Wilhelmgümbelite

Chemical formula: Zn²⁺Fe²⁺Fe³⁺₃(PO₄)₃(OH)₄(H₂O)₅·2H₂O

Wilhelmgümbelite is a very rare, hydrated zinc and iron phosphate, forming microscopic, bladed crystals of a dark green color.

## Characteristics Wilhelmgümbelite is a very rare mineral from the phosphate group, chemically classified as a hydrated zinc and iron phosphate. It occurs as very small, bladed or thin, tabular crystals, which rarely exceed 0.2 mm in length. These crystals often form rosette-like or radial aggregates. Its color is dark green, and due to the small size of the crystals, detailed visual features are only discernible under high magnification. ## Physical Properties Wilhelmgümbelite crystals exhibit a vitreous luster. They are transparent to translucent. The Mohs hardness is estimated at 3, classifying it as a soft mineral. The density calculated based on the chemical formula and unit cell parameters is approximately 3.01 g/cm³. ## History and Name The mineral was named in honor of Carl Wilhelm von Gümbel (1823–1898), a distinguished Bavarian geologist who made significant contributions to geological research in the region. The name honors his merits in Earth sciences. ## Applications Due to its extreme rarity and microscopic size, wilhelmgümbelite has no commercial or industrial application. Its significance is purely scientific and collectible, being an object of interest for specialized collectors of rare minerals (so-called micromounts).

Properties

Mohs hardness
3
Color
Light yellow-brown to orange-red
Luster
Vitreous
Streak
pale yellow-brown
Density
2.89
Cleavage
parallel to {010}
Fracture
Irregular/Uneven
Transparency
Transparent to Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of wilhelmgümbelite is only possible using advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD) in conjunction with chemical analysis (EDS). Visual identification in the field or even in a collection is impossible due to its microscopic size and similarity to other secondary phosphates. ## Distinguishing from Similar Minerals It can be confused with many other green, secondary phosphate minerals found in oxidation zones, such as other minerals from the laueite group, strengite, or rockbridgeite. Definitive differentiation requires specialized laboratory equipment. ## Crystal Forms Wilhelmgümbelite forms very small, bladed or thin, tabular crystals, elongated along one axis. These crystals often arrange into small, radial or rosette-like aggregates on the surface of the host rock.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones of ore deposits, particularly within granitic pegmatites containing primary phosphate minerals (like triphylite) and sulfides (like sphalerite). It forms as a result of hydrothermal alteration of these minerals under low-temperature conditions. ## Mineral Associations This mineral co-occurs with other secondary phosphates, such as laueite, pseudolaueite, stewartite, strengite, rockbridgeite, as well as with quartz, limonite, and other iron oxides. ## Localities The only confirmed locality of wilhelmgümbelite in the world (type locality) is the Hagendorf-Süd quarry in Waidhaus, in the Upper Palatinate Forest, Bavaria, Germany. This is a classic locality for many rare phosphate minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a wilhelmgümbelite specimen is primarily assessed based on the abundance and development of its microscopic crystals. The most prized specimens are those that display well-formed, radial aggregates of an intense green color, contrasting with the rock matrix. Crystal size, even if sub-millimeter, also increases the specimen's value. ## Popular Localities The only source of specimens is the historical Hagendorf-Süd locality in Germany. Material from this site is extremely rare and sought after by collectors specializing in mineral systematics or rare species collectors.

Care and storage

## Cleaning Specimens containing wilhelmgümbelite, which are typically micromounts, should not be cleaned mechanically or chemically. If necessary, dust can be removed very carefully using a photographic air blower or compressed air from a safe distance. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. The mineral is soft and brittle, so it should be protected from impacts, vibrations, and abrasion. It should not be heated or exposed to sudden temperature changes. ## Storage Specimens should be stored exclusively in specialized micromount boxes that protect them from dust, mechanical damage, and humidity. Storage under stable room conditions is entirely sufficient.

External references

Sources

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