**1. Introduction**

Thorium is a naturally occurring, slightly radioactive element. It is found in small amounts in most rocks, where it is about three times abundant than uranium. Thorium is relatively enriched in acid igneous rocks, especially in granites. The most common thorium mineral is monazite. In uranium ore deposits, thorium is concentrated in thorite and thorianite. In magmatic carbonate-enriched rocks (carbonatites), thorium is associated with rare earth elements (REE) in bastnaesite. However, the most important reserves of thorium occur in placer deposits, which contain monazite. Monazite is in placer deposits mined together with other heavy

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

minerals, such as rutile, zircon, ilmenite, and cassiterite. The principal monazite-producing countries are India, Brazil, Malaysia, and Thailand. Major end uses of thorium are refractories, lamp mantles, and aerospace alloys. Relatively restricted is using thorium in energy production. Although research into thorium-fuelled nuclear reactors continues, there exist no industrial-scale nuclear reactors using thorium. However, India continued its plan for a development of its nuclear power program based on the thorium-fuelled nuclear reactors.
