The Mineral META-URANOCIRCITE

Chemistry: Ba(UO₂)₂(PO₄)₂ - 6-8H₂O, Hydrated Barium Uranyl Phosphate
Class: Phosphates
Group: Meta-autunite
Uses: A minor ore of uranium and as mineral specimens.
Specimens

Meta-uranocircite is a dehydration product of its close cousin, uranocircite, hence the name. When the mineral uranocircite loses water and converts to meta-uranocircite, it becomes what is known as a pseudomorph. A pseudomorph is generally an atom by atom replacement of one mineral's chemistry in place of another mineral's chemistry, while the original crystal's outward shape remains largely unchanged. The process leaves the crystal shape of the original mineral intact, but the original mineral is no longer there. Pseudomorph translated means false shape (pseudo=false; morph=shape).

In this case, the conversion is not so dramatic since it involves only the loss of a few water molecules; therefore, a good pseudomorph is likely. The conversion from uranocircite to meta-uranocircite is irreversible and ongoing, and all collection specimens of a certain age are almost certainly partially to totally converted. If accuracy is demanded, all uranocircite collection pieces of a few years of age or more should be labeled as meta-uranocircite. Meta-uranocircite can be found naturally in nature, but these specimens probably started out as uranocircite.

The structure of meta-uranocircite is composed of phosphate tetrahedrons linked to uranium-oxygen groups that form distorted octahedrons. The phosphate and uranium groups form sheets that are weakly held together by water molecules. This structure produces the tabular habit, the one perfect direction of cleavage, and the relative softness. It is an analogous structure to that of the phyllosilicates. Its symmetry is monoclinic although it outwardly appears to be tetragonal (i.e. pseudotetragonal).

Remember, this is a radioactive mineral and should be stored away from other minerals that are affected by radioactivity and human exposure should always be limited.

PHYSICAL CHARACTERISTICS:

Colors include yellow to yellow-green.
Luster is vitreous or pearly to dull.
Transparency: Crystals are translucent to opaque.
Crystal System is monoclinic (pseudotetragonal).
Crystal Habits include tabular square crystals dominated by two pinacoid faces. Crystals can look cubic (pseudocubic) too. Also as crusts, micaceous, foliated and earthy. Almost all meta-uranocircite crystals are pseudomorphs of uranocircite.
Cleavage is perfect in one direction.
Fracture is uneven.
Hardness is 2 - 2.5
Specific Gravity is approximately 4.1 (well above average for translucent minerals and higher than uranocircite due to loss of water molecules).
Streak is a pale yellow.
Other Characteristics: Radioactive, fluorescent green and cleavage sheets are surprisingly brittle.
Associated Minerals include limonite, quartz, meta-autunite, meta-torbernite, autunite, torbernite, uranocircite, uraninite and other uranium minerals. Some meta-uranocircite is found in petrified wood in Arizona.
Notable Occurrences include the Cameron area of Coconino County and Dripping Springs Quartzite in Gila County , Arizona; Kern County, California and Harding County, South Dakota, USA; Bergen, Saxony, Germany; Banat, Romania and Antsirabe, Madagascar.
Best Field Indicators are color, crystal habit, fluorescence, higher density than uranocircite, radioactivity, associations, cleavage and brittle cleavage sheets.