Metarauchite

Chemical formula: Ni²⁺(U⁶⁺O₂)₂(As⁵⁺O₄)₂(H₂O)₅·₃H₂O

Metarauchite is a very rare, secondary uranyl and nickel mineral, forming small, yellowish-green tabular crystals.

## Characteristics Metarauchite is a hydrated nickel uranyl arsenate. It occurs as very small, thin, tabular crystals with a square or octagonal outline, usually not exceeding 0.2 mm. It also forms rosette-like aggregates and coatings. It is a transparent mineral with a vitreous luster. ## Physical Properties Metarauchite has a Mohs hardness of approximately 2.5. Its calculated density is 3.96 g/cm³. It is a brittle mineral, exhibiting perfect cleavage in one direction. ## Colors and Varieties This mineral has a characteristic yellowish-green color. The streak is pale yellow. No varieties have been distinguished. ## History and Name Metarauchite was first described in 2013 by Jakub Plášil and his team. The name refers to its relationship with the mineral rauchite, from which it differs by a lower degree of hydration (it is its meta-analogue). The name "rauchite" in turn honors Frank Rauch, a German mineral collector specializing in uranium minerals. ## Uses Due to its extreme rarity, small crystal size, and radioactive element content, metarauchite has no industrial applications. It is of purely scientific interest and is an object of interest for specialized collectors of uranium minerals.

Properties

Mohs hardness: 2
Color: Yellow to light greenish yellow
Luster: Vitreous
Streak: light green to pale yellow strea
Density: 3.81
Cleavage: (011)
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Metarauchite can be identified by its characteristic yellowish-green color, tabular crystal habit, perfect cleavage, and occurrence in the oxidation zones of uranium deposits. A key diagnostic feature is its strong, green fluorescence under shortwave ultraviolet (UV-SW) light. ## Differentiation from Similar Minerals Metarauchite is visually almost identical to other minerals of the meta-autunite group, such as metakahlerite, metanovacekite, or meta-autunite. Definitive differentiation is possible almost exclusively using advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD), which allow for the identification of chemical composition (presence of nickel and arsenic). ## Crystal Forms It forms thin, tabular crystals with a square or octagonal outline. Crystals often combine into rosette-like or fan-shaped aggregates and form thin coatings and crusts on the host rock.

Geological environment

## Genesis Metarauchite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal uranium-arsenic deposits rich in nickel. It crystallizes at low temperatures from aqueous solutions. ## Mineral Associations This mineral co-occurs with other secondary uranyl arsenates. At the type locality (Jáchymov), it was found in association with metakahlerite, metanovacekite, uranophane, gypsum, hematite, and quartz. ## Localities The only confirmed and described locality for metarauchite in the world is its discovery site – the Geschieber vein in the Svornost mine, in the Jáchymov (St. Joachimsthal) district in the Czech Republic.

Rarity

Extremely rare

For collectors

## Quality Criteria The most prized metarauchite specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates on a contrasting rock matrix. Due to the microscopic size of the crystals, their abundance and visual quality under magnification are crucial. Specimens rich in crystals, covering a significant surface area of the matrix, are the most desirable.

Care and storage

## Cleaning Metarauchite specimens should not be cleaned wet. A soft brush or compressed air from a safe distance can be used to remove dust. ## What to Avoid Contact with water should be strictly avoided, as it can damage the mineral due to its solubility and potential change in hydration state. All chemicals, ultrasonics, and high temperatures should also be avoided. As a uranium mineral, it is radioactive and requires careful handling – avoid inhaling dust and wash hands after each contact. ## Storage Store in a dry place, preferably in a sealed, closable "membrane box" to protect fragile crystals from mechanical damage and moisture. It should be stored away from other radiation-sensitive minerals.