Hallimondite

Chemical formula: Pb<sup>2+</sup><sub>2</sub>(U<sup>6+</sup>O<sub>2</sub>)(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·nH<sub>2</sub>O

Hallimondite is a very rare, secondary uranyl mineral, intensely yellow in color, occurring as microscopic crystals and coatings.

## Characteristics Hallimondite is a hydrated lead uranyl arsenate. It forms very small, bladed or tabular crystals, rarely exceeding 0.1 mm in length, which often occur in radial or disordered aggregates and coatings. Due to the microscopic size of the crystals, its visual features are difficult to observe without magnification. It has a characteristic bright yellow to canary yellow color. ## Physical Properties Hallimondite crystals exhibit a vitreous luster. Hardness and density have not been precisely determined due to the small size and rarity of the material. It is a brittle mineral. ## Colors and Varieties This mineral occurs in a uniform, bright yellow color. No varieties are distinguished. ## History and Name The name hallimondite comes from Arthur Francis Hallimond (1890-1968), a British mineralogist who made significant contributions to the development of ore mineralogy and polarizing microscopy. The mineral was first described in 1965 based on samples from the Wheal Gorland mine in Cornwall (Great Britain). ## Uses Due to its extreme rarity and microscopic size, hallimondite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or uranium minerals (so-called "uraninites").

Properties

Mohs hardness
2-3
Luster
Vitreous
Streak
Light yellow
Density
6.1
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Identification of hallimondite is based on its characteristic bright yellow color, specific form of microscopic, bladed crystals, and co-occurrence with other secondary lead and uranium minerals. Positive identification is almost exclusively possible using advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from similar minerals Hallimondite can be confused with many other yellow secondary uranium minerals, such as autunite, torbernite (in degraded form), carnotite, or kasolite. Distinguishing them with the naked eye is practically impossible. Key factors are the examination of chemical composition (presence of lead and arsenic) and crystallographic analysis. ## Crystal forms Hallimondite forms very small, thin, bladed crystals, elongated in one direction and flattened. These crystals often combine into small, radial aggregates or disordered clusters forming coatings on the host rock.

Geological environment

## Genesis Hallimondite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic deposits rich in uraninite, arsenopyrite, and galena. It forms as a result of complex chemical processes in which solutions containing uranium, arsenic, and lead react with each other under conditions of low temperatures and pressures. ## Mineral associations This mineral most often occurs in association with other secondary arsenates and uranyl minerals. Typical associated minerals include mimetite, olivenite, chenevixite, cornwallite, as well as quartz and hematite. ## Localities The most important and classic occurrence (type locality) is the Wheal Gorland mine in St Day, Cornwall, United Kingdom. Its presence has also been confirmed in several other locations worldwide, including the Clara mine in the Black Forest (Germany) and the Gozaisho mine in Fukushima Prefecture (Japan).

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the most important criterion is the abundance and visibility of crystals on the rock matrix. Although the crystals are microscopic, specimens with dense, well-formed aggregates of intense, yellow color are most desirable. Contrast with dark host rock also enhances the specimen's attractiveness. Due to their size, specimens are evaluated under a microscope. ## Popular Localities Specimens from the type locality – Wheal Gorland mine in Cornwall – are by far the most valued and sought after by collectors. Material from this historic locality is considered classic and fetches the highest prices in the collector's market.

Care and storage

## Cleaning Specimens with hallimondite should generally not be wet cleaned. Dust can only be removed with a soft brush or a photographic air blower. Contact with water can lead to damage of the delicate crystals. ## What to avoid Hallimondite is a brittle and sensitive mineral. Contact with chemicals, especially acids and bases, should be avoided. It should not be heated or exposed to prolonged moisture. As a uranium mineral, it emits radiation, so direct exposure should be limited, and hands should always be washed after contact with the specimen. ## Storage Specimens should be stored under stable conditions, in closed, padded "micromount" boxes to protect them from mechanical damage and dust. It is recommended to store them away from other minerals that could be damaged by radiation.

Sources

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