Metanováčekite

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

Metanováčekite is a rare, secondary uranium mineral of the meta-autunite group, forming characteristic, yellow, tabular crystals with strong fluorescence.

## Characteristics Metanováčekite is a hydrated uranyl magnesium arsenate, belonging to the meta-autunite group. It occurs as small, tabular or scaly crystals with a square outline, often forming rosette-like or fan-shaped aggregates. It can also be found as coatings and earthy masses. It is a secondary mineral, formed in the oxidation zones of uranium deposits. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5. The crystals are brittle and exhibit perfect cleavage in one direction, causing them to easily separate into thin lamellae. The luster on cleavage surfaces is pearly, and on other faces, it is vitreous. A characteristic feature is its strong, yellowish-green fluorescence under ultraviolet light (both shortwave and longwave). ## Colors and Varieties Metanováčekite ranges in color from pale yellow, through greenish-yellow, to yellow. No named color varieties or commercial varieties are distinguished. The variation in shades is slight and depends mainly on the degree of hydration and the presence of any impurities. ## History and Name The mineral was named in 1964 by Paul F. Kerr in honor of Radim Nováček (1905-1942), a Czech mineralogist from Charles University in Prague, who first described its synthetic analogue and the natural mineral (later named nováčekite), from which metanováčekite forms by dehydration. The prefix "meta-" refers to a lower degree of hydration compared to nováčekite. The type locality is the Anton mine in the Heubach Valley (Baden-Württemberg, Germany). ## Uses Due to its rarity and small accumulations, metanováčekite has no industrial application. It is of purely scientific and collector interest as a secondary mineral of uranium deposit oxidation zones.

Properties

Mohs hardness: 2.5
Luster: Pearly, Vitreous
Streak: yellowish white
Density: 0
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Key diagnostic features of metanováčekite are its yellow color, square, tabular crystal habit, perfect cleavage in one direction, and very strong, yellowish-green fluorescence under UV light. It is a secondary mineral, occurring as coatings and aggregates in oxidation zones. ## Distinguishing from Similar Minerals Metanováčekite is visually almost identical to other minerals of the autunite and meta-autunite groups, such as autunite, meta-autunite, torbernite, metatorbernite, zeunerite, or metazeunerite. Certain differentiation from its magnesium analogue, nováčekite, is only possible by controlling the degree of hydration under laboratory conditions. Distinguishing it from other minerals in the group requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Crystals are thin-tabular, with a square or rectangular outline, consistent with tetragonal symmetry. They often form fan-shaped, rosette-like, or packet-like aggregates. They also occur as scaly coatings, crusts, and earthy masses on the surface of host rocks.

Geological environment

## Genesis Metanováčekite is a secondary mineral that forms in the oxidation (weathering) zones of hydrothermal and sedimentary uranium deposits. It forms as a result of dehydration (loss of water) of primary nováčekite under conditions of low air humidity. It can also crystallize directly from aqueous solutions under appropriate temperature and pressure conditions. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as nováčekite, autunite, meta-autunite, uranophane, as well as with uraninite (as a primary mineral), quartz, hematite, and clay minerals. ## Localities The most important occurrences of metanováčekite are in Germany (Anton mine in Wittichen - type locality; Schneeberg in Saxony), the Czech Republic (Jáchymov), and the USA (Utah, Arizona, South Dakota).

Rarity

Rare

For collectors

## Quality Criteria Specimens most valued by collectors are those with well-formed, sharp crystals of intense yellow color, forming aesthetic, fan-shaped or rosette-like aggregates. Contrast with a dark host rock (matrix) is important. Large, undamaged crystals are rare and command higher prices. Due to the small size of the crystals, it is often collected as micromounts. ## Popular Localities Specimens from the classic type locality at the Anton mine in Germany are historically important and sought after. Good quality crystals also come from some localities in the Czech Republic and the USA.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its brittleness and solubility, contact with water and any cleaning fluids should be avoided. If wet cleaning is absolutely necessary, use a minimal amount of alcohol and dry it immediately. ## What to Avoid The mineral is sensitive to changes in humidity – it readily absorbs water, transforming into nováčekite, or loses it in a dry environment, which can lead to its destruction. Direct sunlight, which can cause fading and further dehydration, should be avoided. It is soluble in acids. As a uranium mineral, it is radioactive and should be handled with appropriate precautions (avoid inhaling dust, wash hands after contact). ## Storage It is recommended to store specimens in sealed, transparent containers (e.g., "micromount" boxes) that stabilize humidity and protect fragile crystals from mechanical damage. They should be kept away from heat sources and direct light, in a place with stable temperature and humidity.

External references

https://www.mindat.org/min-2681.html