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Mechanism of chemical weathering

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Mechanism of chemical weathering

Post  bejoy on Tue Mar 04, 2014 1:51 am



Igneous rocks are generally termed as fire rocks, which are formed either underground or above ground. Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface and by solidification of molten rock material. Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rocks are commonly classified by their composition and texture.

There are two basic types

1) Intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface.
2) Extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.

Igneous rocks can be classified according to chemical or mineralogical parameters


Felsic igneous rocks containing high silica content
Intermediate igneous rocks containing between 52-63 per cent SiO2
Mafic igneous rocks
Ultramafic rock igneous rocks


Most of the igneous rocks are composed of both major and minor important elements in the Earth's crust.

Major elements found in igneous rocks include


Silicon dioxide
Aluminum oxide
Sodium oxide
Potassium oxide
Phosphorus pentoxide


Minor elements in igneous rocks include


Lithium
Beryllium
Scandium
Zinc
Copper
Lead
Fluorine
Chlorine
Sulfur
Barium


Facts and process of chemical weathering

The primary agents in chemical weathering are water, oxygen, and acids. These react with surface rocks to form new minerals that are stable in, or in equilibrium. Any excess ions left over from the chemical reactions are carried away in the acidic water.

When a rock is brought to the surface millions or billions of years after it has formed, the original minerals that were crystallized deep in the crust under high pressures and temperatures are unstable in the surface environment and eventually break down.

Rocks are not only changed by water, wind (oxygen), acids and ice but can also be changed by chemicals. Chemical weathering is most intense in areas that have abundant water. Acids are chemical compounds that decompose in water to release hydrogen atoms. Hydrogen atoms frequently substitute for other elements in mineral structures, breaking them down to form new minerals that contain the hydrogen atoms.

Rainwater commonly contains dissolved carbon dioxide and is slightly acidic in nature. The burning of coal, oil, and gasoline elements releases carbon dioxide, nitrogen, and sulfur into the atmosphere, which react with rainwater to form much stronger carbonic, nitric, and sulfuric acids that damage the environment. This process of releasing acids into the environment is well known as acid rain.

One of the more common and visible chemical weathering reactions is the combination of iron and oxygen to form iron oxide (rust).

Chemical weathering definition

Chemical weathering is the process by which the mineral compositions of rocks are changed. Chemical weathering can cause minerals to decompose and even dissolve.

Chemical weathering is much more common in locations where there is a lot of water. It is the most important process for soil formation. Chemical weathering becomes more effective as the surface area of the rock increases. Since the chemical reactions occur largely on the surface of the rocks, therefore the smaller the fragments, the greater the surface area per unit volume available for reaction.

Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions. These reactions include hydration, hydrolysis, carbonation, oxidation and reduction. These processes either form or destroy minerals, thus altering the nature of the rock’s mineral composition. Temperature and, especially, moisture are critical for chemical weathering.

Chemical weathering processes

Following processes are involved in chemical weathering

Hydration
Hydrolysis
Carbonation
Oxidation
Reduction

Hydration

Hydration is one of the most common processes in nature and works with secondary minerals, such as aluminum oxide and iron oxide minerals and gypsum. Soil forming minerals in rocks do not contain any water and they undergo hydration when exposed to humid conditions.

a) 2Fe2O3 + 3HOH   →     2Fe2O3 .3H2O
  (Hematite) (Red)            (Limonite) (Yellow)

b) Al2O3 + 3HOH     →     Al2O3 .3H2O
  (Bauxite)                      (Hyd. aluminum Oxide)

Hydrolysis

Hydrolysis is the most important process in chemical weathering. It is due to the dissociation of H2O into H+ and OH- ions which chemically combine with minerals and bring about changes, such as exchange, decomposition of crystalline structure and formation of new compounds.

KAlSi3O8 + H2O       →        HAlSi3O8 + KOH
(Orthoclase)                       (Acid silt clay)

HAlSi3O8 + 8 HOH  →  Al2O3 .3H2O        +      6 H2SiO3
(Recombination)          (Hyd. Alum. oxide)        (Silicic acid)

Some substances present in the rocks are directly soluble in water. The soluble substances are removed by the continuous action of water and the rock no longer remains solid and form holes, rills or rough surface and ultimately falls into pieces or decomposes. The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids.

NaCl + H2O → Na+, Cl-

Carbonation

Carbon dioxide when dissolved in water it forms carbonic acid.

2H2O + CO2    →   H2CO3

This carbonic acid attacks many rocks and minerals and brings them into solution. The carbonated water has an etching effect up on some rocks, especially lime stone. The removal of cement that holds sand particles together leads to their disintegration.

CaCO3       +       H2CO3   →     Ca(HCO3)2
(Calcite)                                 (Ca bi carbonate)

Oxidation

Oxidation is the process of addition and combination of oxygen to minerals. The absorption is usually from O2 dissolved in soil water and that present in atmosphere. The oxidation is more active in the presence of moisture and results in hydrated oxides.

4FeO (Ferrous oxide) + O2     →   2Fe2O3 (Ferric oxide)
4Fe3O4 (Magnetite) + O2        →   6Fe2O3 (Hematite)
2Fe2O3 (Hematite) + 3H2O     →   2Fe2O3 .3H2O (Limonite)

Reduction

The process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil color to grey, blue or green as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes place.

2Fe2O3 (Hematite) - O2 → 4FeO (Ferrous oxide)

Reference

[1] © From, http://www.agriinfo.in/?page=topic&superid=4&topicid=249
[2] © From, http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1124303183&topicorder=4&maxto=7
[3] © From, http://www.ux1.eiu.edu/~cfjps/1300/weathering.html
[4] http://www.worldofchemicals.com/wocart/443/mechanism-of-chemical-weathering.html

bejoy

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