Metanatroautunite

Chemical formula: Na(U₆+O₂)(PO₄)·₃H₂O

Metanatroautunite is a hydrated sodium uranyl phosphate, a secondary mineral from the autunite group, distinguished by its intensely yellow-green color and strong fluorescence.

## Characteristics Metanatroautunite is a mineral belonging to the autunite group, chemically classified as a hydrated sodium uranyl phosphate. It forms characteristic, tabular crystals with a square or octagonal outline, often with truncated corners. Crystals are usually very small and rarely exceed a few millimeters. It occurs as thin, brittle flakes, rosette-like aggregates, and also as coatings and crusts on host rocks. Its most recognizable features are its intense, bright, yellow-green color and strong fluorescence under ultraviolet light. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2 to 2.5, meaning it can be scratched with a fingernail. It is brittle, and its crystals are easily damaged. It has a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. Its density is approximately 3.63 g/cm³. Like other uranium minerals, it is radioactive. ## Colors and Varieties Metanatroautunite occurs in characteristic shades of yellow and green. Its color can be lemon-yellow, bright yellow, yellow-green, or greenish. There are no named color varieties or commercial varieties. ## History and Name The mineral's name refers to its chemical composition and structure. The prefix "meta-" indicates a lower degree of hydration compared to hypothetical, fully hydrated natroautunite. The component "natro-" refers to the presence of sodium (Latin: *natrium*), and "autunite" to its structural and chemical similarity to autunite. The mineral was first described by A.A. Chernikov in 1957. ## Uses Due to its rarity and small crystal size, metanatroautunite has no industrial applications. It is of purely scientific significance and is valued by specialized collectors of uranium minerals.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous, Pearly
Streak: Light yellow
Density: 3.63
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Metanatroautunite is most easily recognized by its characteristic, bright, yellow-green color, tabular crystal habit, and very strong, green fluorescence under ultraviolet light (both shortwave and longwave). Excellent cleavage in one direction, causing thin, square flakes to separate, is also a key feature. A radioactivity test with a Geiger counter will immediately confirm the presence of uranium. ## Distinguishing from Similar Minerals Metanatroautunite is visually almost identical to many other minerals from the autunite group, such as autunite, meta-autunite, torbernite, metatorbernite, or uranocircite. Distinguishing it from autunite and meta-autunite (calcium analogues) is impossible without advanced chemical analyses (EDS/WDS) to confirm the dominance of sodium over calcium. It differs from torbernite and metatorbernite (copper minerals) in color – torbernite is usually purely green, not yellow-green. ## Crystal Forms Crystals are thin-tabular, with a square or octagonal habit, often with truncated corners. They usually form fan-shaped or rosette-like aggregates. They also occur as scaly aggregates, coatings, and crusts on rock surfaces.

Geological environment

## Genesis Metanatroautunite is a secondary mineral, forming in the oxidation (weathering) zones of uranium deposits. It forms as a result of the alteration of primary uranium minerals, such as uraninite, in the presence of phosphate- and sodium-rich waters. It is a low-temperature mineral, crystallizing in fissures and fractures of sedimentary rocks (sandstones, shales) or granites containing uranium. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as autunite, meta-autunite, uranophane, schoepite, as well as with gypsum, limonite, and clay minerals. It can also be associated with primary ore minerals, such as uraninite and pyrite. ## Localities Significant specimens come from several locations worldwide. The most important include deposits in the Chkalovskoye Lake region in Russia (type locality). It also occurs in the United States, mainly in sandstones of the Morrison Formation in Utah, Colorado, and Arizona, for example, in mines in San Juan County (Utah). It is also known from uranium mines in the Menzenschwand area in the Black Forest (Germany).

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals of intense, bright color are most valued by collectors. Rich groups of crystals forming rosettes or fans on a contrasting rock matrix are highly prized. Crystal size is also important, although they rarely exceed a few millimeters. Purity and lack of mechanical damage to the brittle flakes significantly increase the specimen's value. ## Popular Localities For collectors specializing in uranium minerals, the most sought-after metanatroautunite specimens come from classic localities in the USA, especially from mines in San Juan County, Utah. Specimens from these locations often feature beautiful, fan-shaped aggregates on sandstone. Specimens from Menzenschwand, Germany, are also valued for crystal quality.

Care and storage

## Cleaning Metanatroautunite specimens are extremely delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove dust. Avoid contact with water, which can alter the mineral's hydration state and lead to its disintegration. ## What to Avoid Absolutely avoid water and other liquids, including ultrasonic chemical cleaners. The mineral is sensitive to changes in humidity and temperature, which can cause dehydration or further hydration, leading to crystal destruction. Protect it from direct sunlight, which can cause fading. As a uranium mineral, it is radioactive – avoid inhaling dust and wash hands after each contact. ## Storage Specimens should be stored in stable conditions, preferably in tightly sealed containers (e.g., "micromount" type), which protect against mechanical damage and humidity fluctuations. Due to its radioactivity, it should be stored away from areas of permanent human habitation and separately from other minerals that could be damaged by radiation.

Sources

IMA Mineral List (CNMNC)