Obradovicite-KCu

Chemical formula: [K<sub>2</sub>(H<sub>2</sub>O)<sub>17</sub>Cu<sup>2+</sup>(H<sub>2</sub>O)<sub>6</sub>][Mo<sup>6+</sup><sub>8</sub>As<sup>5+</sup><sub>2</sub>Fe<sup>3+</sup><sub>3</sub>O<sub>34</sub>(OH)<sub>3</sub>]

Obradovicite-KCu is a very rare molybdate-arsenate mineral with a complex composition, forming microscopic, platy, black crystals.

## Characteristics Obradovicite-KCu is an extremely rare, complex mineral from the molybdate group. Its composition includes potassium, copper, molybdenum, arsenic, and iron. It forms very small, platy or bladed crystals, which typically do not exceed 0.2 mm in length. Observed under a microscope, the crystals are black and opaque, often gathered into small, radial aggregates or rosettes. ## Physical Properties The crystals exhibit a metallic to submetallic luster. Due to the microscopic size of the crystals, many physical properties, such as hardness, density, or cleavage, have not been precisely determined. The mineral is brittle. ## Colors and Varieties This mineral is black, and its streak is gray. No colored or commercial varieties are known. ## History and Name Obradovicite-KCu was first described in 2011 by Anthony R. Kampf and co-authors. The name honors Martin Obradovic (born 1953), a mineral collector from Washington, USA, who provided the first samples for study. The "-KCu" suffix in the name refers to the dominant cations in its composition: potassium (K) and copper (Cu), distinguishing it from other potential minerals in this group. ## Uses Obradovicite-KCu has no industrial application. Its significance is purely scientific and collectible, being an object of desire for specialized collectors of rare minerals.

Properties

Mohs hardness
2
Luster
Metallic
Streak
Grey
Density
3.45
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Opaque
Crystal system
Triclinic

Diagnostic features

## Identification Identification of Obradovicite-KCu is impossible without advanced analytical methods, such as energy-dispersive X-ray spectroscopy (EDS/EDX) combined with X-ray diffraction (XRD). Visually, at the occurrence site, it can be suspected based on its black color, metallic luster, and occurrence as small, platy crystals in paragenesis with other rare molybdates. ## Distinguishing from Similar Minerals It can be confused with many other black, fine-grained secondary minerals, such as manganese oxides, tenorite, or other rare molybdates. Definitive distinction requires specialized chemical analysis of its composition. ## Crystal Forms Crystals are very small, usually thin-platy or bladed, elongated in one direction. They often form small, radial or rosette-like aggregates on the surface of the host rock.

Geological environment

## Genesis Obradovicite-KCu is a secondary mineral that forms in the oxidation zones (gossans) of polymetallic deposits. It forms as a result of the weathering of primary sulfides, mainly molybdenite, chalcopyrite, and pyrite, in the presence of arsenic minerals. ## Mineral Associations It co-occurs with other rare secondary minerals. In its type locality (Chuquicamata mine), it was found in association with gypsum, chalcanthite, jarosite, lindgrenite, powellite, wulfenite, as well as other rare molybdates like szenicsite and hummerite. ## Localities The only confirmed location of this mineral in the world is its type locality: the Chuquicamata copper mine, located in the Antofagasta region of the Atacama Desert in Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of Obradovicite-KCu specimens is primarily assessed based on the abundance and development of microscopic crystals. The most valued samples are those on which well-formed, even if very small, radial aggregates or single, undamaged crystals are visible. The aesthetics of the rock matrix and the contrast created by the black crystals on it are also important. ## Popular Localities The only source of specimens is the Chuquicamata mine in Chile. Material from this locality is extremely rare and sought after by collectors specializing in "microminerals" (micromounts) and systematic minerals.

Care and storage

## Cleaning Specimens of this mineral are extremely delicate and small. Mechanical or chemical cleaning is not recommended. Any attempts at cleaning may destroy the microscopic crystals. It is best to leave the specimen undisturbed. ## What to Avoid Avoid contact with any liquids, chemicals, ultrasound, and sudden temperature changes. The crystals are very brittle and sensitive to vibrations and shocks. ## Storage Specimens should be stored exclusively in specialized, sealed "micromount" boxes, protecting them from dust, moisture, and mechanical damage. It is not suitable for display outside of such protection.

Sources

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