Why would India have so much thorium on its beaches?

Question

The recent BBC News article Why India Wants to Turn its Beaches into Nuclear Fuel includes the passage:

The tropical beaches of India probably bring to mind sun-dappled palms, fiery fish curries and dreadlocked backpackers, but they also hold a surprising secret. Their sands are rich in thorium – often hailed as a cleaner, safer alternative to conventional nuclear fuels.

The country has long been eager to exploit its estimated 300,000 to 850,000 tonnes of thorium – quite probably the world’s largest reserves – but progress has been slow. Their effort is coming back into focus amid renewed interest in the technology. Last year Dutch scientists fired up the first new experimental thorium reactor in decades, start-ups are promoting the technology in the West and last year China pledged to spend $3.3bn to develop reactors that could eventually run on thorium.

The caption to a stock image of a beach says:

Kerala's beautiful beaches hold 16% of India's thorium reserves, a major potential power source - if only scientists can find the right way to exploit it.

Question: why would India have so much thorium on its beaches, apart from the fact that India has so many beaches of course?

Answer 1

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.