The abundance of thallium in the Earth crust is about 1 gram per ton. It is interesting to notice that, according to this estimation, thallium is more abundant than other well-known metals, such as mercury, antimony, bismuth, cadmium and silver. The element is widely spread throughout the earth crust but its direct extraction is very expensive and ineffective. Thallium usually occurs associated with potassium minerals and is extracted as a by-product.