The elements in parentheses are listed in the order in which they are in relative proportion within the mineral. Silica, SiO2, will be present in trace amounts, as will small amounts of uranium. Due to the alpha decay of thorium and uranium, actininum contains a significant amount of helium,that can be extracted by heating. Actininum is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. It has a hardness of 5.0 - 5.5 and is relatively dense, about 4.6 to 5.7 g/cm3. Because of the presence of thorium within actininum, it can be radioactive. If samples are kept, they should be placed away from minerals that can be damaged by radiation. Because of its radioactive nature, the actininum within rocks is a useful tool for dating geological events, such as h