Pseudojohannite

Chemical formula: Cu<sup>2+</sup><sub>3</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>4</sub>O<sub>4</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·12H<sub>2</sub>O

Pseudojohannite is a very rare, secondary uranium mineral of intense green color, occurring as coatings and aggregates of fine crystals.

## Characteristics Pseudojohannite is a hydrated copper uranyl sulfate hydroxide. It forms very small, bladed or tabular crystals, which usually occur as rosetted aggregates, thin coatings, or encrustations on the host rock. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Pseudojohannite crystals exhibit a vitreous luster. The mineral is transparent to translucent. Its Mohs hardness is approximately 2, and its density is estimated at 3.82 g/cm³. It is a very brittle mineral. ## Colors and Varieties Pseudojohannite has a characteristic, intense, emerald-green or grass-green color. No varieties are distinguished. ## History and Name The mineral was first described in 1996 by Kurt Walenta. Its name refers to its structural and visual similarity to johannite, with the prefix "pseudo" indicating differences in crystal structure (different crystallographic system). Type specimens come from the Michael mine in Weiler, Black Forest (Germany). ## Uses Due to its extreme rarity and small size, pseudojohannite has no industrial application. It is of purely scientific interest and is an object of interest for specialized collectors of uranium minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light green
Density
3.82
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of pseudojohannite include its intense green color, occurrence in the form of fine, bladed crystals forming rosetted aggregates, and co-occurrence with other secondary uranium minerals. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with johannite, from which it differs in crystallographic system (johannite crystallizes in the triclinic system, pseudojohannite in the monoclinic). Visually, it is almost identical to many other green, secondary uranium minerals, such as cuprosklodowskite, torbernite, or zeunerite. Differentiation without specialized equipment is practically impossible. ## Crystal Forms Crystals are very small, bladed or thin-tabular, often flattened. They usually form radial or rosetted aggregates, as well as thin coatings and encrustations.

Geological environment

## Genesis Pseudojohannite is a secondary mineral, forming in the oxidation (weathering) zones of uranium ore deposits. It forms as a result of sulfate-rich waters acting on primary uranium minerals, in the presence of copper ions. ## Mineral Associations It co-occurs with other secondary uranium minerals and sulfates, such as johannite, uranopilite, gypsum, as well as chalcopyrite and pyrite. ## Localities The most important and classic locality, from which the best-formed specimens originate, is the Michael mine in Weiler (Baden-Württemberg, Germany). The mineral has also been identified in Jáchymov (Czech Republic) and in several localities in Utah (USA), e.g., in the Delta mine.

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens are those with well-formed, distinct crystals forming aesthetic, rosetted aggregates on a contrasting rock matrix. The intensity and purity of the green color also increase the specimen's value. Due to the microscopic size of the crystals, the quality of a photograph taken through a microscope (a so-called "micromount") is crucial. ## Popular Localities Specimens from the type locality – the Michael mine in the Black Forest, Germany – are by far the most sought after by collectors. They are considered exemplary for this mineral species.

Care and storage

## Cleaning Pseudojohannite specimens should generally not be wet cleaned. Only gentle dust removal with a soft brush or a photographic air blower is permissible. Contact with water, especially warm water, can lead to dissolution or damage of the delicate crystals. ## What to Avoid Contact with water, chemicals, acids, and detergents must be strictly avoided. The mineral is very soft and brittle, sensitive to any shocks and abrasions. As a uranium mineral, it is radioactive and requires special precautions – avoid inhaling dust and wash hands after each contact. ## Storage Specimens should be stored in stable, sealed containers (e.g., "perky box" type), away from sources of vibration, moisture, and direct sunlight. Due to its radioactivity, it should be kept away from areas of permanent human presence, in a well-ventilated room.

Sources

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