Paulkellerite

Chemical formula: Bi₃+₂Fe₃+O₂(PO₄)(OH)₂

Paulkellerite is a very rare bismuth and iron oxyphosphate, forming aggregates of small, tabular crystals with a vitreous luster.

## Characteristics Paulkellerite is a very rare mineral from the phosphate group, chemically classified as a bismuth and iron oxyphosphate. It occurs as small, tabular or bladed crystals, rarely exceeding 1 mm in length. These crystals often form rosette-like or radial aggregates. Due to its composition and rarity, it is a mineral of purely scientific and collectible interest. ## Physical Properties Paulkellerite crystals are characterized by a vitreous luster and are transparent. This mineral is relatively soft, with a Mohs hardness of 3. It is also quite dense, which is typical for bismuth-containing minerals. ## Colors and Varieties Paulkellerite occurs in shades from light green to dark green, and can also be greenish-yellow. No varieties have been distinguished. ## History and Name The mineral was first described in 1988 by Pete J. Dunn, Donald R. Peacor, Sturges W. Bailey, and Robert A. Ramik. The name "paulkellerite" was given in honor of Paul Keller (born 1940), a professor of mineralogy at the University of Stuttgart (Germany), in recognition of his contributions to the study of pegmatite minerals from Tsumeb and Hagendorf, and of secondary phosphates and arsenates. ## Uses Paulkellerite has no industrial applications. It is valued solely as a collectible mineral and an object of scientific research.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Pale green
Density: 5.55
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Paulkellerite is identified by its characteristic appearance – small, green, tabular crystals forming rosette-like aggregates. Its specific occurrence environment, namely the oxidation zones of polymetallic deposits rich in bismuth, is also key. ## Distinguishing from Similar Minerals It can be confused with other secondary green phosphate or arsenate minerals, such as mixite, agardite, or some varieties of pyromorphite. However, definitive differentiation requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS), due to its unique chemical composition (combination of bismuth, iron, and phosphorus). ## Crystal Forms Crystals are thin, tabular, flattened parallel to {001} and elongated along [010]. They typically form radial or rosette-like clusters and aggregates.

Geological environment

## Genesis Paulkellerite is a secondary mineral, forming in the oxidation zones of hydrothermal polymetallic deposits. Its formation is associated with the weathering of primary bismuth minerals in the presence of phosphate-rich solutions. ## Mineral Associations This mineral co-occurs with other secondary bismuth minerals and phosphates. In the type locality (Schneeberg, Germany), it was found in association with bismutite, atelestite, preisingerite, valentinite, and quartz. In the Tsumeb mine, it co-occurs with quartz, galena, tennantite, pyrite, chalcocite, beudantite, and anglesite. ## Localities The most important and confirmed occurrences of this very rare mineral are: - "Weißer Hirsch" Mine in Schneeberg, Saxony, Germany (type locality). - Tsumeb Mine in Namibia, known for exceptional secondary mineral specimens. - Locality in the Jáchymov area in the Czech Republic.

Rarity

Very rare

For collectors

## Quality Criteria The most prized paulkellerite specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. An intense, green color and contrast with the rock matrix (most often quartz) are important. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. Purity is not a critical criterion, but the absence of damage to delicate aggregates is highly desirable. ## Popular Localities Specimens from Schneeberg (Germany) and Tsumeb (Namibia) are the most classic and sought after by collectors specializing in rare and systematic minerals.

Care and storage

## Cleaning Due to the small size and delicate nature of the crystals, mechanical cleaning is highly risky. If absolutely necessary, compressed air can be used to remove loose dust. Contact with water and any chemicals should be avoided. ## What to Avoid Avoid ultrasonic cleaners, chemical agents, acids, and sudden temperature changes. The crystals are brittle and easily susceptible to mechanical damage. ## Storage Specimens should be stored in stable conditions, in a closed "micromount" box, to protect them from dust and mechanical damage. Exposure to vibrations and shocks should be avoided.

Sources

IMA Mineral List (CNMNC)