Thorium is a fissionable material used in thorium-based nuclear power. Thorium has been suggested for use in thorium-based nuclear power. Mill tailings often contain radium, thorium and trace amounts of uranium. MOX fuel containing thorium and plutonium oxides is also being tested. Tobacco contains traces of thorium, polonium and uranium. The deposits were mined for uranium, thorium, and rare earths. Thorium and plutonium fluorides have also been used. They occur in the decay chains of primordial isotopes of uranium or thorium. Four radioactive elements were known in 1900&colon; radium, actinium, thorium, and uranium. Fuels with plutonium and thorium are also an option. Less common are the niobates of calcium, uranium, thorium and the rare earth elements. It is also high in uranium and thorium. Monazite is an important ore for thorium, lanthanum, and cerium. In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. Additionally, cerium dioxide also acts as a catalyst for the combustion of thorium oxide.