Thorium - Occurrence

Occurrence

Thorium is found in small amounts in most rocks and soils; it is three times more abundant than tin in the Earth's crust and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO₄), thorianite (ThO₂ + UO₂) and monazite. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. Monazite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium. Thorium-containing minerals occur on all continents. Thorium is several times more abundant in Earth's crust than all isotopes of uranium combined and thorium-232 is several hundred times more abundant than uranium-235.

232Th decays very slowly (its half-life is comparable to the age of the universe) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible.