Autunite
Chemical formula: Ca(U⁶⁺O₂)₂(PO₄)₂·10-12H₂O
Autunite is a hydrated uranyl calcium phosphate, valued for its intense yellow-green color and very strong fluorescence under ultraviolet light.
Description
## Characteristics Autunite is a uranium mineral belonging to the phosphate group. Its most characteristic feature is its bright, lemon-yellow to greenish-yellow color. It typically forms thin, tabular crystals with a square or rectangular outline, which often group into fan-like aggregates or structures resembling open books. Due to perfect cleavage in one plane, its crystals easily separate into thin, brittle flakes, similar to mica. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2-2.5, meaning it can be scratched with a fingernail. It is relatively light, with a density of about 3.05 g/cm³. The luster on cleavage surfaces is distinctly pearly, while on other faces it can be vitreous. It is transparent to translucent. Autunite is an unstable mineral in dry air – it loses water from its structure, transforming into meta-autunite, which often causes the crystals to become cloudy and more brittle. ## Colors and Varieties Autunite's color palette is limited to shades of yellow and green, from lemon-yellow, through bright yellow, to yellow-green. No distinct color or commercial varieties are recognized. A key feature is its transformation into meta-autunite, which is a natural and irreversible process. Meta-autunite specimens have the same crystal forms but are usually less transparent and have a weaker luster. ## History and Name The name autunite comes from the town of Autun in France, where it was first identified. The mineral was described in 1852 by Henry-Jules Brooke and William Hallowes Miller. ## Uses Due to its uranium content, autunite was historically a minor ore of this element. Today, because of its beautiful crystals and spectacular fluorescence, it is almost exclusively of collector's interest.
Diagnostic features
## Identification The most important diagnostic feature of autunite is its extremely strong, bright yellow-green fluorescence under ultraviolet light (both shortwave and longwave). The combination of this property with its characteristic yellow-green color, tabular crystal form, and pearly luster on cleavage planes is practically unambiguous. Additional confirmation is its radioactivity, which can be measured with a Geiger counter. ## Differentiation from Similar Minerals Autunite is sometimes confused with other secondary uranium minerals. It is most often confused with torbernite, which has similar crystal forms but is green and does not fluoresce. Uranocircite is visually almost identical but fluoresces slightly weaker and with a greener hue. Sabugalite is also very similar but usually forms microcrystalline coatings. In collecting practice, the fluorescence test is crucial for distinguishing autunite from torbernite. ## Crystal Forms Autunite crystallizes as thin, square or rectangular tablets. Crystals rarely occur individually, most often forming fan-like, radial, or layered aggregates resembling stacks of paper or books. They can form spectacular, dense coatings on host rocks.
Geological environment
## Genesis Autunite is a secondary mineral, formed in the oxidation (weathering) zones of uranium deposits. It forms as a result of the alteration of primary uranium minerals, such as uraninite, under the influence of phosphate-rich waters. It most often occurs in granites, granitic pegmatites, and in some uranium-bearing sandstone sediments. ## Mineral Associations It often co-occurs with other secondary uranium minerals, primarily with meta-autunite (into which it transforms), torbernite, uranophane, phosphuranylite, and sabugalite. It is also accompanied by host rock minerals such as quartz, feldspars, micas, and fluorite. ## Localities Historically important specimens come from the type locality in Autun, France, and from German deposits in Schneeberg and Johanngeorgenstadt in Saxony. Significant quantities of beautiful specimens have been provided by deposits in Portugal (e.g., in the Viseu and Guarda regions). Some of the most spectacular specimens in the world come from the Daybreak Mine in Washington State (USA), where it formed large, rich plates. It also occurs in many other places around the world, including Brazil, Argentina, Australia (Ranger Mine), and Poland (Karkonosze, Sudetes).
Rarity
Not very common
Collector aspects
## Quality Criteria For collectors, the most desirable specimens are those with large, well-formed, sharp crystals of intense, bright color. Rich groups of crystals forming aesthetic fans or "books" on a small, contrasting rock matrix are highly valued. Transparency and strong luster are also important, indicating that the mineral has not yet undergone advanced dehydration. Specimens that are not fractured and have retained their integrity are much more valuable. ## Popular Localities Specimens from the Daybreak Mine in the USA are considered classic and most prized, setting the standard of quality for this mineral. Beautiful, though usually smaller, crystals come from Portugal and France. German localities in Saxony have also provided many historically important specimens.
Care and storage
## Cleaning Autunite is extremely sensitive to water, which accelerates its decomposition. Cleaning should be limited to very gentle dust removal using a soft, dry brush or blowing it off with compressed air from a safe distance. ## What to Avoid Contact with water and other liquids must be absolutely avoided. The mineral is very soft and brittle, so it must be protected from impacts and scratches. Storage in dry air, heat, or direct sunlight accelerates the dehydration process to meta-autunite, causing a loss of transparency and crystal integrity. As a uranium mineral, it is radioactive – avoid inhaling dust and wash hands after each contact. ## Storage The best storage method is to place the specimen in a sealed, transparent container (e.g., an acrylic box), which slows down water loss and maintains constant humidity. The container also protects against dusting and limits radon emission. Specimens should be stored in a cool, dark place, away from other minerals that may be sensitive to radiation (e.g., smoky quartz, amethyst, fluorite). Each specimen should be clearly labeled as radioactive.