Meta-autunite

Chemical formula: Ca(U⁶⁺O₂)₂(PO₄)₂·6H₂O

Metaautunite is a uranium mineral from the autunite group, formed by the dehydration of autunite and characterized by intense, yellow-green fluorescence under UV light.

## Characteristics Metaautunite is a secondary uranium mineral belonging to the autunite group. It forms as a result of partial dehydration of autunite, which leads to changes in its crystal structure and water content. Typical specimens form thin, platy crystals with a square or octagonal outline, often aggregated into fan-like or layered clusters. It also occurs as earthy, scaly aggregates or as coatings and crusts on host rocks. It is a highly radioactive mineral. ## Physical Properties Metaautunite crystals are brittle and soft, with a Mohs hardness of 2-2.5. The luster on crystal faces is vitreous to pearly, and on fractures, it can be earthy or dull. The mineral is usually translucent, rarely transparent. Its most characteristic feature is strong, bright green or yellow-green fluorescence under ultraviolet light (both shortwave and longwave). ## Colors and Varieties Metaautunite exhibits colors ranging from lemon-yellow, through yellow-green, to green. Its color is directly related to the presence of uranyl (UO₂)2+ ions in its structure. There are no named commercial or gemological varieties. ## History and Name The name "metaautunite" refers to its connection with autunite and its lower water content in the structure (the prefix "meta-" in mineralogy often denotes a lower hydration state). The process of autunite transforming into metaautunite was observed as early as the 19th century, and metaautunite itself was formally described as a distinct mineral species later, when analytical techniques allowed for precise differentiation of the two phases. ## Uses Due to its high uranium content, metaautunite, along with other secondary uranium minerals, is significant as an indicator of uranium deposits and can be a minor ore of this element. For collectors, it is a valuable and sought-after mineral due to its intense color, crystal form, and strong fluorescence. Due to its radioactivity, its applications outside the nuclear industry and collecting are negligible.

Properties

Mohs hardness
2-2.5
Color
greenish yellow, lemon yellow
Luster
Vitreous, Pearly
Streak
pale yellow
Density
3.35
Cleavage
{001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Tetragonal

Diagnostic features

## Identification The most important diagnostic features of metaautunite are its characteristic yellow to greenish-yellow color, platy crystal habit, and very strong, bright green fluorescence under UV light. This feature allows it to be easily identified in the field and distinguished from many other yellow minerals. Further confirmation is its high radioactivity, which can be measured with a Geiger counter. ## Distinguishing from Similar Minerals - **Autunite:** Metaautunite forms from autunite and is almost visually identical. The main difference is that autunite fluoresces slightly more strongly and has a subtly different hue. Differentiation is certain only with X-ray diffraction (XRD) analysis, although autunite is stable only in environments with nearly 100% humidity. Under room conditions, it spontaneously transforms into metaautunite, so most specimens in collections are actually metaautunite. - **Torbernite:** Has a similar crystal habit but is green and does not exhibit UV fluorescence. - **Uranophane and beta-uranophane:** Have a similar color but form acicular or fibrous aggregates, not platy crystals. Their fluorescence is much weaker or absent. ## Crystal Forms Metaautunite crystallizes in the tetragonal system. It forms thin, platy crystals with a square or octagonal outline. Crystals often arrange into fan-like, foliated, or stacked aggregates. It also occurs as scaly aggregates, coatings, and crusts on other minerals or rocks.

Geological environment

## Genesis Metaautunite is a secondary mineral, meaning it does not crystallize directly from magma or hydrothermal solutions. It forms in the oxidation (weathering) zone of uranium deposits. It is created by the oxidation of primary uranium minerals, such as uraninite, by groundwater containing phosphates. Most often, however, it forms as a result of the spontaneous, partial dehydration (loss of water) of autunite under conditions of low air humidity. ## Mineral Associations As a mineral of the oxidation zone, metaautunite often co-occurs with other secondary uranium minerals. Its most common associations include: autunite (from which it directly forms), torbernite, metatorbernite, uranophane, phosphuranylite, as well as primary minerals such as uraninite. It may also be accompanied by quartz, micas, feldspars, and iron minerals such as goethite and hematite. ## Localities Significant and classic localities for metaautunite (often described as autunite) are found worldwide. The most important include: - **France:** Autun (type locality for autunite), Auvergne region. - **Portugal:** Mines in the Viseu and Guarda regions (e.g., Urgeiriça mine). - **Germany:** Schneeberg and Johanngeorgenstadt in Saxony. - **USA:** Daybreak Mine in Mount Spokane, Washington, where large, spectacular aggregates were found; numerous localities in Utah, Colorado, Arizona, and New Mexico. - **Democratic Republic of Congo:** Katanga Province (Shinkolobwe). - **Australia:** Rum Jungle in the Northern Territory.

Rarity

Not very common

For collectors

## Quality Criteria The collector appeal of metaautunite depends on several factors. Highly prized are specimens with well-formed, sharp crystals with a distinct platy habit. The intensity and saturation of color are very important – vibrant, lemon-yellow and greenish-yellow hues are preferred. The size of crystals and entire aggregates also increases value, as does their arrangement on the rock matrix, forming an aesthetic composition. Specimens without mechanical damage, with a clear luster and minimal foreign growths, are most sought after. Due to strong fluorescence, its intensity and uniformity also influence the quality assessment. ## Popular Localities The most valued specimens by collectors come from historical localities that have provided material of exceptional quality. Classic examples are crystal aggregates from the Daybreak Mine in Washington State (USA), which are considered among the best in the world. Specimens from Portugal (Viseu region) and France are also highly valued. In recent years, interesting specimens from China have also appeared on the market.

Care and storage

## Cleaning Metaautunite specimens are extremely delicate, brittle, and sensitive to water. Cleaning should be kept to an absolute minimum. To remove dust, a soft brush can be used, or dust can be carefully blown off with compressed air from a safe distance. Any contact with water or other liquids should be avoided, as they can cause further changes in the mineral's structure or its dissolution. ## What to Avoid - **Water and moisture:** Contact with water can lead to further dehydration or rehydration, destroying the crystals. Avoid storing in humid places. - **Sunlight:** Prolonged exposure to direct sunlight can cause color fading and further dehydration. - **High temperature:** Heating leads to water loss and can destroy the crystal structure. - **Vibrations and impacts:** The mineral is very brittle and has perfect cleavage, making it susceptible to mechanical damage. - **Chemicals:** Avoid contact with acids and other chemical agents. ## Storage Metaautunite is a highly radioactive mineral and requires special precautions. Specimens should be stored in sealed, lead-lined or thick-glass containers, away from areas of permanent human habitation, especially bedrooms. The container should be clearly marked with a radioactivity symbol. Display should be behind a protective screen (e.g., plexiglass) that blocks alpha and beta particles. Limit direct contact time with the mineral and always wash hands after touching it.

External references

Sources

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