Why would India have so much thorium on its beaches?
why would India have so much thorium on it's beaches...?


Because monazite. Monazite is a rare earth element phosphate, with the formula CePO4 (where Ce stands not only for cerium, but all of the are earth elements and yttrium as well). It is one of the main ore minerals for the rare earths. It's a very common mineral in granites and similar rocks, and it's very resistant to erosion.

Therefore, it commonly accumulates in placer deposits, together with other resistant minerals such as quartz, magnetite, zircon, rutile, etc. Essentially, nature breaks down the rocks and concentrates those minerals in mineral sands (sometimes known as black sands). These sands are not unique to India, and many examples can be found across the world. Essentially you find them where ever you have sandy beaches developed in areas of granite mountains (in the case of monazite).

Why thorium? Because monazite shares the same crystal structure with a mineral called cheralite: CaTh(PO4)2, and the two components (monazite and cheralite) can exist in the same mineral via solid solution. Some monazites can contain 20 and 30% thorium! Granite-derived monazite is particularly rich in Th, unlike carbonatite-derived monazite which usually has little Th. The Th is usually an unwanted by-product, with monazite mined primarily for the rare earths. But, if you're looking for the Th - this is something that you actually want.