An introduction to geology
An introduction to geology

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

An introduction to geology

4.6 Rare earth elements

Dredging isn’t the only cause for concern when it comes to extracting minerals from the Earth. Cast your minds back to the mobile phone in Week 2. Do you recall the term ‘rare earth elements’ (REE)? They are a group of elements that aren’t that rare in the Earth’s crust, but there are not many places where they occur in concentrations great enough to exploit economically and they are difficult to separate from each other.

REE are not found as native metals like copper and gold, but bound up in minerals. Often they are found in low concentrations in some common minerals, but in higher concentrations they can form their own minerals. The most economically viable being the interestingly named bastnäsite, monazite and xenotime. These minerals contain many other elements in addition to the REE, many of them toxic, such as arsenic, and radioactive, such as thorium.

Described image
Figure 4.7 Bastnäsite, xenotime, monazite

REE are found in igneous, metamorphic and sedimentary rocks and you will look briefly at each, as their origin determines the way that they are mined and the environmental impact.

In igneous rocks, REE are most abundant in the veins and pegmatites (igneous rocks made of large crystals, seen in Week 1) around magmatic intrusions, especially in areas where continental crust is being pulled apart, a modern example being the East African Rift Valley. Here, we find rocks with unusual chemistries, rich in alkali elements such as sodium, potassium and calcium, and even carbonate igneous rocks called carbonatites, where instead of being dominated by silica they are over 50% carbonate. A modern carbonatite volcano is Ol Doinyo Lengai in Tanzania.

There are also REE not associated with alkali igneous rocks and carbonatites, they are often associated with iron and the REE are extracted as by-products of the iron industry.

The above examples are called primary deposits because the REE are mined from the location that they were originally emplaced. Secondary deposits are where an economically viable mineral has been transported and redeposited, and is mined from there. The most common source of REE deposits are placer deposits, over 350 of which have been identified worldwide. Placer deposits are where minerals that are denser than the surrounding grains (normally quartz) are concentrated by winnowing, by wind or water. These deposits are either mined using diggers or, if the deposit is underwater, they are dredged like aggregates.

Described image
Figure 4.8 Placar deposit

Another secondary deposit is bauxite, the ore of aluminium, that you looked at in Week 2. If the parent rock was rich in REE, the bauxite is too. In fact, just like the aluminium, the weathering process concentrates the REE from perhaps 0.1–0.2% REE to up to 40%, as is found in the Mount Weld deposit, Australia.

The final and still unexploited sources are deep sea muds. Two areas of the Pacific Ocean are reported to contain high concentrations of REE, which is thought to come from mid-ocean ridge hydrothermal activity.

Almost all REE are actually a by-product of mining for other materials. Only Mountain Pass, California (which is currently not in production) is mined solely for REE.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371