Johannite
Chemical formula: Cu<sup>2+</sup>(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·8H<sub>2</sub>O
Johannite is a rare, secondary uranium mineral, characterized by its emerald-green color, strong radioactivity, and intense fluorescence under UV light.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 3.32
- Cleavage
- Perfect on {010}
- Fracture
- Conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Johannite is relatively easy to identify based on its unique characteristics. Key indicators include its intense emerald-green color, characteristic crystal forms (small, bladed, often in radial aggregates), and occurrence in the oxidation zones of uranium deposits. The most important diagnostic features are strong green fluorescence under UV light and high radioactivity, easily detectable with a Geiger counter. ## Distinguishing from Similar Minerals - **Zippeite:** Has a similar yellowish-green color and is also fluorescent, but usually forms earthy masses or very fine aggregates, less often distinct crystals. Johannite more often forms visible, bladed crystals. - **Torbernite/Zeunerite:** Have a similar green color but form characteristic, square, tabular crystals and do not exhibit fluorescence under UV light. - **Uranophane:** Usually yellow to yellowish-green and forms acicular crystals in radial aggregates. Its fluorescence is weaker than that of johannite. ## Crystal Forms Johannite crystals are small, flattened, tabular or bladed, often with a hexagonal outline. They usually occur as radial or stellate aggregates, rosettes, and also as coatings and crusts on the host rock.
Geological environment
## Genesis Johannite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal uranium deposits. It forms as a result of sulfate-rich waters acting on primary uranium minerals, such as uraninite. It is one of the later minerals to crystallize during the weathering process, often appearing as coatings on the walls of mine workings in uranium mines. ## Mineral Associations Johannite often co-occurs with other secondary uranium minerals. Its most common associations include: gypsum, zippeite, uranopilite, brochantite, chalcanthite, as well as primary uraninite. ## Localities The most important and classic locality, from which historical specimens originate, is Jáchymov (St. Joachimsthal) in the Czech Republic. Other known localities worldwide include: - **USA:** numerous mines in Utah (e.g., Delta, Happy Jack Mine), Colorado, Arizona, and New Mexico. - **Germany:** mines in Saxony (Schlema-Hartenstein, Johanngeorgenstadt). - **France:** around Lodève. - **Italy:** Val Rendena in Trentino.
Rarity
Rare
For collectors
## Quality Criteria The most prized johannite specimens are characterized by well-formed, sharp crystals of an intense emerald-green color. Rich groups of crystals forming radial or stellate aggregates on a contrasting rock matrix are particularly sought after. Crystal size is crucial – specimens with crystals exceeding a few millimeters are rare. The clarity and transparency of the crystals also increase the value of the specimen. ## Popular Localities Classic, historical specimens from Jáchymov in the Czech Republic are considered exemplary and fetch high prices. Currently, the collector's market is dominated by specimens from mines on the Colorado Plateau in the USA (especially from Utah), which provide specimens with well-defined crystals. Specimens from German and French localities are rarer and also highly valued.
Care and storage
## Cleaning Johannite specimens are very delicate and sensitive to water. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to dry-dust or compressed air from a safe distance. Avoid contact with water, which can damage or dissolve the crystals. ## What to Avoid - **Water and moisture:** Johannite is water-soluble. It must be protected from humid air, and especially from direct contact with water. - **Chemicals:** Avoid contact with acids and all cleaning agents. - **Shocks and friction:** The mineral is very soft and brittle, easily susceptible to mechanical damage. - **Direct sunlight:** Prolonged exposure may slightly affect the color, although the main threat is temperature and humidity changes. ## Storage Johannite is a strongly radioactive mineral and requires special precautions. It should be stored in a tightly sealed, lead or glass container, away from other minerals that could be damaged by radiation. The container should be clearly marked with a radioactivity symbol. Store in a dry place, away from areas of frequent human presence, especially bedrooms and living rooms. When handling specimens, wear gloves and avoid inhaling dust.