Chemistry of Sahara Sand: Elements in the Dunes


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The Sahara is huge – but the sand holds links to the very beginnings of Planet Earth. Image by Wikigab

The Sahara is a large expanse of land that few of us will ever visit – it has been a desert for a long time, and it’s getting bigger.

Processes such as desertification spread the landscape adjacent to semi-arid regions. As the sand spreads, it’s instructive to examine the grains to understand more about the desert as a whole.

What is the Sahara Desert?

So, exactly what is the Sahara? Merriam-Webster online defines the Sahara as the following:

“desert region N Africa N of the Sudan region extending from the Atlantic coast to Red Sea.”

It further states:

Sahara   (Concise Encyclopedia)

Largest desert in the world, encompassing almost all of northern Africa. Covering an area of about 3.3 million sq mi (8.6 million sq km), it is bounded by the Atlantic Ocean, the Atlas Mountains, the Mediterranean Sea, the Red Sea, and the Sahel region.

While examining the geographic and geologic record to understand the Sahara, we may not know where to begin. The ‘physical record’ is buried under layers of sand, which composes part of the physical record in itself.

What Makes Up Sahara Sand?

The Sahara sands are primarily derivatives of silicon Si and carbonates CO3. Technically, the following constituents are most prevalent:

Sahara sand components – it’s a little more complicated than you’d think. Image adapted from Diaz-Hernandez et al, by John Jaksich, all rights reserved.

The major components of the Sahara sand, the clay minerals: kaolinite, smectite, and dolomite, occurred as a result of weathering. These consist of the elements aluminum, silicon, magnesium  and oxygen. The presence of oxygen (or the oxidative states in aluminum, silicon, and magnesium) is indicative of the long term exposure to water and oxygen related species, O2, CO2 and H2O.

The chemical reactions of weathering in the inorganic species (magnesium, aluminum, and silicon)  with O2, CO2, and H2O might be better viewed as oxidative and reductive coupling.

The magnesium, aluminum and silicon served to donate their electrons while the oxygenated species accepted the electrons. The other components, calcite (CaCO3, calcium carbonate) and halite (NaCl, sea salt) also resulted from the oxidative/reductive coupling (or weathering).

It is suggestive that the original molecular species (magnesium, silicon  and aluminum) would be difficult to place (or find) on earth.

We can say that the presence of water altered the original landscape permanently.

Selective Crystallization and Buoyancy

In order to understand how and why these are the most common components, we need to understand the ground beneath us. The birth of the Sahara region involved processes such as selective crystallization and buoyancy.

The early Earth was a magma ocean. As it cooled, parts of the inner crust and mantle crystalized selectively. According to geochemists, certain molten materials cannot solubilize (or mix effectively). Metal globules of iron and nickel sank towards the core. Sulfurous compounds did not mix with the less dense alumino-silicates and carbonates and sank as well.

Other processes were in play, such as differential crystallization – a process in which materials with higher (freezing) melting points crystallized first as the earth cooled. Counterintuitively, the outer crust became solid last (due to its cooler melting point). The elements we find in Sahara sand are part of that process – surface elements which became solid last – during the earliest years of Earth.

Natural Laws Govern the Sahara

The physical and chemical processes of the Sahara are governed by known natural laws that at time seem other-worldly.  Despite its reputation  of being empty and non-livable, the Sahara is a dynamic, intriguing environment.

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