metaautunite

Metaautunite is a hydrated uranyl phosphate mineral within the autunite group. It is considered a dehydration product of autunite, forming when autunite loses water through drying, heating, or long-term weathering. Its composition is typically described as a reduced-hydration form of the autunite formula, with fewer water molecules in the interlayer region.

Formation and occurrence

Metaautunite occurs as a secondary mineral in oxidized zones of uranium-bearing deposits and in environments where uranyl phosphates accumulate, such as hydrothermal or sedimentary settings near uranium sources. It commonly appears as crusts, tabular crystals, or earthy masses on fracture surfaces and can arise by alteration of autunite or related uranyl phosphates. Under continued drying or instability, metaautunite can transition further to metaschoepite or other lower-hydration species.

Physical properties

Metaautunite is typically yellow to yellow-green in color and can be transparent to translucent in well-formed crystals or appear as earthy aggregates. It has a relatively low hardness and is radioactive due to its uranium content. The mineral commonly forms in association with other uranyl phosphates such as autunite, metaschoepite, and schoepite.

Relationship and context

As part of the autunite group, metaautunite shares the characteristic uranyl phosphate framework with water molecules occupying interlayer sites. The term “meta-” reflects its status as a dehydrated form relative to autunite. Hydration levels in the environment can influence stability and visibility of metaautunite, with the potential rehydration to autunite under humid conditions.