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Thorium dioxide
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<p>Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is mainly a by-product of <a href="/facts/Lanthanide/rt3XIRCl">lanthanide</a> and <a href="/facts/Uranium/n2sGWBzU">uranium</a> production. <a href="/facts/Thorianite/Xy7LefW9">Thorianite</a> is the name of the mineralogical form of <a href="/facts/Thorium/ysdWH470">thorium</a> dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including <a href="/facts/Tungsten/WYwbNDYX">tungsten</a> and <a href="/facts/Carbon/ukrgQexo">carbon</a>) and a few compounds (including <a href="/facts/Tantalum_carbide/9JcAp42f">tantalum carbide</a>) have higher melting points. All thorium compounds, including the dioxide, are radioactive because there are no stable <a href="/facts/Isotopes_of_thorium/lRb3opzg">isotopes of thorium</a>. </p>