Weathering

Chemical weathering

Primary minerals in igneous rocks, synthesized at high temperatures in the absence of oxygen, are generally unstable at the earth's surface and slowly decompose to produce secondary minerals. This is the process of chemical weathering, and it is the first step in the formation of soil material.

Here, we will look at the weathering of potassium feldspar, a common mineral in granite and other igneous rocks. Similar weathering reactions occur for many other primary aluminosilicate minerals.

Hydrolysis

Chemical weathering occurs when the surface of a mineral crystal is attacked by percolating water containing some H⁺.

As the crystal slowly dissolves, its constituent cations (like K⁺, Al³⁺, and Si⁴⁺) are released into the soil solution.

To understand what happens next, you need to know about the solubility of each of these cations.

K⁺

K⁺ is soluble in the soil solution and may be leached from the soil. For more information, look at Leaching in this module.

Si⁴⁺

Si⁴⁺ reacts to form Si(OH)₄ (silicic acid), which is slightly soluble in the soil solution:

Si⁴⁺ + 4OH⁻ → Si(OH)₄

Being slightly soluble, it may be leached or retained in the soil profile, depending on environmental conditions.

Al³⁺

Al³⁺ is insoluble except in strongly acidic soil environments:

Al³⁺ + 3OH⁻ → Al(OH)₃

It is usually retained in the soil profile.

Clay mineral synthesis

The hydrated Si⁴⁺ and Al³⁺ cations released by weathering react together to form secondary clay minerals. The type of clay mineral that is produced depends on the amount of leaching taking place in the soil.

To see why this is so, consider three different leaching environments.

Very strong leaching

The large quantities of water moving through the profile are sufficient to remove all the Si(OH)₄ as fast as it is released by weathering. The clay mineral produced therefore comprises sheets of octahedral Al(OH)₃ (gibbsite).

Moderately strong leaching

Leaching is strong enough to remove most, but not all of the tetrahedral Si(OH)₄. Some remains and combines with the octahedral Al to produce layers of a 1:1 silicate clay mineral like kaolinite.

Weak leaching

Under weak leaching conditions, most of the Si(OH)₄ is retained in the soil, and there is sufficient for clay minerals to form with two tetrahedral sheets to one octahedral sheet. These are the 2:1 clay minerals like montmorillonite.

Thus, the effect of different leaching environments on the ratio of Si to Al is the main determinant of clay mineralogy in soils. Other cations are also found in some clay minerals, and so their presence may sometimes exert an influence.

For more information on clay minerals, look at the Soil Minerals module.