What is Chemical Weathering of Rock?

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It is a process of mineral alteration, which consists of a number of chemical reactions, whereby the primary minerals (i.e. the original silicate minerals of igneous rocks) are converted into new compounds, the secondary minerals, which are stable in the surface environment.

The sedimentary and metamorphic rocks are also substantially affected by the chemical processes of weathering. Chemical weathering is the result of interaction of rocks of the superficial layers of the lithosphere with chemically active constituents of the atmosphere.

Most important of these constituents are water, carbon dioxide and oxygen. The effectiveness of these constituents depends on the following factors:

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(i) Size of the particles

The smaller the particles, the greater is the surface areas which may come in contact with the atmosphere. Since substances react chemically along surfaces, the greater the surface area, the more pervasive is the chemical weathe­ring.

(ii) Composition of the rock

According to the general order of stability of minerals, as tabulated by Goldich (1938), quartz is very stable. As such, rocks composed primarily of quartz de­compose very slowly. Similarly, rocks composed mostly of ferromagnesian minerals like olivine, augite etc. are highly suscep­tible to chemical weathering.

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(iii) Favourable temperature and humidity condition. The chief chemical weathering processes are ,

(a) Hydration;

(b) Hydrolysis;

(c) Oxidation;

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(d) Carbonation; and

(e) Solution

(a) Hydration :

The term hydration refers to the chemical union of water with a mineral. In this process, certain minerals take up water, which lead to a change in the mineral composition of rocks.

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Due to the absorption of water, the minerals expand, causing more stresses within the rock. The most well-known examples of hydration occurring in nature is the altering of anhydrite to gyspum, hematite to limonite as shown below:

(i) CaS04 + 2H20 ——–àCaS04.2H20 (Gypsum)

(anhydrite) (Water)

(ii) Fe203 + nH20 ———–àFe2O3.nH2O

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(hematite) (Water)

It is to be remembered that when a mineral is hydrated, it is the (0H-) ion that is built into the new crystal lattice.

Since the process of hydration causes swelling of some miner­als, this in turn produces a considerable mechanical effect on the enclosing rocks and causes even local dislocations.

(b) Hydrolysis

It is the process of exchange reactions between the bases of the minerals and the hydrogen ions of the elecuolytically dissociated part of the water. As we know, water is an active chemical agent because it is always to some extent dissociated into H* and (OH)” ions.

The acidity and alkalinity of water is measured by the concentration of hydrogen ions i.e. the pH value. Water having the pH value greater than 7 is alkaline and less than 7 is acidic in nature. The higher the r H value, the more dissociated is the water and the stronger its action as a chemical weathering agent.

The most characteristic example of hydrolysis is that of thepotash- feldspar(orthoclase), where the reaction can be shown as

KAISi308 + HOH ——à HAISi308 + KOH

(orthodase) (Water) (alumino-silicic acid) (potassium-hydroxide)

But since carbon dioxide (C02) is almost invariable present in the atmosphere, the rain water usually contains dissolved C02. Water is more readily dissociated when it contains free C02. This reacts with the potassium hydroxide giving rise to potassium-carbonate and water.

H20 + Co2— ► H+ + (HCO,)-

Carbonic acid

2KOH + H2C03 > K2C03+ 2H2O

The alumino-silicic acid, formed through the hydrolysis of potash- feldspar, is unstable and breaks down with the formation of clay mineral and collidal silica.

Another example of hydrolysis is the formation of magnesium hydroxide from olivine, as

MgFeSiO” + 2HOH »Mg(OH)2 + H2Si03 + FeO

(Olivine) (Water) (Magnesium (Silicic acid) (Ferrous oxide)

-hydroxide)

(c) Oxidation

The process of oxidation involves the chemical union of oxygen atoms with atmos of other metallic elements. Thus the minerals are altered with the production of oxides.

The free oxygen of the atmosphere and of the air dissolved in water is a most active chemical reagent. It acts actively upon many minerals, especially those containing iron.

The ferromagnesian minerals like pyroxenes, hornblende and olivine etc. rapidly un­dergo oxidation in the surface conditions, producing a brown crust consisting largely of oxides of iron. The oxidation effects are most conspicuous in warm, moist climates.

An example of oxidation of pyrite may be shown as follows:

FeS2 + n02 + mH2O -► FeSO,-► Fe2(SO4)3–àFe2O3.nH2O

(Pyrite) (Oxygen) (Water) (Ferrous (Ferric (limonite)

sulphate) sulphate)

(d) Carbonation

Carbon-dioxide is a gas and is a common constituent of the earth’s atmosphere. This is the process by which carbon dioxide is added to minerals to form certain carbonates.

Rain water in course of its passage through the atmosphere, dissolves some of the carbon-dioxide present in the air. It thus turns into a weak acid called carbonic acid

H2o + c02 -► h2co3

(Water) (Carbon-dioxide) (Caibonic acid)

This water containing carbondioxide is capable of reacting with several minerals. It dissolves carbonates with relative ease. Thus, sometimes a limestone may be entirely removed leaving behind the insoluble materials it contains.

CaC03 + H20 + C02—–àCa (HC03)2

The process of carbonation is more effective with minerals con­taining alkali metals like sodium and potassium as well as calcium and magnesium.

A characteristic example of decomposition of the mineral feldspar through the process of carbonation, can be illustrated as follows:

2KAlSi308 + H2CO3 + H20 = K2CO3 + Al2Si205(0H)4 + 4Si02

(Orthodase) (Carbonic (Water) (Potassium (Kaociinite)’ (Silica)

acid) carbonate)

In this illustration, some of the components of feldspar are removed in solution leaving the rest in the form of clay (Kaolinite is one of the common mineral in clay).

The effect of this process is well noticed in the limestone or chalk areas in the humid regions of the world.

(e) Solution

Some of the minerals get dissolved by water and thus removed in solution. Even though the process of solution is wide spread in natural environments, pure water is not an effective solvent for any of the common rocks.

Solution takes place at different rates for different rocks. Maximum degree of solution is found to be with chlorides (halite (NaU), sylvite (KCL) etc. Sulphates and carbonates are less soluble.

But the joint action of carbon dioxide and water enhances the effectiveness of the process of solution. As for example, although calcite (CaCO3), the chief mineral in limestone is very slighdy soluble in pure water, it has been noticed that vast quantities of limestone have been dissolved and carried away by the water containing carbon dioxide.

It has also been found that silica even though is a mineral of high stability under surficial condition undergoes solution in alkaline fluid.

The process of removal of soluble material from the rocks in solution by percolating water, is termed Leaching. These processes of chemical decomposition of rocks go on side by side resulting in the weathering of the rock masses.

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