Karenwebberite

Chemical formula: NaFe<sup>2+</sup>(PO<sub>4</sub>)

Karenwebberite is a rare, dark green to black phosphate mineral of the alluaudite group, found in granitic pegmatites.

## Characteristics Karenwebberite is a rare mineral of the alluaudite group, belonging to the phosphate class. It forms anhedral (lacking well-developed crystal faces) grains and aggregates, typically up to 1 cm in size. Its crystals are usually embedded in a matrix of other phosphates. It is a brittle mineral, dark green to black in color, with a vitreous luster. ## Physical Properties The Mohs hardness of karenwebberite is approximately 5. This mineral is characterized by a vitreous luster and is opaque. Its density, calculated based on the chemical formula and unit cell parameters, is 3.83 g/cm³. ## Colors and Varieties Karenwebberite occurs in dark green to black colors. No distinct color varieties or commercial varieties have been identified. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2011 under number 2011-066. The name honors Dr. Karen L. Webber, Professor of Petrology at the University of New Orleans (Louisiana, USA), for her contributions to research on pegmatites, including those from the Black Hills region of South Dakota, where this mineral was discovered. ## Uses Due to its rarity and small crystal size, karenwebberite has no industrial application and is solely of interest to collectors and scientists.

Properties

Mohs hardness
5
Luster
Vitreous
Streak
Greenish gray
Density
3.83
Cleavage
Good on {010} and {110}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Karenwebberite is difficult to identify visually. It is recognized by its dark green to black color, vitreous luster, and occurrence as anhedral grains in granitic pegmatites. Its co-occurrence with other phosphates is characteristic. ## Differentiation from Similar Minerals It can be confused with other dark minerals of the alluaudite group, such as alluaudite or ferroalluaudite, as well as other black phosphates. Definitive differentiation from these minerals is almost exclusively possible using advanced analytical methods, such as X-ray diffraction (XRD) or X-ray microanalysis (EDS/WDS). ## Crystal Forms This mineral forms anhedral, i.e., irregular, grains and granular aggregates. Well-formed, idiomorphic crystals are not observed.

Geological environment

## Genesis Karenwebberite is a primary mineral, crystallizing in the late stages of the evolution of complex LCT-type (lithium, cesium, tantalum) granitic pegmatites. It forms in phosphate-rich zones. ## Mineral Associations This mineral co-occurs with other phosphates, such as arrojadite-(KFe), fluorapatite, graftonite-(Fe), as well as quartz, microcline, and muscovite. It has been found in paragenesis with other minerals of the alluaudite group. ## Localities The only confirmed occurrence (type locality) of karenwebberite is the Big Rock Quarry, located in the Custer Mountain pegmatite district in Custer County, South Dakota, USA.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of a specimen primarily depends on its richness – the larger and more distinct the mass of karenwebberite relative to the host rock, the more valuable the specimen. Since this mineral does not form well-developed crystals, their form is not a criterion for evaluation. However, a well-documented paragenesis with other rare phosphates is important. ## Popular Localities All known specimens come from a single location worldwide – the Big Rock Quarry in South Dakota, USA. This means that material from this locality is the only one available to collectors and research institutions.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Water and any chemical agents should be avoided, as the mineral may react or be damaged. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is brittle, so it must be protected from impacts, drops, and scratches. It should not be subjected to ultrasonic cleaning. ## Storage It is recommended to store specimens in stable, dry conditions, away from direct sunlight and heat sources. They are best kept in sealed collector boxes, which protect them from dust and mechanical damage.

Sources

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