Tyuyamunite with a Geiger counter detecting radioactivity. Tyuyamunite is a very rare uranium mineral and a member of the carnotite group. Its high uranium content gives it a bright yellow colour and its radioactivity. The uranium it contains is mostly (>99%) in the form of uranium-238, which decays by alpha decay to thorium-234. Alpha decay involves the emission of an alpha particle, or helium nucleus, comprising two protons and two electrons.