Uraninite is a highly radioactive and interesting mineral.
It is the chief ore of uranium and radium, which is
found in trace amounts. Helium was first discovered
on the earth in samples of uraninite. Radium and helium
are found in uraninite because they are the principle
products of uranium's decay process. Weathered or otherwise
altered uraninite produces some wonderful by-products
such as the beautiful uranyl phosphate minerals like
autunite and torbernite as well as uranyl silicates
like sklodoskite and cuprosklodowskite.
The structure is analogous to the structure of fluorite,
CaF2. The structure of fluorite is highly symmetrical
and forms isometric crystals such as cubes and octahedrons.
Flourite also has four directions of perfect cleavage
that produces octahedrons. However, in uraninite, crystals
are rare and the cleavage is not usually observable.
A variety of uraninite is called pitchblende which is
a combination of mostly uraninite and some other minerals.
It is generally softer and less dense and usually botyroidal
or earthy. Remember, this is a highly radioactive mineral
and should be stored away from other minerals that are
affected by radioactivity and human exposure should
definitely be limited.