Density
3.09
Cleavage
{100}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Straczekite can be identified by its characteristic mode of occurrence – as felted, fibrous, or bladed aggregates of greenish-black color. Key diagnostic features include its extremely low hardness (1-2 on the Mohs scale) and greenish-black streak. It usually occurs as thin coatings or small aggregates on the host rock. ## Distinguishing from Similar Minerals It can be confused with other dark, fibrous vanadium minerals, such as hewettite or barnesite. Differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). In field conditions, precise identification of associated minerals and locality can be helpful. ## Crystal Forms Straczekite crystals are very small, in the form of blades or fibers. They usually form dense, felted mats, radial or rosette-like aggregates, as well as thin efflorescences and coatings.

Geological environment

## Genesis Straczekite is a secondary mineral, formed in the oxidation zones of uranium-vanadium deposits hosted in sedimentary rocks, such as sandstones. It forms under low-temperature conditions as a result of the weathering of primary vanadium minerals, often in the presence of organic matter. ## Mineral Associations This mineral co-occurs with other secondary vanadium and uranium minerals. At the type locality (Arkansas, USA), it has been found in association with minerals such as hewettite, barnesite, metarossite, pascoite, as well as montmorillonite and quartz. ## Localities The most important and best-documented occurrence of straczekite is its type locality – the Union Carbide mine (North Wilson Pit) in Wilson Springs, Garland County, Arkansas, USA. It is a very rare mineral, and reports of its occurrence in other places around the world are scarce and often require confirmation.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of straczekite primarily depends on the richness and aesthetics of the aggregates. Specimens with well-formed, rosette-like or radial aggregates of fibrous crystals, clearly contrasting with the host rock, are most valued. The abundance of the mineral on the specimen and the absence of damage to the delicate fibers are also important. ## Popular Localities By far the most known and valued specimens come from the mineral's discovery site – the mine in the Wilson Springs area of Arkansas, USA. Specimens from this locality are the standard for this species and command the highest prices in the collector's market.

Care and storage

## Cleaning Straczekite specimens are extremely delicate and soft. They should be cleaned only very carefully, using compressed air to remove dust. Any contact with water or a brush can permanently damage the fibrous aggregates. ## What to Avoid Avoid contact with water and chemicals that may react with the mineral. Due to its very low hardness (1-2 on the Mohs scale), it is extremely susceptible to mechanical damage – scratches and abrasions. It should be protected from vibrations and impacts. ## Storage It is recommended to store specimens in closed, padded "micromount" boxes to protect them from dust, light, and physical damage. It should not be displayed in open showcases.

External references

Sources

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