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

2.4 Bauxite – amazing things that a lot of rain can do

Sometimes weather is all it takes to form an ore, so long as the weather is intense enough and you give it enough time!

Described image
Figure 2.3 Bauxite

Bauxite is an aluminium ore that forms just from the weathering of rocks. Weathering is what happens when rocks are left out in the rain, and comes in three forms: physical, chemical and biological. Physical weathering is when bits of rock are attacked by wind, or water, or sometimes just gravity during rock falls. Chemical weathering is when rocks slowly dissolve – rainwater is naturally ever so slightly acidic, and that acid will slowly dissolve the rocks. The third sort of weathering is biological, where rocks are attacked (chemically or physically) by living things.

Described image
Figure 2.4 Weathering processes

For bauxite to form, the important factor is chemical weathering under very specific conditions. Aluminium is the third most abundant element in the Earth’s crust (after oxygen and silicon) and, at 8.4%, the most abundant metal. Aluminium is usually combined very tightly with other elements in some very common minerals, so although there’s a lot of it almost everywhere, it’s very expensive to extract.

Fortunately, the chemical weathering of one of those minerals, feldspar (found in lots of igneous and metamorphic rocks), forms kaolinite, a clay mineral. This process increases the amount of aluminium oxide from less than 28% (in feldspar) to 40% – but that’s still not enough for it to be worthwhile to extract it. However, in really warm climates (like the tropics), the kaolinite is chemically weathered even more, forming minerals like gibbsite, which has high enough concentrations to be worthwhile to mine and process.

The essential thing in the formation of bauxite is that all of the other things which aren’t wanted (like silica – silicon dioxide, quartz, is its best known form) can, in tropical conditions, stay dissolved in the rainwater and be carried away, leaving the aluminium in place. This effect is shown in the figure below – showing the solubility of the aluminium oxide (Al2O3) and silica (SiO2) at different pH levels (a measure of acidity). This means that given enough time, enough starting material and enough weathering, you can go from having a lot of rock with not very much aluminium in it to having not much rock with a lot of aluminium in it!

Described image
Figure 2.5 Solubility at different pH levels

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