Primary and Secondary Minerals | Soil Science

After reading this article you will learn about the primary and secondary minerals present in the earth’s crust.

Primary Minerals:

The primary minerals are those which are formed owing to the crystallization of the molten magma. We have already seen that the earth’s crust contains dominant amount of oxygen (46.60%) followed by silicon (27.72%).

In order to achieve neutrality between the negatively-charged oxygen and the positively-charged silicon, there would be a greater tendency for silicon and oxygen to combine to form the basic compound, called the silicon-oxygen tetrahedron (SiO₄). This explains the dominance (>90%) of silicate minerals (compounds containing silicon and oxygen, and one or more metal cations) in the earth’s crust.

A more complex linkage is in the sheet structure in which all tetrahedral (away from the sheet edge) share three oxygen ions with the neighbouring tetrahedra. This further reduces the overall negative charge imbalance to one only. The sheet structures are phyllosilicates, of which micas are a typical example which is depicted in Fig. 2.4.

The maximum linkage is attained in a 3-dimensional network in which all oxygen ions are shared between tetrahedra. Here each silicon is associated with 4 shared oxygen ions.

This results in 4 positive and 4 negative charges for any tetrahedral unit, thereby achieving an electrically neutral structure. Such minerals with 3-dimensional structures are termed as tectosilicates; typical examples of which are quartz and feldspars.

 Secondary Minerals:

The secondary minerals are formed at the earth’s surface by weathering on the pre-existing primary minerals under variable conditions of temperature and pressure. During weathering, water accompanied by CO₂ from the atmosphere plays an important role in processes, like hydrolysis, hydration and solution. As a result the primary minerals are altered or decomposed.

Feldspar + water → clay mineral + cations + anions + soluble silica

One may observe that because of weathering, many elements are released into solution; a part of which may be used as a source of plant nutrients, a part may be leached out into the ground-water; still another part together with other constituents (like CO₂, H₂O) of the environment may recombine to form secondary minerals.

The most commonly formed secondary minerals are clay minerals (e.g. illite, montmorillonite and kaolinite) and iron and aluminum oxides.

Clay Minerals:

The clay minerals carry a significant negative electrical charge on their surface and have a structure like that of mica. In some cases, the groups of sheets are not firmly bounded together and water molecules can enter in their crystal lattice. This can cause considerable swelling due to change in soil moisture content.

This is the case in Vertisols (Black Cotton Soils) of India and of NE Iraq, where deep and wide cracks on the surfaces are suggestive of the swell-shirk characteristics of soil clays.

Owing to the negative electrical charge on the clay surfaces, the cations are attracted to regions of electrical charge around the clay minerals. These cations do not get bounded permanently and can be exchanged for other cations. The amount of charge varies depending upon the type of clay mineral and it is referred to as the cation exchange capacity.

Because of this exchange there is always a balance between the concentration of cations in soil water and those adsorbed on the surfaces of the particles. Rain water percolating through the soil leaches out many metal cations (K, Na, Ca, and Mg) together with the old soil water and replaces it with new water containing H⁺ ions and may render the soils acidic in reaction.