Geiger counter detecting the radioactivity from a small sample of the mineral autunite. Autunite is a form of calcium uranyl phosphate, with the formula Ca(UO2)2(PO4)2.10-12H2O. 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. Naturally occurring uranium will have partly decayed to its various products over time, including many other radioactive nuclei, including thorium-234, radium-226, radon-222 and polonium-218. This sample is from Mount Painter, South Australia.