Boltwoodite is a hydrated uranyl silicate mineral distinct in crystal structure from sodium boltwoodite, which has an orthorhombic structure rather than monoclinic.  It is formed from the oxidation and alteration of primary uranium ores. It takes the form of a crust on some sandstones that bear uranium. These crusts tend to be yellowish with a silky or vitreous luster.

This specimen finds it's way to me from Namibia, the yellow needle like crystals on this one are covered in a layer of calcite giving it it's unique look.

This mineral is naturally radioactive as it does contain Uranium along with it's daughter isotopes.