Class : Phosphates, arsenates, vanadates
    Subclass : Uranyl phosphate
    Crystal system : Tetragonal
    Chemistry : Ca(UO₂)₂(PO₄)₂ 6H₂O
    Rarity : Quite common

Meta-autunite is a dehydration product of autunite which forms by prolonged exposure to open air of this mineral. Meta-autunite is almost systematically associated with autunite (often pseudomorphs) in the oxidation zone of uranium deposits. It is also found more rarely in pegmatites, granites and episyenites. Its name comes from the Greek meta (after) and autunite, because it is a less hydrated form of this mineral. It also closely resembles it in its habit, in tabular to lamellar crystals with a square or rectangular appearance, and its strong yellowish-green fluorescence under UV light. Only its color is usually more yellow and less green than that of autunite, but this difference is not systematic. Its occasional dark green hue appears to be due to inclusions of uraninite particles. Like autunite, it is an important uranium ore. 

Main photo : Meta-autunite from Nanjingqiao U deposit, Pingjiang Co., Yueyang, Hunan, China

Certain French uranium deposits have also provided samples of meta-autunite of world quality : the Limousin deposits in the first place, with centimeter-sized crystals from the Margnac and Crouzille mines (Haute-Vienne), the Auvergne mines of district of Lachaux (Puy-de-Dôme), with their 2 cm lamellar crystals, and the Huis Jacques deposit (Nièvre) where the 2 cm crystals assemble into decimetric veneers.

No twin known for this mineral species.

No fakes recorded for this species which remains difficult to differentiate from autunite with the naked eye. Most meta-autunite samples are treated to prevent dehydration and therefore ensure good conservation. Old specimens are often varnished, varnish which does not necessarily age very well (yellowing, cracking). Modern treatments are almost invisible, and “plasticize” the sample. We use paraloid B72 to stabilize our autunites and meta-autunites, it exists in a ready-made pre-dosed solution to apply directly with a brush and also in "balls" to dissolve in acetone.